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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.

Getting to know the neighborhood – our solar system

Modern science shows us a world that is frustratingly difficult to comprehend and appreciate. In quantum mechanics we deal with a world that is tiny beyond anything we experience. And in astronomy we deal with a world that is huge beyond anything we experience.

Our experiential knowledge is confined to the middle.  That’s where we live. And the middle appears to be the only place where intelligent life can take place. A life form much larger than us – say the size of our galaxy – would take thousands of years to have a thought. And a life form much smaller than us – say the size of a flea – just can’t be complex enough to create the emergent property of awareness.  So here we are int he middle of things, trying to comprehend the two extremes and having nothing in our experience that helps much.

Still, I think we should make an effort to move from our experiential knowledge – something we know, not simply in our minds, but in our whole bodies because we have experienced it – and try to apply that to the huge numbers we deal with in astronomy so we can at least begin to appreciate the reality they represent. That’s the purpose of this little exercise. Not to test your math knowledge. Not to try your patience. But to help you move a little closer to the wonder that is our solar system.

We will deal with one simple thing – the distances involved – and we’ll start with a basic distance that will serve as a key to all the others, the distance between the Earth and Sun. In good round numbers that distance is 93 million miles.  But, of course, no human being has taken a trip of a million miles, let alone 93 million, so we have no experience that can relate to this number. So let’s first pick a starting point – one your comfortable with – and see how that relates to this number. Choose one of the following travel means, or make up one of your own that is something you have experienced.

  • Brisk walk – 4 miles an hour
  • Automobile – 65 miles an hour
  • Indy race car – 200 miles an hour
  • Maglev train – 300 miles an hour
  • Commercial Jet – 500 miles an hour
  • F-15 fighter – 1900 miles an hour
  • Space Shuttle – 18,000 miles an hour
  • Apollo Moon rocket – 25,000 miles an hour

I’m not sure anyone can really relate to the last three – unless you happen to be a fighter pilot or astronaut – but if they are in your experience, feel free to use them. For me, I relate to the speed of a commercial jet. I have some idea of  how fast that is and how long it takes me to cover the distance between Rhode Island and Florida. That fits my experience.

The question you want to answer is, at your chosen speed, how long will it take you to travel from here to the Sun?

That is,what is 93,000,000 divided by the chosen speed?

Do your answer first in miles an hour, but to get it down to a manageable number, calculate it in years – and round off. we’re not trying for precision here – just trying to get a feel for the distance involved. So when you get your final answer, sit back and try to imagine making such a trip.

Your travel means: __________________________

Your travel time from Earth to the Sun:_______________________________

OK – now let’s apply this to the rest of the Solar System. That’s really a lot easier than it may sound. See, the distance from the Earth to the Sun – 93,000,000 miles –  is known as an “astronomical unit.”  Many other distances can then be stated in terms of that unit. And you’ve just created an astronomical unit of travel time.

Here is the average distance – in astronomical units – between Earth and the other planets in astronomical units. To calculate travel time to each simply multiply the average distance in astronomical units by the number you arrived at above and create your own Solar System Travel Guide.

Average distance in Astronomical Units                Calculated Travel Time

Mercury    .61                    ______________________________

Venus        .28                    ______________________________

Mars        .5                    ______________________________

Jupiter    4.2                    ______________________________

Saturn        8.5                    ______________________________

Uranus    18.2                    ______________________________

Neptune    29.1                    ______________________________

Pluto        38.5                    ______________________________

Bonus Section: Want to try a few more?

How long would it take to carry on a radio conversation with someone on Neptune? Radio waves – and light waves – travel at 186,200 miles a second.  You can calculate an astronomical unit of time by dividing that into 93,000,000 – round your answer off in minutes to get a good approximation, then use the numbers in your table.  (By the way – when you see a TV anchorperson talking to a correspondent half way around the world  and you notice a delay in the conversation, that is NOT due to the speed of radio waves. Radio waves travel around the world  seven times in a second. The delay is due to other issues with the technology used.)

Radio waves to Neptune: ______________________

Use 238,000 miles as the distance to the Moon. How long would it take to get there by your chosen travel speed?

Travel time to the Moon: _________________________

How long would it take you to travel around the Sun – what Earth does every year? Fudge it a bit by considering our orbit circular. You can calculate the distance involved by finding the circumference of that circle – the formula is 2 pi R. In this case “R” is the astronomical unit and since you have already calculated the time of a single astronomical unit at your chosen speed, you can just substitute that for “R.”

