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  • Rapt in Awe

    My Journey through the Astronomical Year

    Think of this as a "companion text" to this, the main web site. Not required reading, butI hope you'll find it interesting and helpful.

Building MESSENGER – the Model

NASA provides a neat little model you can make of the MESSENGER spacecraft by simply printing out the directions and doing a little cutting and folding. You can download it here. And if you have the energy and time, you can build a more sophisticated version by downloading this.

If all goes well, MESSENGER will become the first spacecraft ever to go in orbit about the planet Mercury. It will do so on March 17, 2011.  You can read all about MESSENGER – and how to see Mercury yourself  in March of 2011 –  by going to our March “Events” post.

Here are step-by-step photos of building the first – and simpler – of the two  MESSENGER models.  We’ve added a simple way to effectively display your work.


  • scissors
  • paper glue (we used rubber cement)
  • model knife (optional)


  • 4 sheets of paper – we used  24lb
  • a couple coffee storrers,  pipe cleaners, or something similar (most straws are too big around)
  • short length of  black thread
  • small piece of clear tape
  • paper clip

Building time is about 30-40 minutes.)


1. Assuming you have printed out the sheets, cut out the spacecraft body. (Click on any image to view a larger version.)

2. Cut out the three white circles (indicated by arrows) on the spacecraft body. I found a model knife was best for this. (You can actually do this as the first step – but in any event do it before folding.)

3. Fold along white lines to make the box-like body.

4. Put fast-drying glue on the edges of the folds.

5. Finish spacecraft body and set aside to dry.  Cut out the gold spacecraft sunshade.

6. Fold and  glue sunshade together with color side out.

7. Cut out the strip labeled “bridge,” fold lengthwise, and glue together, color side out.  Set aside to dry.

8. Cut out solar panels.

9. Fold, but do not glue together until you have noted the position the boom (coffee stirrer) will be inserted, This is marked by dotted lines on the dark side of the solar panels.

10. Put glue onthe inside being careful to leave the area marked by the dotted lines free of glue. (A tad tricky, since the lines are on  he other side.)

11.Fold the bridge along the dotted lines and glue one side of it to the spacecraft body in rectangle outlined on it.

12. Put glue on the taps of the bridge, then glue the sunshade to it. Set aside to dry.

13. We found that it was difficult to simply stick the boom into the slots left for them in the solar panels, but these slots were easy to open with any sharp object,. We used a toothpick to do the job.

14. We mounted one solar panel tot he boom,t hen slid the boom through the wholes in the spacecraft body.  We did not glue the solar panels as they seemed to fit tightly enough.

15. Here’s the almost finished spacecraft before  folding the sunshade to give it an arched shape and adding the thread and paper-clip hook to display it. Note the solar panels and gold side of the sunshade are pointed in the same direction – which would be towards the Sun with the instruments aimed at Mercury.

16. Ooops – alomost forgot the boom. Didn’t have a straw, or stirrer as recommended, so we used a pipe cleaner, cut down to size. We also folded the sunshade into a gentle arch that looks more like the pictures of the craft.  However, we found it awkward to display  the spacecraft properly by just sitting it down on something.  So we added a piece of black thread to the top, center edge of the sunshade with clear tape. On the other end I tied a paperclip , bent into a hook to make an easy hanger.  Here’s the thread taped to the sunshade.

17. And here’s the finished model, dangling under the lamp over the dining room table – a fitting. space age centerpiece for March, 2011! Hey – it’s a space craft. It’s not supposed to sit on the table. it’s supposed to be out there flying.

Looking at this  little modest model gives me pause . I try to develop a sense – in myself and in my visitors to Driftway Observatory – of the incredible emptiness of space by having one person hold a soccer ball representing our Sun while another visitor holds a 2mm glass bead representing our Earth.  On this scale Mercury would be less than 1mm in diameter – barely visible even when in your hand.  But I ask the person with the bead to hold it at what they think is the correct distance from the Sun. Usually they guess this to be a foot or two away – sometimes boldly they move several feet away. But no one guesses the correct answer, which is about 75 feet away. Now think of that. A 2mm bead – Earth – out at 75 feet from our soccer ball Sun!   Another 2mm bead – Venus – would be placed at about 54 feet out from the Sun.  And then a third, tiny Mercury – at roughly 29 feet.

And now try to imagine how tiny the real MESSENGER  spacecraft – roughly the size of a table – would have to be made to fit into this scale model! Then close your eyes picture the MESSENGER entering this vast, empty interplanetary ocean  and traveling for seven years in that emptiness, then arriving at just the right spot and just the right time, to be placed in orbit.  This vital little craft with its complex instruments going all that distance – almost 5 billion miles in total at speeds that sometimes exceeded 140,000 miles an hour.  (To put that in perspective, the fastest rifle bullet goes about 2,700 miles an hour and our Apollo astronauts traveled about 25,000 miles an hour during part of their lunar journey.

And all around Messenger is just about nothing except for a hostile emptiness and the incredible heat of our Sun as it moves in close.  Awesome! Just plain awesome. Three cheers for little MESSENGER – and three cheers for us – a species that dares to challenge the hsotile vastness of space, and send it’s robot silicon  and ceramic envoys on a mission of exploration for new knowledge.

Step 3 – Connecting the dots – or why I love asterisms!

When it comes to learning the night sky, constellations can be very confusing and asterisms can be very helpful. But to understand why, let’s make sure we know which is which.


  • Asterism – An informal  collection of stars that form a simple pattern.
  • Constellation – A collection of stars that frequently represents a traditional figure from mythology. The entire sky is divided into 88 such constellations with specific boundaries that are recognized by international agreement.

Many people have never heard the word asterism, but they probably know one or more asterisms, and chances are they think these asterisms are constellations.  Best example: The Big Dipper.  It’s not a constellation. It’s an asterism.  That is, it is an informal collection of seven stars that form the pattern of a water dipper.  But this asterism  is a significant part of a much larger constellation known as Ursa Major, the Great Bear.

Now here’s the problem.  Asterisms are invariably simple. They are usually made of just the brightest stars, and they form patterns that are easy to recognize and remember.  Constellations can be quite complex; to make them look like their names imply, you frequently have to use stars that are too faint for suburban observers to see, and even then they seldom really look like the mythical figures they represent. Even their names can throw the modern ear for a loop. Do you know, for example, what a camelopardalis is? The myths associated with them, while interesting and fun, are rarely known by modern people.  But don’t take my word for it. Here. Give it a try. Take the Constellation Challenge. Use the link below to download a version of the following image suitable for printing, and see how you do. (Of course you can do this in your mind  on the computer screen if you like.)

Click image for larger version.

Click here to download a version of the Constellation Challenge for printing.

Now try the Asterism Challenge.

Click image for larger version.

Click here to download a version of the Asterism Challenge for printing.

