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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.

Events February 2011: A great “Luna-See” month, plus goodby Jupiter, hello Saturn and basking in the Zodiacal Light?

The apparent wobbling of the Moon shown here causes "librations" which result in our being able to see about 59% of the Moon, even though it always keeps the same face towards us. (This work has been released into the public domain by its author, Tomruen.)

 

 

February 2011: A Great “Luna-See” Month

What makes February 2011 a great “Luna-See” month? Well, the cycle of the Moon is pretty close to being in step with the calendar we all use this month. But the truth is, the Moon is special and always worthy of our attention if for no other reason than it is constantly changing, while most everything else in the sky stays the same – or takes more than a human lifetime to change noticeably.

What’s more, people just take the Moon for granted, and I find that even many experienced sky watchers either haven’t noticed – or can’t explain – some of the major motions of the Moon and why it is where it is at any given moment. So let’s engage in a little “Luna-See” this month and see if we can untangle in simple fashion the motions of the Moon and how these motions gives us a constantly changing show night-by-night and moonth-by-moonth!

No – that’s not a typo. A “moonth” is simply my term for the lunar cycle. Our months vary in length. But the lunar cycle – the time it takes to go from one New Moon to the next – is a constant 29.5 days. The Moon actually goes around our planet in 27.3 days – but by the time it completes one orbit our planet has moved along in its path around the Sun and the Moon has to play catch-up. To get to the position we call “New Moon” – the position where it is directly between us and the Sun – takes 29.5 days, or one “moonth!” ( OK, there’s a correct term for this – it’s called the “synodic month” or a “lunation.” It’s just that “moonth” makes more sense to me – maybe i should contact Mr. Webster 😉

Since 29.5 days is not the same length as our calendar month, the “lunations” get, well, out of sync with the calendar. But in February 2011 the New Moon occurs pretty close to the start of the month – February 3 – so that’s where we’ll start this little adventure in “luna-see.”

Ideally, this should be a three-pronged discovery adventure – first, attack the problem with your mind by reading this. But for me abstract descriptions don’t hold much water, so when you’re through reading what follows, I urge you to move on to:

The real fun, though, is to go out, look up at the Moon, and in your mind be able to picture the entire Earth/Moon/Sun tableau – to see those things that are out of sight and understand how they contribute to what is in sight – or more simply, to know why the Moon appears the way it does, in the place in our sky that it occupies at any particular moment. So let’s get started.

 

February 3, 2011, 2:31 UT – New Moon – Everyone Wants to See the New Baby!

Notice the precise time? That’s important for new Moon junkies who like to see what is the youngest Moon they can see. I’ve never been addicted to this game, but I’m a casual player. It’s fun and a challenge open to anyone. And a crescent Moon can be simply beautiful, especially if paired with some nearby bright planets. What you need is an unobstructed western horizon and very clear skies because when you’re looking for something near the horizon, you’re looking through a lot of air and even small clouds that build up over the course of distance to form a haze near the horizon.

The new Moon occurs when the Moon is between the Earth and the Sun. For February 2011 that is at 2:32 UT. (To convert Universal Time – UT – to your local time, go here, find your time zone, then use the chart on the bottom right of that page to see how many hours you should add or subtract for your time zone.) At new Moon we can’t see the Moon because the side facing us is unlit, and even if it were a tiny bit lit, it would be overwhelmed by the glare of the Sun. But because new Moon occurs so early in the Universal Time day, there’s a slim possibility some of us in North America and points west may see it on February 3 – it just depends on where we are – and, of course, the weather.

A young Moon with Jupiter about 30 degrees above it, as seen from 42°N 71°W about 45 minutes after sunset on February 4, 2011. Click image for a larger version. (Stellarium screen shot.)

People have reported seeing Moons as “young” as about 12 hours. A more realistic goal is a 20-hour Moon. Exactly how old the Moon is after sunset on February 3 or 4 – depends on your exact location. I’m roughly at 42°N, 71° W, and on February 3 right after local sunset the Moon will be about 18 hours old. If I’m real lucky I may see that – and if I do, it will be the youngest Moon I have seen. A much more realistic task is for me to look for it the next night, February 4. At sunset on February 4 at my location the Moon will be close to 40 hours old.