Your new “year”: _______________________________

How long would it take to go to the next nearest star, Alpha Centauri, which is 4.3 light years away? (A light year is equal to  63,239.6717 astronomical units – you can round that off to 63,240 astronomical units. So use your “astronomical unit” and multiply it by 63,240 – then remember to multiply that answer by 4.3.

Travel time to the next nearest star: _________________________________

I hope this  begins to help you develop a feel for astronomical numbers. We will return to them another time. Living in the “middle” is nice – but not always easy 😉

Step 8 – Directions in the sky – sometimes east is west!

Knowing the major directions in the sky – north, east, south, and west – can be confusing, but it is easy if you remember these two rules:

The direction the stars appear to move is how we define “west.”
The direction from a star to Polaris is how we define “north.”

Notice that these are new definitions. We are not talking about the cardinal directions – north, south, east, and west – as they appear on the horizon, though these are closely related. These sky directions are a bit different because we are looking at a sphere from the inside – the dome of the sky. They are absolutely essential, however, for talking intelligently and usefully about where things are in the sky in relation to one another.

Terms such as “above” and “below” are relative and not always that helpful when trying to find your way around the sky dome. Instead, learn to think in terms of the cardinal directions, north, south, east, and west.

So face south. This puts east to your left and west to your right.

Hey that was easy! Yes it was.

Now face north. East is now to your right and west to your left.  Wow – there’s nothing to this! Nope. Nothing to it – until you look at the section of sky beneath the North Star.  Now east and west get flipped. Now – in the sky – west is to your right and east to your left!

Remember – this is the special case that applies only to objects that are below the North Star. This chart should help you understand why.

 

West is always the direction the stars appear to move as theyc ircle the north celestial pole, marked approximately by the North Star. (Click image to see a much larger version.)

 

Why the change in direction? Nothing has changed really. Remember Rule 1: The star always appear to rotate to the west.  Since they appear to circle the North Star – the North Celestial Pole really – then beneath it they will appear to move from left (east) to right (west.) It is confusing because the western point of the horizon is still to your left – but you are not dealing with the horizon – you are now dealing with the sky dome.

Now lets look at the second rule. It applies everywhere in the sky, no matter what direction you face. North is always toward the North Star. We’ll illustrate this by looking north.

North is always towards the north celestial pole, marked approximately by the North Star, Polaris.

North is always towards the north celestial pole, marked approximately by the North Star, Polaris. (Click image to see a much larger version.)

Meridian and celestial equator

Let’s examine this sky dome a bit more and draw some imaginary arcs on it. One we’ll call the meridian and the other the celestial equator. Here’s how they’ll appear to us as we look toward the southern horizon.

Facing south at latitude 42° North the celestial equator appears to cross the meridian at a point 48 degrees above the southern horizon. (The  double line marks the meridian. I'm not absolutely sure why it is double in this image, but that's how my version of the free "Stellarium" software represented it. All the image son this post come from Stellarium which can be downloaded here. (Click this image for a much larger version.)

Facing south at latitude 42° North the celestial equator appears to cross the meridian at a point 48 degrees above the southern horizon. (The double line marks the meridian. I’m not absolutely sure why it is double in this image, but that’s how my version of the free “Stellarium” software represented it. All the image son this post come from Stellarium which can be downloaded here. (Click this image for a much larger version.)

Think in  terms of two huge circles.

The meridian is an imaginary circle that runs through the north and south celestial poles. Think of it as starting at the North Pole, running up through the North Star, on over your head, and down to the point that is due south on your horizon.

The second huge circle is a little more difficult to locate precisely. It is a projection of the Earth’s equator onto the sky dome and is called the celestial equator.

The celestial equator runs from the eastern point on your horizon (due east) to the western point (due) west. But it does not cross directly over head. Instead it makes a huge arc and goes through a point on the meridian that is exactly 90 degrees south of the North Star.

Finding the celestial equator in your sky isn’t as difficult as it may seem. Let’s return to that half circle, the meridian. As half a circle, it totals 180 degrees. It starts at the northern horizon and climbs to the North Star. How many degrees is that? It depends on your latitude. Here in Westport, Massachusetts, it is approximately 42 degrees. That’s the first point on our imaginary circle – our meridian.

The next point on this circle will be the point directly overhead. We call it the zenith. That will be 90 degrees above the northern horizon – 48 degrees beyond the North Star.

Now head south from the zenith – the point directly overhead – and you will find that in an additional 42 degrees you have reached the point where the celestial equator crosses the meridian. At this point there are still 48 degrees of meridian left to take you to the south point on the horizon.  This is much easier to see in a simple diagram than it is to imagine from words. But remember this easy rule:

To find the point where the celestial equator crosses the meridian at your location, simply subtract your latitude from 90 degrees. The remainder is the height of your celestial equator above the due south point on your horizon.