When you have done – or attempted to do –  both the Constellation Challenge and the Asterism Challenge, either on paper or mentally on the computer screen, then go to “page 2” which is linked below.

Coloring the stars – an exercise for all seasons

Trying to identify the true colors of stars, as we see them, is fun, challenging, and instructive.

Your assignment, should you choose to accept it, is to develop accurate color swatches that represent the colors of the bright stars, including our Sun, as they actually are seen and in so doing learn:

  • How to see color in the stars
  • What the color tells us about each star

The chart shows the Winter Hexagon because many of the brightest stars can be seen there all at one time, but it also includes swatches for several bright stars that are prominent in the spring, summer, and fall.

This image of the Winter Hexagon was taken by Jimmy Westlake looking at the skies over Stagecoach, Colorado. Look carefully and you can see color in some of the stars - especially if you click on the image to see the larger version. (Copyright © 2007-2011 JRWjr Astrophotography. All rights reserved.)

Star colors are real. They relate to a star’s temperature and from them we can surmise much more about a star. But they also are very subtle. I think of them not as colors, but as tints. I see stars essentially as white lights to which a little color has been added to tint it one way or another. I believe most people don’t see the colors at all when they first look at the stars and this can be frustrating, especially if you’ve read that Betelgeuse, for example, is an “orange” star.

With the naked eye you only will see color on the brightest stars because our eye simply needs a lot of light to detect color. In fact, point your binoculars at those bright stars, and you should find it easier to detect the colors because the binoculars gather more light. You can train yourself to see star colors, though people do differ in this ability. But for most, the colors are really quite obvious on some of the brighter stars, once you know what you can expect to see. And that’s what this little exercise is for – learning what you can expect to see.

Your main tools will be the chart and the color table provided here. You’ll have to provide the primary colors (red, yellow, and blue), plus white in some easily blendable medium, such as common water, tempera, or poster paints. Nothing fancy needed and no special painting skills required.

First, here’s the chart you will be coloring.

You can download a version for printing by clicking here.

And here’s the color table you will use as your guide

Your task is simple.

Next to each star on the star chart is its spectral classification. This consists of a letter and number. The letters go from blue to red stars in this order: OBAFGKM. Each letter gets divided into a numerical sub-classification from 0-9. So a “B0” star would be just at the beginning of the “B” category. A “B9” star would be at the end of that category and almost into the next one. The star chart shows the spectral classification for each star. Match that with what you see in the color table. Then determine its color.

You will notice that there are two different color scales in the table. That’s because the way we see color depends in part upon the environment in which we see it. The “conventional color” is what would be seen if the star were put under high magnification and projected onto a white sheet of paper in the daylight. The “apparent color” is what is seen by the naked eye in a dark sky. That is the color you want. That’s what you’ll try to duplicate by mixing your water colors and painting the swatch next to each star so it matches its classification – and thus what you are likely to see in the night sky.

The result will be a chart that will help you know what color to expect to see when you look at the stars in the sky. I should add that if you see a photograph of these stars, the colors will be similar, but different. That’s because the colors in a photograph depend on the color sensitivity of the film or computer chip used to record them – which is not the same as your eye. So you cannot use a photograph as an absolute guide to what you will see. The chart you make, if done well, will be a much better guide.

When you look for star color, make sure the star is high in your sky – hopefully at least 30 degrees or more above the horizon. All bright stars near the horizon will appear to flash many brilliant colors. Those colors – like the colors in our sunrises and sunsets – are caused by the Earth’s atmosphere. When you are looking at an object near the horizon, you are looking through much more air than when you are looking at a star high overhead.

Of course, you are going to have to use your judgment in making the color swatches, and you might experiment a bit on another piece of paper. That’s why I recommend using some sort of water color for this activity – so it’s easy to blend and thin your colors to get the color you want – the one that is closest to what you actually see. Of course to get orange you mix the yellow and red – and white will come in handy to lighten any of the colors.

Get the idea? Will your colors be perfect? I doubt it. But experimenting this way will give you a much better feel for how subtle star colors are and exactly what you are looking for when you go out at night. Too often people are confused and disappointed because they read that Betelgeuse or Aldebaran is a red or orange star – and when they read that, they are thinking of the conventional red or orange – quite naturally – but look at the chart and look at the difference between conventional colors and apparent colors.

More about a star’s spectral class

OBAFGKM is certainly a crazy order, I know. It started out to be an alphabetical list more than a century ago. But as they learned more about the stars, the letters got scrambled. Here’s an easy way to remember the order:

Oh Be A Fine Girl/Guy Kiss Me

At first they thought letters would be enough, but the more they learned about stars, the more they saw there were many subtle variations that were important. So for each letter there is a sub-classification system that goes from 0-9. Thus an O9.5 star, such as Mintaka, is in the “O” spectral class (blue) but about as close as one can get to being a “B” (blue white) star. Don’t be too concerned about these numbers, however. You’ll find it difficult enough just to get colors that accurately match the letter classifications. Besides, I’ve found that different sources sometimes give different numbers for the spectral classification of a specific star, so I see them as a good rough guide as to how solidly a star is into a specific class but not something to take overly seriously in terms of what we can detect with our eyes.

Mintaka, incidentally, is included here because bright “O” stars are hard to find. Mintaka was one of the easiest “O” star to identify, being the western-most star in Orion’s Belt. But coincidentally, Ainitak, the star at the other end of the belt, is also an”O” and  a bit brighter. But ay O9.7 it, too, just makes it into the “O” class by the skin of its teeth. In fact, it’s a bit closer to being a “B” star than Mintaka – but I guarantee you won’t see any difference.

“M” stars are even more difficult to find. True, Betelgeuse is one in the Winter Hexagon, and  in the summer we have another brilliant “M” star – Antares, the brightest star in the Scorpion.  But these are special. They both are Supergiants – stars that are going through their death throes and have expanded tremendously.  The vast majority of “M” stars are of average size, and in fact, these average-sized “M” stars are the most common stars in the universe – yet there is not a single “normal” class “M” star visible to our naked eye, let alone as bright as the stars that form the Winter Hexagon.

I also added our Sun to the chart. DO NOT LOOK AT THE SUN TO TRY TO DETERMINE COLOR. YOU WILL DAMAGE YOUR EYES. We were all taught as children to color our Sun yellow – and this is correct if you are talking about conventional color. But the Sun is a class G2 star, and I suggest you color its swatch the “apparent” color it would appear to our naked eye were we seeing it as just another bright star in our night sky. This means it would appear the same as Capella.

Binocular and telescope users can see many double stars, and some of these provide striking color contrast, such as the blue and gold of Albireo. Seeing two stars close together that are of different colors makes it even easier to see star colors but also presents a whole new set of challenges, and experienced observers frequently differ on what the colors of the double stars are. John Nanson has explored this in an excellent post to the “Star Splitters” blog that we co-author. To learn more about these stars and the special challenges of determining their colors, read John’s post here.