This looking for the first slither of Moon is a little game that’s fun to play. The sky is usually too bright for you to see it right after sunset. You have to wait a while – but the problem is, the longer you wait, the lower the Moon gets in the sky. So on the one hand, the sky is getting darker making it easier to see the Moon – but the Moon is getting lower making it harder to see! Binoculars help – but use caution. Wait until you are sure the Sun has set. You will be looking in the same section of sky for the Moon as the Sun just occupied, and you don’t want to accidentally view the Sun and burn your eyes!

In my case on February 4, 2011, the Moon sets almost two hours after the Sun. This means that about 45 minutes after Sunset – when the sky is getting pretty dark – the Moon will still be about 10 degrees above the western horizon and just a tad north (3 degrees) of where the Sun set – which, if I’m lucky and have really clear skies to the west – will be easy to see. On February 3 it’s a much different story. Even at sunset the Moon is only eight degrees directly above where the Sun set. (Remember, your fist held at arm’s length covers about 10 degrees.) But the Moon sets in just 45 minutes, so if I catch it at all, it will be while looking through a lot of air and against a bright background, and it will be a very dim sliver. My strategy will be to wait until about 10 minutes after sunset, then start scanning with binoculars.

 

February 11, 2011, 7:18 UT – First Quarter and a Close Encounter of the Pleiades Kind

This is the view on February 11, 2011, about 45 minutes after sunset for my location - 42° N, 71° W. The Moon passes about one degree beneath the Pleaides star cluster and you should be able to see both in the same binocular field of view, but the Pleiades will be washed out and dim because of the brilliant Moon light. Exactly how close the Moon is to the Pleiades depends on your location. Remember, it is constantly moving eastward - while at the same time, it, and all other objects in the sky, appear to be moving westward because of the rotation of the Earth. (Prepared from Starry Nights Pro screen shot.)

Now seeing the Moon is no challenge if your skies are clear. Each day at sunset it has moved about 12 degrees farther east as it circles the Earth. This means it’s climbing up our sky – on a slant, since it follows an arc across the sky, not a straight line – but higher each night. Tonight it will be about 70 degrees high about 45 minutes after sunset. And, of course, more and more of it is lit each night. Now we see about half of the Moon – that is half of what we will ever see, but really just one quarter of it. Nearly half of the Moon remains hidden to us all the time. This is because the Moon keeps the same face towards us. That is, it takes it just as long to spin once on its axis as it does for it to revolve once around the Earth. If the Earth did that as it journeyed around the Sun, half the Earth would be in constant sunlight, the other half in constant darkness! The other half of the Moon isn’t constantly dark, however. It does get sunlight on it, just as the half that faces us does – it’s just that the other half never faces us. Well, not quite – for various reasons we get little peeks at part of it, and when you add all these up we actually see about 59% of the Moon’s surface.

But as you look at the first quarter Moon in our sky, think about where it is at that moment – and more importantly, think about where the Sun is at that moment. The Sun moves about 15 degrees each hour. So if you are seeing the first quarter Moon about an hour after sunset, then the Sun is already about 15 degrees below your horizon. Can you picture it in your mind? Sort of like a huge flashlight, shining on and lighting half of it? Of course, you see only half of the lit portion, which is why this is called “First Quarter.”

For me, in the Eastern Time Zone of the US, first quarter Moon actually occurred at 3:18 am! So when I see it on the night of February 11 it’s about 15 hours later and the Moon is well on its way to being eight days old – it is past first quarter and a little more of it is lit. As I say, it is constantly changing – something you can see for yourself as you follow the “terminator,” the line between sunshine and darkness. Do this with binoculars a or telescope over the course of a few hours, and if you are a careful observer, you will see a difference.