Do this for Westport, MA, and you get 90 minus 42 equals 48 degrees.  So face south and use your fist as a measuring stick, estimate how high 48 degrees is. That is the point where the celestial equator crosses. To see the entire equator in your mind, draw an imaginary arc from due east through the meridian at this point to due west. That’s the celestial equator.

Do you get the concept? To test yourself, try to answer these questions without reading on.

If you were standing at the north pole – latitude 90 degrees, where would the celestial equator appear to you?
If you were standing on the equator – latitude 0 degrees – where would the celestial equator appear to you?

Think you have the answers? Ok, read them here.

At the north pole the celestial equator would appear to be a projection of your horizon running from east, through the due south point on the horizon, to west.  At the equator, the celestial equator would run from east to west, passing through your meridian directly over head – at the zenith. Confused? Return to the simple rules:

The direction the stars appear to move is how we define “west.”
The direction from a star to Polaris is how we define “north.”
The zenith is the point directly over head.
The meridian runs from due north to due south passing through the North Star and the zenith.
The celestial equator runs from due east to due west making an arc that crosses the meridian 90 degrees, minus your latitude, above the due south point on your horizon.

Exercise: Test you sense of direction!

Brilliant Vega is the brightest star in the constellation of the Lyre.  See if you can find the other stars as outlined in the image below. Or, find just the two that form a distinctive triangle with it. These may not be visible in light-polluted skies, but you should see them if you look at Vega with binoculars.

Click on this to get a larger image suitable for printing and using inthe exercise below.

Click on this to get a larger image suitable for printing and using inthe exercise below.

Your assignment is this:

1. Click on the image above and when you get to the larger version, print it.Draw arrows from Vega indicating the directions of North and of West.

2. While observing, determine the cardinal directions in relation to Vega and the associated stars, starting with North.

3. What then is the direction from Vega to Sulafat, the star in the lower right corner  – or from Vega to the star in the triangle that is below and to the right in this drawing.

Scroll down for the answer.

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Click to enlarge.

The direction from Vega to Sulafat is roughly southeast. Click to enlarge.

Vega brings us the Summer Triangle – and continues to do so right into winter!

Vega is the guidepost star for June, heralding the rising of the Summer Triangle which stays with us right until winter! Look for it in the eastern sky about 45 minutes after sunset. (Click for much larger image.)

Vega is the guidepost star for June, heralding the rising of the Summer Triangle which stays with us right until winter! Look for it in the eastern sky about 45 minutes after sunset. (Click for much larger image.)

In these June nights when it isn’t really dark untila fter 10 pm  – at least for those of us on Daylight Savings Time – the Big Dipper pointing  to Arcturus and Spica are still high inthe sky, but brilliant Vega – magnitude zero – is well up in the east and bringing with it the other two stars of the Summer Triangle.  Deneb and Altair.  We’ll focus on those two next month. This month it’s enough to remind yourself of where to find Arcturus and Spica, then move on to Vega. And while you’re at it, see if you can notice the color difference between Arcturus and Vega – but wait until Vega is high enough so it now longer twinkles and changes color in the thick atmosphere near the horizon.

We call these three the “summer” triangle, but the truth is, they dominate our sky for a full six motnhs. You can see them in the east  on the night of the Summer Solstice – June 21 – and in the West around the Winter Solstice near Christmas. In fact, they’re stillt here a month later, but by then Altair is starting to get lost in the twilight, though Vega is still high enough in the northwest to see easily.

Add a couple of asterisms!

While I feel the guidepost stars are the most important to learn, if you really want to find your way about the night sky it’s also helpful to learn some key asterisms.  For June there are two to add to your memoru banks – the Keystone of Hercules and  the half circle of stars thata re the core of Coronoa Borealis – the Northern Crown.

Once the sky has really darkened on a June evening, look for the Keystone and the Crown on a line drawn between the two, bright guideposts stars of Arcturus and Vega.  (Click image for much larger chart.)

Once the sky has really darkened on a June evening, look for the Keystone and the Crown on a line drawn between the two, bright guideposts stars of Arcturus and Vega. (Click image for much larger chart.)

Once the sky has really darkened – between 10 and 10:30 on June evenings in Westport, MA  – draw an imaginary line between Vega and Arcturus.

Now look for our two helpful asterisms along this line.

The Keystone of Hercules is made up of four stars of magnitude 3-4 that form a perfect keystone, its narrow end to the south. (The star in the southeast corner is faintest – magnitude 4. The star in the southwest corner is the brightest.)