What we can surmise from the colors

As you can see from the temperature scale, blue stars are hot – red stars are “cool.” Cool, that is, as far as star temperatures go. They are still very, very hot: 3700 Kelvin is about 6,200 degrees Fahrenheit! (Steel melts at about half that temperature.)

So once you notice a star’s color, what more can it tell you about the star? A detailed answer is beyond this exercise, but it means you can make a very good guess about some other important characteristics of the star.

Here’s a summary in table form of what the spectral classification tells about the size and life expectancy of a star, and even hints at how it will probably die.

  • Ninety-five percent of all stars are on what is called the “main sequence.” Most of the stars that are not on the main sequence are white dwarfs. But a few others are giants or supergiants. Roughly one percent of the stars fall into one of the giant categories, such as Antares.
  • The lower limit for the mass of a star is 1/80th the mass of our Sun – or about 13 times the mass of Jupiter.
  • Temperatures are for a star’s surface. The interior is much hotter.
  • Age – “O” stars have short lives and thus die first, then “B,” etc. No “dwarf” “K” or “M” star has died yet – the universe isn’t old enough.

Luna-see II: Frame it!

For closely related links, see:

The purpose of this little exercise is to drive home the significance of the Earth being tilted on ita axis by 23.5 degrees.  That fact accounts not only for our seasons, but for the rapidly changing, north-south position of the Moon in our sky – changes that many sky watchers are either unaware of, or can’t explain if they are aware of them. But it’s not that complicated – it’s all in the way we lean, and we can see this plainly if we make ourselves a simple window frame with which to view the heavens.

To do that simply click on this text to download and print a proper frame. Then cut out the center portion with scissors. It’s a minor exercise, but it gets you past one those counter-intuitive realities where things go up in your view when you think they might go down.

We’ll use this frame first to illustrate the movement of the Sun in our sky at different seasons. Pick a small object a reasonable distance away and pretend that object is the Sun.


1. Hold the frame by the sides with thumb and forefinger of both hands.

2. Stretch out your arms in front of you so you’re looking through the frame and your eyes are level with the center of the frame.

3. Center your “Sun” in the frame. Your horizon is down, the zenith up.


4. Being careful to keep the center of the frame in line with your eyes, tilt your head back - raising your arms to keep your eyes in the center of the frame as you tilt your head. Did the Sun appear to move up or down when you did this? That is, did it appear to get closer to your horizon, or closer to the zenith?

When you did Step 4 you were simulating the relationship between Earth and Sun in the northern hemisphere winter.  It is low in our southern sky as it crosses from east to west each winter day.


5. Repeat Step 4, but this time tilt your head forward. Does the Sun move up or down?

When you did Step 5 you were simulating the relationship between Earth and Sun in the northern hemisphere summer. Then it crosses high in our sky.

In the starting position – with your head level – you are simulating the relationship of the Sun in spring and fall when it  rises pretty much due east, sets due west,  and crosses at a midway point half way between the low extreme of winter and the high extreme of summer.

There’s one problem with our little exercise though – and it’s critical to understand.  We changed our view  by leaning forward or leaning back. The Earth never changes the tilt of its axis. It is always leaning the same way. What changes, of course, is we’re going around the Sun. So when we’re on one side of the Sun, we in the northern hemisphere are leaning towards the Sun – and when we’re on the other side of it, we’re leaning away from the Sun. It’s not because we change the way we lean. it because we moved from one side of the Sun to the other.

The lesson this exercise should drive home is how the way we are leaning changes the apparent height of the Sun in our sky. That is, how far above our southern horizon it appears to be when it reaches the highest point in its east-west path across the sky. Or simply put, how high it is at noon.

Framing the Moon

If you are comfortable with how our leaning changes the position of the Sun in the sky as we go around the Sun, now consider the Moon.

We are not going around the Moon.

It is going around us – every month!

But the effect is the same as when the Earth goes around the Sun – when the Moon is on one side of us, we are leaning towards it. When it is on the other side of us, we are leaning away from it.

We haven’t changed our leaning. The Moon has simply gone around us. But the impact is the same as with the Sun. From our perspective, the Moon appears to be highest in the sky when we are leaning towards it – and lowest when we are leaning away from it.

So the motions we observe with the Sun – the way it gets lower in our sky in winter and higher in summer – are duplicated by the Moon – EVERY MONTH.

Simple? Yes, but . . .

Here’s where it gets a little complicated.

At the same time that the Earth goes around the Sun, the Moon is going around the Earth – but much quicker. It makes 12 trips for our one. So to really understand where the Moon is going to be on any given night, you have to combine your knowledge of the Earth’s relationship with the Sun, the Moon’s relationship with the Earth, and how the Moon goes through phases.

Don’t let the changing west-to-east position of the Moon confuse you. As the Moon goes from New Moon to First Quarter it is higher in our sky at sunset each month. That is because of the west-to-east movement (counterclockwise) of the Moon around the Earth each month.   When we say “higher”  or “lower” in what follows we are not talking about this west-east movement. We are talking about how far north or south the Moon is in our sky – for northern hemisphere observers we are talking about  how high it is above our southern horizon when it crosses the central meridian, the highest point it gets to each 24 hours. You can think of that as how high it is at high noon! (Or maybe we should say “high Moon! 😉

It’s not hard to understand if you take it one season at a time, but it does help if you have built the Lunar-See model and made one important addition to it – you should mark the north pole of the Earth with a dot using pen or pencil so you can set the Earth in the center of your model  and lean that pole in the correct direction.

If the Earth on your Lunar-See model is wood, put a black-dot to indicate the northpole. If it is clay, stick a piece of a tooth pick in it to indicate the axis of the Earth .

Let’s try winter.

Black dot on Earth on this "Luna-See" model indicates the north pole, so in these images the Earth is tilted to the right. Notice that this tilt doesn't change from New Moon to Full Moon. What changes is the position of the Moon. At New Moon the northern hemisphere of the Earth is leaning away from it, and at full Moon it is leaning towards it. (Click image for larger view.)

In winter the Earth is leaning away from the Sun, so the Sun appears to be low in our sky.  That means at new Moon, the Moon will be between us and the Sun, and like the Sun, will be low in our sky – to the southwest.

At Full Moon the opposite will be true. The Moon will be opposite the Sun in our sky. So while we are leaning away from the Sun, we are leaning towards the Moon.

And that is why the full Moon in December rises in the northeast, appears to go nearly overhead, and then sets in the northwest.

Summer? It’s just the opposite. In June the Full Moon rises very low in the southeast as the sunsets opposite it in the northwest.  Like the winter Sun, the summer Full Moon  never gets very high above the southern horizon, and sets in the southwest.

It’s all for one simple reason – we are leaning on our axis 23.5 degrees! And how high the Moon or Sun is in our sky depends on whether we’re leaning towards it or away from it.