The Moon is also about as far north as it will get this month – and thus almost as high in our sky as it’s going to get in February. Actually, for me it reaches its highest point a couple of hours after sunset on February 12 when it is 72 degrees high. Why? Because at that point in its travels around the Earth, it’s on the side where the Earth is leaning towards it – at least for us Northern Hemisphere observers. This is explained in detail in the projects:


 

February 18, 2011, 8:36 UT – Full Moon – Great for Lovers but Not Always Loved

Now here we are at full Moon - as illustrated by the "Luna-See" model.

Ah – the moment we’ve been waiting for – when we see the full face of the Moon – well, the full face that faces us is lit because the Moon is opposite the Sun in our sky. Focus on that word “opposite.” It’s very important for understanding the north-south motions of the Moon, and here’s a good thing to remember: The full Moon always rises directly opposite the setting Sun. This is most dramatic in June and December. Again, sticking to my own Northern Hemisphere as an example, the December Sun sets in the southwest – so the Full Moon in December rises in the northeast – directly opposite the setting Sun – not just on the other side of the sky, but in the other quadrant. And in June the Sun sets in the northwest, so the full Moon rises in the southeast. That summer full Moon mimics the path of the winter Sun – that is, it crosses our sky in a low arc to the south. And the winter full Moon? It mimics the path of the summer Sun and crosses our sky in an arc, high above us.

The abbreviated explanation for this is that the Moon goes through the same north/south motions in our sky every month – well, every “moonth” or lunation – that the Sun goes through every year. Why? It’s all because the Earth is tilted on its axis. The Sun is high in our sky when that tilt has us leaning towards it. It’s low when we’re leaning away from it. But we don’t change the way we lean – what changes is that we go around the Sun and so sometimes we’re leaning towards it, sometimes away from it. And the Moon? It goes around us each month. So sometimes it is on the side of us that leans towards the Moon, and thus it’s high in our sky, and sometimes it’s on the side that leans away from it.

Again – this explanation may be sufficient for you, but if not, I urge you to build the simple models and explore these motions on your own. You should find it much easier to get the geometry clear in your mind if you do the projects.

The full Moon is, of course, a beautiful sight – but amateur astronomers won’t always agree. For them – especially if they use telescopes – the full Moon is a bother. If you look at the Moon with your telescope, it is rather uninteresting because the sunlight is shining straight down on it and so there are no shadows to indicate how high a lunar mountain is or how deep a lunar crater. Worse yet, the Moon is so bright, its light is washing out many of the other fine telescopic sights that the night sky holds.

However, sky watchers just learning the bright stars may welcome a full Moon because the sky is less confusing. Instead of seeing a couple of thousand stars – which can be very confusing – they see only the brightest stars, and these are easier to get straight.

During the first half of the lunation – the “moonth” – the Moon is in the sky right after sunset. As it went from new Moon to full, it got higher in the sky each night and it stayed with us longer, setting roughly about 50 minutes later each night. By the time it was full, the Moon rose as the Sun set, then set in the early morning, just before the Sun rose.

 

February 24, 2011, 23:26 UT – Last Quarter Fun for Insomniacs and Early Risers

Last quarter Moon rises in the southeast near the bright star, Anatares, in the early morning of February 26. Here's how it would appear at about 2:30 am EST from 42°N 71°W. Click image for much larger version. (A Stellarium screen shot.)

OK, last quarter is not the same as first quarter. I say that with emphasis because I see too many books that treat the Moon in detail from new Moon to full Moon – then dismiss the second half of the cycle as if it were just a repeat of the first half. Anyone who believes that’s the case is guilty of what Holmes accused Dr. Watson of when he said: “Quite so . . . you see, but you do not observe.”

Top image shows how the waxing Moon reveals the Sea of Crisis on its eastern limb as sunrise moves from east to west across the Moon. In the second half we see the Moon a few days after full when it has started to wane. Notice that it shuts down the eastern side first - the Sea of Crisis was the first major sea revealed and it is the first major sea to return to darkness as the sunset line moves from east to west across the Moon.