The second asterism to seek out, the Northern Crown, has one really bright star of second magnitude. Its other stars are magnitude four and five, so your eyes must be dark adapted to see this. If you live in suburban, light-polluted skies, try this. Find its brightest star, then with binoculars look first for the arc of three stars trailing off to the east, then the arc of two stars to the west of this dominant star.

Note: The typical binocular field will capture just half of this asterism at a time.

A dreamy, planet-filled, midsummer night!

The sky will be filled with planets about 45 minutes before daybreak - BUT, only Jupiter and Venus will be easy to spot witht he naked eye. You might spot Mars, but binoculars will help and they certainly are needed for the rest. Neptune will be right next to Jupiter looking like a faint star, Uranus will be brighter, but refer to the chart for it. Mercury? You'll need ideal conditions to see it and that slither of Moon.

The sky will be filled with planets about 45 minutes before daybreak on the morning of June 21 - BUT, only Jupiter and Venus will be easy to spot with the naked eye. You might spot Mars, but binoculars will help and they certainly are needed for the rest. Neptune will be right next to Jupiter looking like a faint star, Uranus will be brighter, but refer to the chart for it. Mercury? You'll need ideal conditions to see it and that slither of Moon. (Click for much larger view.) This an all other charts here are screen shots taken from Starry Nights Pro software and slightly modified for this use.

June 20-21, 2009 – Midsummer Night –  is the shortest night of the year, but it is chock full of planets – all of them! ( Yes, we had a similar opportunity in May, and that was great fun – but it gets a tad better in June and there’s something a bit magical about Midsummer Night!)

In the hours before midnight you can enjoy Saturn in the western sky and as it sets shortly after midnight,  you can enjoy Jupiter and Neptune rising in the East. Pluto is there as well, low in the west, but it is so faint you’ll need a large telescope and lots of patience to track it down. Uranus will be the next one up, rising about 1 am, but not becoming easy to see for another hour or two. By 3 am Mars and Venus will have broken the eastern horizon. I don’t know how easy it will be to spot Mars with the naked eye. It will be just one degree from brilliant Venus and although it will be first magnitude, that will still be five magnitudes dimmer than Venus! In binoculars, however, it should be easy.

The difficult catch will be Mercury. My best guess is about 45 minutes before local sunrise (about 4:25 am for me in Westport, MA) it may be high enough if there are no clouds on the eastern horizon and the sky may still be dark enough. The thinnest of crescent moons may be a guide, assuming you can see it! Mercury will be about six degrees below the Moon and to the right – close enough to fit with the Moon in a wide-field binocular view, though typical 10X50 binoculars will not fit the two in the same field. Still, at zero magnitude it will be bright. Both of these are north of east – Mercury at about azimuth 67° and the Moon at  about azimuth 62°.

Frankly, I’ll consider myself very lucky if I spot Mercury – and I probably won’t have the patience to hunt for Pluto. If the skies are clear enough to see it, there will be too many other things to catch my interest, particularly near the southern Milky Way. But it would be kind of fun to pull an all-nighter – especially since this is the shortest night of the year – and to see all of what are currently called “planets” on the same evening. So if the weather is cooperative, I’ll probably start observing about 11 pm EDT.  And I’ll do my observing at a favorite location near the ocean where I have a great view of the eastern horizon – in fact, all horizons!

The charts which follow provide a guide to finding Neptune and Uranus.

Finding Neptune is easy, as long as you remember that at magnitude 8 it is much fainter than 5th magnitude Mu Capricorni, or the moons of Jupiter, all of which will be visible in a small telescope, or good binoculars held steady.

Finding Neptune is easy, as long as you remember that at magnitude 8 it is much fainter than 5th magnitude Mu Capricorni, or even the four brightest moons of Jupiter, all of which will be visible in a small telescope, or good binoculars held steady. Jupiter itself is second only to Venus in brightness at magnitude -4.6 (Click for much larger view.)

Uranus is a challenge to find with binoculars, though at magnitude 6 it will appear reasonably bright. Theproblem is, it's in a section of the sky without bright, maked eye guide stars. I dentify thegeneral search region by looking at the large chart at the start of this post. Then use this chart and search for the rectangle of 5th magnitude stars. Find themand it's a short star-hop up to Uranus.

Uranus is a challenge to find with binoculars, though at magnitude 6 it will appear reasonably bright. The problem is, it's in a section of the sky without bright, naked-eye guide stars. It does fit on aline between Jupiter and Venus and is about 27 degrees from Jupiter on that line. Identify the general search region by looking at the large chart at the start of this post. Then use this chart and search for the rectangle of 5th magnitude stars. Find them and it's a short star-hop up to Uranus which will appear to be the twin of the star closest to it. (Click for a much larger image.)

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