What about the First Quarter Moon? That’s a favorite of many lunar observers because the terminator cuts across very interesting areas and the shadows in those areas are especially long  at that time making them stand out in dramatic relief.  So when will the First Quarter Moon be highest in our sky? It will be highest when we are leaning towards it, of course. And for northern hemisphere observers that happens in March near the Spring Equinox. At that time the Earth is leaning towards the Moon just as it reaches the First Quarter phase – so that First Quarter Moon gets very high in our sky.  Of course this means the Last Quarter Moon is low in our sky in spring because when the Moon gets around to that phase it is on the other side of us and thus we are leaning away from it.  If you want to get the best look at the Last Quarter Moon, that will come in Septmber when it is highest in our sky.

So now when you look at the Moon you should be able to get all the dynamics clear in your mind. Ask yourself first, where is the Sun at that particular moment – what direction is it coming from to strike the Moon? Then ask yourself how is the Earth leaning? Towards the Moon? Away from it? Or somewhere inbetween.  You may find this a lot to swallow in the abstract – even with models to move around – but if you make the effort to put it all together when you’re out under the night sky, you’ll gain a greater sense of just where you are and what the various players in this magnificent medley of motions, are doing at that moment. If you;re really lucky yu may get one of those special, “aha!” moments when you feel like your part of the action and it all make wonderful sense in an ineffable way.

For closely related links, see:

Luna-see: Your own Earth/Moon(s) model

Yes, that’s “Moons” plural because in this little project we make two just as a matter of convenience.

The idea here is simple. I believe that concrete demonstrations stick with us and allow us to internalize abstractions – so I found the time invested in developing a simple Earth/Moon model deepened my understanding of why we see what we see, and I hope it will do the same for you. So here it is. The major abstract ideas that are made concrete here include:

  • The relative sizes not only in diameter, but volume, of the Earth and Moon.
  • The phases of the Moon and why it changes from night to night and changes position in our sky as well.
  • The true-to-scale distance between the Earth and Moon and why this simply isn’t shown in most books.
  • The reason why eclipses of the Moon do not occur at each full Moon, but are relatively rare.

Step 1

First you need to gather a few simple materials and tools. Here’s what I used, but for the wood you could easily substitute cardboard – or some other material – and use clay for the Earth ball as well as the two moons. I just like wood and found what I wanted in a local craft store that’s part of a national chain and so I assume, commonly available. Clay for the Moon balls works best because shaping them to size is a learning experience and because they cling to the wires that are used as stands – no glue needed, and you can adjust their position quickly.

Tools and materials for model - click image for larger view.

Materials needed:

1 large disc approximately 6 inches in diameter
2 small discs about 3/4-inch in diameter
1 small disc about 1/2-inch in diameter
1 1-inch ball
1 piece of string 30 inches long
2 thin, stiff wires such as used in floral arrangements – one should
be 1-inch long, the other 3-inches long

Tools needed:

pen or pencil
black felt-tipped pen (fine or very fine tip works best for writing)
small flashlight

Step 2 – Prepare a “month disc”

Using a pencil or fine ballpoint pen and protractor, carefully mark off 15 points along the perimeter of the disc, each 12 degrees apart, starting at “0”. These marks will cover half the disc.

Putting marks every 12 degrees starting at 0 and going a full 180 degrees.

Use the ruler to draw 15 lines on your disc, each going from a mark at the perimeter, through the center and clear to the other edge – this will divide the disc into 30 equal spaces, separated by 12 degrees each. Twelve degrees is the approximate amount the Moon covers in our sky each 24 hours, and the 30 divisions mark out a lunar month from New Moon through First Quarter, Full, Last Quarter and back to New.

Choose one line as your zero point, and about halfway between the perimeter and the center, place an arrowhead pointing towards the center (see highlighted area on picture above) – on this line
write “SUN” – the arrowhead indicates the direction of sunlight which for our purposes will remain constant through the month.

Considering the line just labeled as your zero point, the other lines can be numbered going counterclockwise 1-29 – the days of a lunar month – the period between two “new” Moons.

The space either side of this “0” line can be labeled “NEW.” On the other side of this line, near the perimeter, you can label the space either side of it “FULL” – Notice “NEW” moon is between the Earth and Sun; the “FULL” Moon is always opposite the Sun. You can label space 7 “FIRST QUARTER,” and on the opposite side to it, “LAST QUARTER.”

Step 3 – Adding the Earth

In the center of the disc put a small mound of clay about half-an-inch high and about an inch in diameter – take care to center this – and using the ball that is the Earth, make a depression in the top of this mound to hold the “Earth” in position at the center. (You could make the Earth ball of clay, in which case the raised mound isn’t necessary – it’s just there to keep the Earth ball from rolling away.)

If you’re satisfied everything is marked correctly, you may want to go over your labels with the black, felt tip pen to make them more prominent.

Step 4 – Adding the Moons

Make two Moons.

Take a small pinch of clay and roll it into a ball 1/4-inch in diameter. Repeat so you have two small clay balls. These represent scale models of the Moon. (The Earth is about 8,000 miles in diameter, the Moon about 2,160.) Did have trouble estimating how little clay you would need to come out at exactly one-quarter-inch in diameter? Many people do. It’s a good lesson in the difference between the diameter of a sphere and the volume.)

Put another small mound of clay about half-an-inch high on the 1/2-inch disc.

Place the short wire in the center of this mound so it is sticking straight up.

Place one of your Moons on this wire.

Step 5 – And now the shadow

Make an Earth-shadow disc

Take a 3/4-inch disc and using a marker, crayon, or whatever – color it black on both sides. This disc represents the Earth’s shadow at the distance of the Moon from the Earth.

Position the shadow by placing a small mound of clay about half-an-inch tall on the line marked “FULL” about 3/4 of the way between the “Earth” and the perimeter of the month disc.

Stand your Earth shadow on its edge in this clay.

Time to demonstrate lunar phases

For these demonstrations we use just the month disc, Earth, and the Moon on the one-inch wire – oh, and you’ll need a flashlight, and while the room doesn’t need to be pitch black, it’s good to lower the lights.

There are two keys here:

  1. Always point your flashlight -which represents the Sun – in the same direction – the direction indicated by the arrowhead you put on the New Moon line.
  2. Always position yourself as if you were standing on the side of the Earth looking up at the Moon in your sky. Another way to think of this is if the Moon is placed at Day 3, you should place yourself so you are looking along the line that connects Day 18 and Day 3 and runs through the Earth.

Move your Moon around the perimeter of the disc. To see its phase on any given night, shine the flashlight on it to simulate sunlight – and sight along the line from the Earth to the Moon for that particular night.

Position yourself so you are looking in the direction of the arrowhead, and you will see a "new" Moon - completely dark and lost in the glare of the Sun!

The person holding this flashlight is positioned to see a 2-day-old crescent Moon - the photographer was at a somewhat different angle and so saw a larger crescent. Remember - keep flashlight pointed in the same direction and position yourself along the line that is nearest to where you have placed the Moon.