On a broad scale, to the naked eye, things are happening in reverse during the second half of the”moonth.” The first crescent Moon we see a few days after new Moon reveals a beautiful, round sea – Mare Crisium (Sea of Crisis) – near the Moon’s eastern limb. After full Moon, this will be the first area to go dark.

Yes, at last quarter we are seeing just half of the face of the Moon again, but this time the Sun is coming from the other direction. From new Moon to full Moon the Sun is rising. The terminator – the line between darkness and light – marks the sunrise point on the Moon. This change becomes most obvious when viewing with binoculars and small telescopes, for it means that when a crater is highlighted it is the inside, western wall of the crater that is lit and the outside eastern wall of the same crater. From full Moon to new moon, that sequence is reversed. Now the same crater will have the inside of its eastern wall lit and the outside of its western wall. So what? So things have a different feel. Seeing is our most powerful sense – but we also tend to take things for granted – take a “been there, seen that” attitude, and the result is that in the fine words of Holmes, we see, but we do not observe.

When you observe the Moon carefully, you will notice that many familiar features look quite different during the second half of the lunar month. In fact, I frequently find myself disoriented when I look at the Moon in its waning phases. I have to go to my charts to be sure I know what I am looking at because sometimes very familiar craters and mountain ranges look quite different under the different lighting.

All of this should be clear if you simply stop and think for a moment about where the major players are – where the Moon is at the moment and where the Sun is at the moment. When we did this with the waxing crescent Moon we knew the Sun was below the western horizon. With the waning crescent Moon the Sun is below the eastern horizon, throwing its light from the west side of the Moon towards the east. That’s the opposite of what it was doing with the waxing crescent Moon.

Since the Moon comes up about 50 minutes later each night, it is not long after the full stage that the Moon isn’t visible in the night sky until several hours after sunset. By the last quarter it’s generally not visible until after midnight. And the waning crescent Moon rises just a few hours ahead of the Sun.

This, of course, means that if you want to observe the Moon during these later phases you need to either get up early – or not go to bed. That works for insomniacs, but is difficult for the schedule of most people.

 

The Moon in Daylight

From time to time someone comes to me quite surprised that s/he saw the Moon in daylight. Yes, you can see it in daylight. It is visible most days. It is just not obvious. And some days are better than others if you want to look for the Moon in daylight. At new Moon and full Moon the Moon is not likely to be visible in daylight. The rest of the time it is – you just have to know where to look and have, of course, clear skies. And you have to be prepared to see a pale shadow of what you see at night – although I must say it can be quite bright and impressive when near the first quarter or last quarter stages and when seen within a few hours of sunset or sunrise when the section of the sky the Moon occupies is still comparatively dark.

So here are a few good times to see the Moon during the day in February 2011.

February 11-15, 2011 – This is when the Moon is waxing between first quarter and full. Two hours before sunset – until sunset – look for the Moon high in the east. On each successive day it will be lower in the eastern sky at that time, but more of it will be lit. By full Moon it is below the eastern horizon until near the time of sunset.

February 20-24, 2011 – This is when the Moon is waning between full and last quarter. Look for it high in the western sky from sunrise until a couple of hours after sunrise. With each successive day the Moon will get smaller, but higher in the western sky.

At either end of the “moonth” the Moon is in a crescent phase and near the Sun. While it is in the daytime sky much of the day, it is very difficult to see and looking for it with binoculars or a telescope at such a time would be dangerous since you could accidentally look at the Sun instead and damage your eyes. That’s why we look for the crescent Moon in the hour or so after sunset, or the hour or so before sunrise.

 

Goodby Jupiter, Hello Saturn!

Not quite. Saturn has been with us for months, but only in the morning sky. And Jupiter will still be with us in March, but it will be getting too low in the west to observe it easily. So in practical terms, this will be the last month to get a real good look at Jupiter and its dancing Moons with binoculars or a small telescope. And it will be the first month that Saturn will get high enough in the sky before midnight to get a good look at this most beautiful of planets – most beautiful, that is, for those using a small telescope. I’m afraid Saturn’s rings are not within the reach of those using binoculars, and it’s the rings that make this planet so astonishing. Even when you know exactly what you will see, the 3-D impact of those rings going around the planet are a sight you will never forget, so if you get an opportunity to see Saturn in even the smallest telescope, do take that opportunity.