Now here we are at full Moon - oops, but we forgot the earth's shadow!

Put the Earth's shadow in place and it should be clear that - ooops, the shadow is blocking the Moon! But if it does that we would have an eclipse every full Moon - every month! Clearly we don't, so ...

You should notice one problem. When you get to full Moon, the shadow of the Earth blocks the Moon from view. This would mean there would be an eclipse every month at full Moon – but we know there isn’t. What’s wrong with our model?

Step 6 – Going full scale and setting things right!

Our model is convenient for showing the phases of the Moon – and actually keeping track of them each month by advancing the Moon on the monthly disc each day. But it has two problems. First, it doesn’t show the distance between the Earth and Moon to scale – and second, it doesn’t show that the Moon’s orbit is tilted about five degrees to the orbit of the Earth!

So here’s how we’ll correct that situation.

Take the second 3/4-inch disc and place a clay mound on it about half-an-inch high.

Place the long wire (3 inches) vertically in this piece of clay.

Place your second clay Moon on top of this wire.

Now use the string to place your new model of the Moon 30 inches from the Earth. You have now created a scale model of the Earth/Moon system. But why is the Moon so much higher than the Earth in this model? Actually, the Moon could be that much lower than the Earth as well – we are showing it in one direction only because we’re building our model on a table. The Moon, at any given moment, could be just as far below the tabletop as it is above it – or anywhere in between these two extremes!.

To show the distance to scale, place the Moon about 30 inches from the Earth.

Placing the Moon 3 inches above the table seems high – does a five-degree tilt in the Moon’s orbit really amount to that much at the distance of the Moon from the Earth?

If the table edge represents the plane of the Earth's orbit, then the string will represent the plane of the Moon's orbit, tilted five degrees to the orbit of the Earth.

Five degrees doesn’t sound like much – but this is how much the Moon’s orbit tilts with respect to the Earth. You can get a rough idea of what this means on the scale of the Earth/Moon system by using your protractor and the string. Line your protractor up with the edge of a table. Then have your string come out at five degrees from the center of the protractor. Thirty inches later you’ll find that five degrees is now represented by about three inches – the height of our second model of the Moon.

That’s why our second wire was three inches long.

And now it should be clear why we don’t have an eclipse each month. Place your Earth’s shadow out near your Moon, and you can see that most of the time the Moon is going to miss the shadow – it will either be above it or below it.

At full the Moon may be well above the Earth's shadow, well below it, or on relatively rare occasions, pass right through it - and at those time we see an eclipse of the Moon.

You can also see that the Moon might pass through the shadow briefly, or it may take nearly three hours to get through it. But it won’t take 24 hours. Three is about the maximum. And so when an eclipse occurs at full Moon, the Moon for those few hours may, or may not, be in your night sky. That’s why eclipses are visible from only part of the Earth, and they may occur at any time of the day or night.

Simply mind-boggling: Universal Measuring Sticks and Observing Logs

Measuring an 11-foot (meters) strip. (Click image for larger version.)

Measuring an 11-foot (3.4 meters) strip. (Click image for larger version.)

While simple, this project is next to impossible to depict well in a photograph because each “measuring stick” is just a few inches wide and more than 10-feet long. But build one and I bet you’ll find it a mind-bending experience!

I call this project the  Universal Measuring Sticks and Observing Logs and together these “measuring sticks” serve a simple function – they put into perspective the distance of each object you observe. And even if you don’t observe, they’ll help you get a handle on the incredible distances to the planets, stars, and other galaxies.

To do this our basic measuring unit will be the speed of light – 186,200 miles a second (300,000  kilometers a second). That, according to Einstein, is the speed limit for the universe. Nothing can go faster. So we simply ask ourselves how far will light go in a minute? An hour? A year?  Just starting with the distance travelled in a second boggles the mind – the distance that light travels in a single second would take it all the way around the Earth more than 7 times.  That is, it’s 24,902 miles around the Earth at the equator and if you divide that into 186,200 you get (rounded) 7.5 ( 40,076 km divided into 300,000 kmps gives you 7.5 as well). So our most basic unit, the light second, is already far larger than anything most of us have experienced. But at least it gives us a starting point to begin to get even more mind-numbing distances into perspective.

Materials needed for this project:
(2) 11-foot (or 3.3meter) lengths of adding machine tape*
pen or pencil(s)  – different colors helpful, but not necessary
calculator (helpful) or scrap paper

*You  can use any 11-foot strip of paper you have or create – but I found adding machine tape the easiest way to do this and it’s commonly available. Some might want to use four lengths and not use each side – or you might want to use a dozen sheets of ordinary paper. The goal is to make four scales each 10 feet (120-inches) long with little extra paper on each end to keep it neat.

We will actually make four measuring sticks, each a bit over 10 feet ( 3 meters) long. We need four because it is impossible to fit everything on a single scale and still have it readable.

Well, not absolutely impossible.  For example, the first and smallest scale used is for the solar system. On that scale, one inch  equals two light minutes. (If you’re using the metric system, then start with a scale of 25mm equals two light minutes – very nearly the same. ) That scale puts the moon just 1/100th  of an inch (a quarter of a millimeter) from Earth at one end with Neptune (and Pluto) near the other end.  But if we were to include the nearest star we observe in our northern hemisphere skies other than the Sun, it would  require a piece of adding machine tape 36 miles (57.9 kilometers) long! Possible – hardly practical. Oh – and were we to include the nearest galaxy we observe, the Andromeda Galaxy, we would need about 10 million miles (16 million kilometers)  of tape. Sort of defeats the purpose of a scale model! So it is for very practical reasons that we have created four measuring scales.

To use each scale start by putting a vertical line across your tape about 6-inches from the left hand edge. This is your starting point, which is, in all cases, Earth. To the left of this, put down the name of your measuring stick and the scale being used for that stick. To the right of this, calculate the distance to each item you observe, mark it and identify it on the scale. If you like, include the date observed.The result should look something like this measuring stick for the solar system – keep in mind this is just the first part – the whole “stick” goes onf or 10 feet.

This is the starting end of a solar system measuring stick.

This is the starting end of a solar system measuring stick.

General notes that apply to each stick

  • Light travels at 186,282 miles (300,000 km) a second. We use the speed of light as our measuring unit.
  • On the second and third scales in particular you may find that several objects are at, or near, the same distance, so to mark and identify them you will need to use the full width of the paper.
  • Distances up to 1,000 light years are pretty well known now and reasonably accurate because of measurements taken by the Hipparcos* satellite. Distances beyond this get increasingly fuzzy with many different indirect methods to determine them. For this reason you should regard all these distances as reasoned approximations. For close objects (within 1,000 light years) use sources written after the Hipparcos* measurements which were published in 1997.
  • Our distances are also an indicator of time – each distance tells us how long ago the light we see left an object. You may find it fun to mark all but the first scale with historical, evolutionary, and geological events on Earth. Such time references add to your perspective.

You can look up distances and calculate them to scale any time, but the real goal here is to reinforce the observing experience, so if you are observing , I suggest you use this more as a log and  mark your scale either when planning an observing session, or when reflecting on that session after observing. That way the abstract experience of learning and calculating distances is in your mind along with the real-life experience of observing the object.

The Scales – (If you prefer to work in metric, just change “1-inch” to 25mm – it will be close enough for these purposes.)

#1 The Solar System

Scale: 1-inch = 2 light minutes (or for those who want more precision, 120 light seconds*

Minimum distance in light hours, minutes, and seconds,  from the Earth to the moon and planets are:

  • Moon: 00:00:01.2  (that is 1.2 light seconds)
  • Venus 00:02:07  ( 2 minutes, 7 seconds)
  • Mars  00:03:02
  • Mercury  00:04:18
  • Sun 00:08:19
  • Jupiter  00:32:43
  • Saturn 01:06:28  ( 1 hour, 6 minutes, 28 seconds)
  • Uranus  02:23:35
  • Pluto 03:58:07**
  • Neptune 03:59:25

To calculate the distance on your Universal Measuring Stick, simply divide the time in minutes by 2, or the total time converted to seconds, by 120.

Example: Jupiter is 32 minutes, 43 seconds away. In seconds that is (32X60) + 43 or 1,920 seconds plus 43 which is 1,963 seconds. 1963/120 = 16, so Jupiter will be 16 inches away.

*Use 2 light minutes for reasonable approximations, or get more precise with seconds.

** yes, Pluto when closest to us is closer than Neptune when closest to us!

If you’ve made the solar system measuring stick, you should have the basic idea how and find the others easy.

#2 Our Stellar Neighborhood – to 2,600 light years

Scale: 1-inch = 21.6 light years

This scale covers most of what you can see with your naked eye –  as well as many things you can not see with the naked eye because they are too faint, but still fairly close to us. Well, close as astronomical objects go, but incredibly far away when it comes to what we’re used to.

(The entire solar system scale would be so small, it would be impractical to represent it on this scale with anything except the thinnest of lines right at the start.)

We’ll use some of our bright guidepost stars just for starters. Here are their distances in light years:

  • Polaris  430
  • Arcturus  37
  • Spica 262
  • Antares 600
  • Vega  25.3
  • Altair 16.8
  • Deneb 1,400
  • Big Dipper  80*

*This is an approximation covering the main stars of the Dipper which are really part of an open cluster.  With most asterisms the stars would be at various distances.

#3 Our home galaxy, the Milky Way – to 100,000 light years

Scale: 1-inch = 833 light years

(The previous scale would take up little more than the first three inches of this scale.)

This measuring stick takes us to some of the more distant open clusters, typical globular clusters, and some nebulae that are easy to observe with the naked eye, binoculars,or small telescopes.


  • Pleiades M45 440
  • Dumbell Nebula M27  1250
  • Orion Nebula M42  1,300
  • Ring Nebula M57 2,300
  • Open Cluster M37 4,400
  • Globular Cluster M13 25,000

#4 Our observable universe – to 100 million light years

Scale: 1-inch = 833,000 light years

(The entire previous scale would take up about the first one eighth of an inch on this one.)

While the Andromeda galaxy can be detected with the naked eye and observed with ordinary binoculars, most of what we include on this measuring stick takes us to the limit of what we usually observe with a backyard observatory that includes at least a  6-inch telescope. We can reach farther into the universe than this, but with anything past the middle point on this scale you see very little – and most of what you see at these distances justifies the term amateur astronomers usually use for these objects – faint fuzzies!


  • Andromeda Galaxy M31  2.5 million
  • Pair of galaxies beahind the Great Bear’s ears –  M81, M82  12 million
  • Whirlpool Galaxy M51 23 million
  • Leo Triplet Galaxies M65, M66, NGC 3628 35 million

Finally, getting the measure of the universe – here’s a brief tribute to the measurers – ancient and modern . . .

“HIPPARCHUS OF NICEA must have been an interesting fellow. He was a second-century B.C. mathematician, philosopher and astronomer. Using the only astronomical instrument available to him — his eyes — Hipparchus took on the daunting task of measuring the positions of the stars and planets as they passed overhead each night. He came up with a catalog of 1,080 stars, each of which he described simply as “bright” or “small.”
“Hipparchus wasn’t the first astronomer to pursue the science of astrometry, as the astronomical discipline of positional measurement is now called. However, his star catalog was the first of many compiled over the centuries by astronomers using ever-better instruments and techniques. From those measurements — all made from the Earth’s surface — astronomers have derived everything from basic stellar properties to estimates for the age of the universe.
“On August 8, 1989, the science of astrometry took a long-awaited leap to the stars. Riding aboard an Ariane rocket was the High Precision Parallax Collecting Satellite, otherwise known as Hipparcos. For the next three and a half years, Hipparchus’s 20th-century namesake measured the parallaxes and brightnesses of more than a million stars — despite a potentially crippling accident that sorely challenged the project’s architects.”

The above is quoted from this Web site: http://tinyurl.com/yurcq2 Go there for more details.

Be the first on your block to build your very own Milky Way Galaxy!

Editors note: This is a companion project to the post on viewing the summer Milky Way found here.

OK – the universe beat you to it by roughly 13 billion years. But you can build a scale model of the Milky Way, and in doing so you’ll develop a better feel for its size, its relationship to other galaxies, and why the Milky Way looks like the Milky Way when you see it in your sky. Essentially, all this project entails is printing the image below and gluing it to a disc that is the appropriate thickness. How thick is that? About 2 mm – a little more than one-eighth of an inch. I glued it to cork, but cardboard of similar thickness would be fine.  Two millimeters will seem mighty thin – but it is believed that our galaxy – at least in the spiral arms where our Sun is located, is actually just 1,000 light years thick.  Since it is believed to be 100,000 light years across, that means the thickness is 1/100th the diameter.  If your printed version of thegalaxy image is dramatically different in size  – say 150 mm (6-inches) in diameter rather than 200 mm (8-inches), then simply reduce the thickness of your backing to 1.5mm or about 1/th of an inch. The exact size will depend on what image you print from and how your computer handles the printing. But don’t get all fanatical about these dimensions. They are much more than guesses, but something less than precise. After all, no one has ever been outside our galaxy to look in at it, and it would take millions of years to send a space probe out of the galaxy, take a picture, and send it back.

CLick on this image to get a scale image suitable for printing. This is the basis for your scale model of the Milky Way. When printed it should be approximately 200 mm in diameter. The scale is 10 mm equals 50,000 light years.  This is created from an artist's conception published by NASA. All the instructional images are clickable as well, so if you need a larger image, just click it.