Finding Jupiter should not be an issue – simply look for the brightest “star” in the west southwest. The only real star you could confuse it with is Sirius – and Sirius will be dimmer and half a sky away in the southeast at that time. About an hour after sunset Jupiter will be about 30 degrees above the horizon – certainly easy to see and view with binoculars or a small telescope. But it sets about four hours after sunset and long before that it will become difficult to view. By the end of the month it will be about 13 degrees above the horizon an hour after sunset and will set in another hour or so. It will make a pretty sight in the southwestern twilight sky to the naked eye but will not be a good target for binoculars and telescopes when that low.

Saturn, however, is a good replacement, making another bright “star” for anyone to pick up with the naked eye and a wonderful sight for the small telescope user.

To find Saturn you need to wait until about six hours after sunset at the start of the month, then look south of east. By the end of the month this is the scene about four hours after sunset. Click image for larger view. Chart is for those in mid-northern latitudes. (Prepared from Starry Nights screen shot.)

To find Saturn you look east about six hours after sunset at the start of the month – four hours by the end of the month. To the north of east you should see brilliant Arcturus rising and a bit higher than Saturn. It also is about half a magnitude brighter. Saturn will have a slight yellowish cast and be a bit south of east. The two will be the brightest stars in that section of sky at the time, although in less than an hour they will be joined by Spica, a bit dimmer than Saturn and just below it.

Venus continues to dominate the morning sky to the southeast, but it is starting to get close to the Sun. It rises about three hours before the Sun at the start of the month – two hours before sunrise by the end of the month. Can’t be missed, though. At magnitude -4.2 it’s about twice as bright as Jupiter and will be outshone only by the Moon and the Sun. On February 28 and March 1, Venus will make a nice pairing with the waning, crescent Moon.

 

Basking in the Zodiacal Light of Almost Spring

The last 10 days or so of February 2011 will be a good time to start looking for that most elusive of Solar System sights, the Zodiacal Light.

Now this is something much different. You don’t need a totally clear horizon to see the zodiacal light, or binoculars, but you do need total darkness and that means little-to-no light pollution and no Moon. So you want to wait until a few days after full Moon to begin this quest. I feel I have a good shot at it from my favorite ocean-front observing point where I have a clear horizon to the west with no cities to create light domes there. Evenings in February and March – and mornings in September and October – are the best time for folks at mid-northern latitudes to look for this.

The zodiacal light is roughly the same intensity as the Milky Way, so if you can see the Milky Way from your chosen location, then you should be able to pick up this faint glow. Like the Milky Way, it stretches over a good deal of sky. It is widest near the horizon and gets narrower as it rises towards the zenith. You want to look for this roughly 80 minutes after sunset. You can check for an exact time for your location by getting information from here on when astronomical twilight ends. (The drop-down menu on that page specifies the times for astronomical twilight.) As astronomical twilight ends you want to start looking. As with any faint object, your eyes need to be dark adapted, so I am assuming you have been out for at least 15 minutes with no white light to dazzle you. If you try to look for this earlier, you may confuse it with twilight. Much later and it is not as bright, for what we are seeing is sunlight reflecting off interplanetary dust particles – dust particles that orbit in the same plane as the planets – the area we call the zodiac – and thus the name for this phenomena, zodiacal light.

If you see it, reflect on this explanation from Wikipedia:

The material producing the zodiacal light is located in a lens-shaped volume of space centered on the Sun and extending well out beyond the orbit of Earth. This material is known as the interplanetary dust cloud. Since most of the material is located near the plane of the Solar System, the zodiacal light is seen along the ecliptic. The amount of material needed to produce the observed zodiacal light is amazingly small. If it were in the form of 1 mm particles, each with the same albedo (reflecting power) as Earth’s Moon, each particle would be 8 km from its neighbors.

For the metrically-challenged (that includes me) that means one dust particle every five miles! And that causes all that light?! Awesome!