Click on this image to get a scale image suitable for printing. When you get to that image you might want to simply print the web page, or you could right click - or control click - to save the image to your computer, then open it in the appropriate software and print. This image is the basis for your scale model of the Milky Way, so you need to print it one way or the other. When printed it should be approximately 200 mm in diameter. The scale is roughly 100 mm equals 50,000 light years. This is created from an artist's conception published by NASA. I added the green dot - showing the approximate location of our Sun - as well as the tabs which will be explained in the text. All the instructional images that follow are clickable as well, so if you need a larger image, just click it.

Click image to get alarger version of this text for printing. This text will be printed,t hen pasted on the back of your scale model.

This text - to be pasted on the back of your model - is really an image. Click the image to get a larger version of this text for printing.


Materials: I used cork board that was little more than 2mm thick and about 200 mm x 250 mm (3/32 x 8 x 10 inches). I used both white glue and rubber cement and scisors for paper,  as well as stronger sheers to cut the cork. The only other materials are the two print outs shown above and a small piece of your backing material - can be cut from scrap - to use to make the galaxy core appear thicker.

Materials: I used cork board that was little more than 2 mm thick and about 200 mm x 250 mm (3/32 x 8 x 10 inches). I used both white glue and rubber cement; scissors for paper, as well as stronger shears to cut the cork. The only other materials are the two print outs shown above and a small piece of your backing material - can be cut from scrap - to use to make the galaxy core appear thicker.

Step 1

Step 1: Cut out the image of the galaxy, being careful to leave the tabs in place. These tabs tie to asterism mentioned in the observing the Milky Way article found here - as well as take note of the positions of "0" and "180" degrees of galactic longitude. We'll explain how to use them once the model is finished.

Step 1: Cut out the image of the galaxy, being careful to leave the tabs in place. These tabs help you locate asterisms mentioned in the observing the Milky Way article found here - as well as take note of the positions of "0" and "180" degrees of galactic longitude. We'll explain how to use them once the model is finished.

Step 2

Cut out a small piece of your backing material about the size and shape of the tellow galactic core. Position the image on your backing and slide this core piece into position underneath it. Trace around the core with pen orpncil to indicate where it will be glued.

Step 2: Cut out a small piece of your backing material about the size and shape of the yellow galactic core. Position the image on your backing and slide this core piece into position underneath it. Trace around the core with pen or pencil to indicate where it will be glued.

Step 3

Step 3: Glue the "core" layer onto your backing sheet. With the cork I had to put a coating of white glue on, let it dry a little, then add a second coating.

Step 3: Glue the "core" layer onto your backing sheet. With the cork I had to put a coating of white glue on, let it dry a little, then add a second coating.

Step 4

Step 4: While you;re waiting for the core backing to dry, cut out the text that will go on the back. This is a tight fit, so trim close to the words.

Step 4: While you're waiting for the core backing to dry, cut out the text that will go on the back. This is a tight fit, so trim close to the words.

Step 5

Step 5: Avoiding the tabs, but glue (I used rubber cement) on the back of the galaxy image.

Step 5: Avoiding the tabs, put glue (I used rubber cement) on the back of the galaxy image.

Step 6

As you glue the image to the base, bend the tabs up out of the way - and, of course, be sure to position this so the yellow galaxy core is over the raised area - aside fromthat there is nothing crucial about the positioning.

Step 6: As you glue the image to the base, bend the tabs up out of the way - and, of course, be sure to position this so the yellow galaxy core is over the raised area - aside from that there is nothing crucial about the positioning.

Step 7

Carefully cut the base around the image. Be sure not to cut the tabs which should be bent up, out of the way.

Step 7: Carefully cut out the base around the image. Be sure not to cut the tabs, which should be bent up, out of the way.

Step 8

Flip you model over and bend each of the tabs down and glue themt o the back side. I was able to do this with ruber cement, though I suspect white glue might work better.

Step: 8: Flip your model over and bend each of the tabs down and glue them to the back side. I was able to do this with rubber cement, though I suspect white glue might work better.

Final Step

Final step: Glue the text in place on the back.

Final step: Glue the text in place on the back.

The finished scale model of our Milky Way Galaxy . . .


. . . and how to use it.

First, the green dot not only marks the approximate location of our Sun and solar system, but it also covers the approximate area where you will find just about all of the naked eye stars that you see. That alone should give you pause for thought.

But the main point of this model is to drive home the basic shape of the galaxy – not unlike a pizza pie – and to help you see why the Milky Way makes a thin cloud of stars across our sky. Imagine yourself at the area of the green dot. Now imagine you’re in the middle of it – that is, down one millimeter from the surface, or about 500 light years by this scale. Now if you look up, you are looking through a thickness of 1 mm – 500 light years of stars. And if you look down, the same thing. In fact, just about any direction you look, you don’t see many stars before you get to the surface – the outer reaches  – of our galaxy.

BUT . . . if you look along the plane of the galaxy toward the core, you are now looking through about 75,000 light years of stars – so you see many more – but they are very distant and also very faint. They make a faint, hazy cloud across our sky – a thin line. a river – what we call the Milky Way.  And if you look outward along the plane in the opposite direction, then you are looking through perhaps 25,000 light years of stars, so you also see a Milky Way – but I think of this one – the winter Milky Way – as skim milk, for it’s much thinner 😉

And this is where the tabs come in. One says “Teapot.” When you look from the green dot toward the “Teapot”  tab, you are looking through the core of our galaxy – and this what you are doing when you look at the Teapot asterism in our summer sky. Similar tabs mark the “Cross” and the “W” asterisms and show you the direction you are looking when you see those in your sky.  (If these asterisms are not familiar to you, be sure to read the post on looking at the Milky Way in August. )

In the winter time we also see the Milky Way, but it is not as bright. That’s because you are looking outward in the general direction of the constellation “Orion,” which is the other tab.

Whatever time of year it is, try this. Bring your scale model of the Milky Way outside under the stars with you. Find the Milky Way overhead. Then hold your model up and orient it so the edge of your model aligns with the Milky Way. That should give you a sense of the plane of the galaxy and where we all are in this vast river of stars.

What about M13 and many other objects you look at with binoculars and telescope? You can find many of them if you understand the galactic coordinate system explained below. But even if you don’t understand this system, there’s a wonderful – and free – computer program that works on any computer that will help. It’s called “Where is M13?” It will show you exactly where you are looking relative to our galaxy when you look at any of the Messier objects.  It’s easy to download and install. Just go here.

How about the next nearest galaxy? Well, there are several very small galaxies that are very near, but usually what we think of as the next nearest galaxy is one that’s close to being our twin – the Great Andromeda Galaxy, M31.  What’s most interesting to me is to consider these two questions in tandem: How far is it to the nearest star after our Sun? And how far is it to the Andromeda Galaxy? If we reduce our Sun to an eight-inch ball – about the diameter of our galaxy model – then the next nearest star  – the next nearest eight-inch ball – is roughly the distance between Boston and Hawaii – essentially half an earth away! That’s a whole lot of empty space betwen stars. So where is the next large galaxy? Well, the Andromeda Galaxy is about 2.5 million light years away. That would be about 200 inches away on the scale of our model – a bit less than 17 feet! That’s mighty close – and, by the way, getting closer. In fact, scientists feel we’re heading for a collision with the Andromeda Galaxy – but don’t lose any sleep over it – it won’t happen for about 3  billion years! (But you can see a simulation of it right here and now!)

Galactic coordinate system

Editor’s note: The best explanation of the galactic coordinate system I have found is in the brief manual for “Where is M13.” Its author, Bill Tschumy, graciously gave permission to reproduce it here with its accompanying diagram.

The galactic coordinate system is the key to understanding where objects are located within the Galaxy. It was established in 1958 by the International Astronomical Union and is useful for specifying an object’s location relative to the Sun and the galactic core of the Milky Way.

The galactic coordinate system is a 2-D spherical coordinate system with us (or the Sun) at its center.  It has latitude and longitude lines, similar to Earth’s. In fact, a good analogy is to imagine yourself standing at the center of a hollow Earth looking at the latitude and longitude lines on the Earth’s surface. The galactic coordinate system is similar except we are looking out at the celestial sphere.

There is a one-to-one mapping between the galactic coordinate system and the more familiar equatorial coordinate system. Relatively simple equations can be used to convert from one to the other.


The galactic equator (i.e., 0º galactic latitude) is coincident with the plane of the Milky Way Galaxy and is shown as the red circle in the image above.  Galactic latitude is the angle above or below this plane (e.g. the yellow angle above).  Thus, objects with a galactic latitude near 0º will be located within the Milky Way’s spiral arms. Objects with a positive galactic latitude will be above the arms in the northern galactic hemisphere.

Galactic longitude is measured from 0º to 360º, counter clockwise as seen from the north galactic pole. 0º galactic longitude is arbitrarily defined as the direction pointing to our galactic center.  Within the plane of our galaxy (0º galactic latitude), the main points of longitude and the Milky Way constellations which lie in their directions are as follows:

  • 0º is in the direction of Sagittarius
  • 90º is in the direction of Cygnus
  • 180º is in the direction of the galactic anti-center in Auriga
  • 270º is in the direction of Vela

Now consider an object and its galactic coordinates. Any other object lying along the same line of sight will have the same coordinates but only differ in its distance component. An object’s distance is not part of its galactic coordinates. However, knowledge of an object’s galactic latitude, longitude and distance, does allow us to uniquely locate it within the 3-D space around the Milky Way.  Where is M13? uses this information to plot deep sky objects in its Galaxy View.

Build an inexpensive, simple, one-tooth-pick, global, equatorial, elegant and smaller than an iPod, wristdial!

A day in the Sun – a brief timelapse video of the garden-size version of the wristdial in action. (See if you can discover what time did the bird land on the dial, casting it’s shadow on the face!)  This is the same basic design as the wrist dial, only larger.

Is the wristdial really all those things – inexpensive, simple, one-tooth-pick, global, equatorial, elegant and smaller than an iPod? Yes, and with no moving parts to break. Instead it depends on the motion of the Earth which, ponderous as it is (6.6 sextillion tons) moves like – well, like clockwork!

And simple?

Yep! Here’s an image of the final product in action in the northern hemisphere early on a summer morning.  (Oh, it can also find north for you, so it can double as a compass. More on this in the full directions in PDF format 😉


Click for larger image.

The wristdial travels in a neat, folded package (see inset at upper left). The toothpick is inserted at a right angle to the dial face, and this can be checked with the “setting triangle.” The same triangle is then used to set the dial face so its plane points to the celestial equator. Get those two set correctly, and the dial works anywhere on Earth that the Sun is shining.  clay_winterThe dial has two faces, the one shown in the preceding picture is for use in spring and summer. The other side –  shown in the picture at right – is for use in fall and winter. The design can easily be scaled up and faces are included for a larger version, or you can use the instruction included here to design your own.

Sundials are simple things that point to profound truths about the motions of Earth and Sun. They’ll teach you about your position on this rapidly spinning sphere and put you in direct touch with some awesome forces of nature. That’s what I love about them. But right now you’re probably more interested in how to make your own wristdial, so let’s do it!

You can download the full directions – with many color photos – from the link below. This is a large Acrobat PDF file, so allow several minutes for it to download.  Because of the numerous color image, I suggest you read  it on screen – but you’ll want to print either page 4 or page 5, depending on the hemisphere in which you will use the dial.

Download complete directions for wrist dial here: directions_wristdial_f4

You might want to get a jump on things by first finding and jotting down a few useful facts.

Latitude and longitude (http://www.getlatlon.com/) – You don’t have to be super precise. All can be rounded to the nearest degree. For Westport, MA I round my latitude to 42° N, and my longitude to 71° W.

Central meridian (http://www.travel.com.hk/region/timezone.htm) – Time zones are set every 15 degrees of longitude so you’ll see the central meridian for yours at the top of the map on the web page linked above. Westport, MA, is in the Eastern Standard time zone which is centered on 75 degrees longitude.

Compass deviation (http://www.geo-orbit.org/sizepgs/magmapsp.html) – I suggest you find your compass deviation only because I’m assuming you might use a magnetic compass to find north. If you have another way to determine north, you can ignore this. But a magnetic compass is not precise. In the case of Westport, MA the deviation is 16° east, which means that if my magnetic compass says it is pointing north, it is really pointing 16 degrees to the west of north, so to point true north I have to correct by pointing 16 degrees to the east of what it says is north. Of course, I might use a GPS, or call the local airport to learn the compass deviation.

Southern hemisphere dial at work in winter.

Southern hemisphere dial at work in winter.

The wristdial has now been tested in the southern hemisphere by my friend Dom in Sydney, Australia. Dom took some photos of his wristdial in action, next to a larger, traditional garden sundial in Centennial Park.  You will note three things about these photos. First, the shadow is on the underside of the dial face because when these photos were taken in mid-July it was winter in the southern hemisphere. Second, the time indicated by the dial is almost exactly the same as the time indicated by Dom’s watch. That’s because Sydney is not on daylight savings time in the winter. Also. Sydney’s longitude is just one degree – four minutes – east of the central longitude for its time zone.   Because of that the time should be four minutes fast. But, the equation of time for July  is six minutes slow. When you apply the equation of time,  the four minutes “fast” caused by a difference in longitude is subtracted from the six minutes “slow” of the equation of time and the dials solar time is within about two minutes of standard clock time.  (This kind of calculation is described in detail  for your location in the directions you can download. )

Southern hemisphere wristdial showing solar time as compared to clock time while sitting on a traditional sundial in a park in Sydney, Australia.

Southern hemisphere wristdial showing solar time as compared to clock time while sitting on a traditional sundial in a park in Sydney, Australia.

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