 

Algol takes a Dip, or Two, or Three

I wrote about Algol the “Demon Star” in the posting for October, but it’ s still well placed for viewing in February, and if you look at the right time, you’ll catch it in mid-eclipse, which is cool.

Every 2.3 days Algol dims like clockwork, but it is only at its dimmest for about two hours, so to see it in this condition you really need to be watching at the right two hours. Fortunately, there are several places that will give you a list of times when this occurs – but many of them will be while normal people are sleeping – and many more will be during daylight hours. However, each month there should be one or two dates when it is really a good time for you to catch Algol doing its thing.

Most of the listings I know of for Algol “minima” give date and time in Universal Time. What I like about the one at Sky and Telescope magazine, is it will calculate a list of coming Algol minima for you – and give you the Universal Time, plus your local time. So it’s easy to glance over it and see when it will be most convenient – weather permitting – for you to take a look. In my case, February 2011 gives me a couple of opportunities worth noting:

  • 02/09/2011 @ 10:18 pm
  • 02/12/2011 @ 07:07 pm
  • 03/04/2011 @ 08:53 pm

Yes, that last one is for March, but why not plan ahead a little – with winter weather it’s easy to get clouded out, so the more opportunities the better your chance of seeing something. I find these eclipses amazingly elusive and rarely see one, maybe because I think there’s always going to be another opportunity.

 

Also new this month:

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.

Now:

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:

Look North in February 2011 – Watch the Great Bear Come out of his Cave!

When you look to the northeast early on a February evening do you see this:

or maybe this:

Used by permission from the Linda Hall Library of Science, Engineering & Technology.

or perhaps this?

It all depends, of course, on your imagination, but for me I see something like the last image. Even that doesn’t quite capture what my imagination wants to do with these stars. What I see is a huge and rather grumpy bear, emerging from his cave a bit early after hibernating through a few rough months, and now he’s stretching – and clawing – his way up my sky, and he is magnificent!
But I admit, for years it wasn’t that way. I saw instead what I suspect many people see – the Big Dipper rising. And I knew, sort of vaguely, that this asterism – one of the most familiar in the world – was a major portion of the constellation of the Great Bear, Ursa Major.  But really, large as the Dipper is, it’s just the hind quarter of the Big Bear, which is really large, and when I finally took the time to trace out his head and ears and front and rear paws, he quickly became one of my favorite constellations – one of the rare ones like Orion and Scorpius that really look like what they depict.  And funny – I can’t explain why, but I seldom see it as a bear except at this time of year when it is rising. Then it seems to dominate my northern sky and my imagination.

Oh – did I say it looks like a bear? No – I should have said it looks like a bear no one has seen except in the sky – a bear with a long tail! I don’t know why that is. I assume it is because of the second depiction, which is how Johann Bayer pictured the Great Bear in his “Uranometria,” a breakthrough star atlas published in 1603.  Bayer was a lawyer, not a hunter. Maybe he had never seen a bear?

The first depiction, a Stellarium screenshot, is the best one to use as a guide for finding the correct stars. Besides the Dipper stars, there are a dozen more that trace out his main features, and all of these are either third magnitude, or on the brighter side of fourth magnitude – that is between 3.5 and 4, so they should be visible from most locations – assuming, of course, you are in mid-northern latitudes.  The chart that follows gives a view of the Bear in context with the rest of the northern sky in February.

About one hour after sunset, look north and you should see a sky similar to the one shown in our chart . The height of Polaris, the North Star, will be the same as your latitude. Polaris stays put.  Everything else appears to rotate about it, so our view of all else changes in the course of the evening – and from night to night. It’s a good idea to check the north sky every time you observe to get a sense of how things are changing and to orient yourself.  Notice that the “W” now looks more like an “M” as it starts to roll on down into the northwest.

Click image for larger view. (Chart derived from Starry Nights Pro screen shot.)
Click here to download a printer-friendly image of the above chart.

 

Look East in February 2011: Two dogs rising in a star-spangled spectacular known as the Winter Hexagon

We have two “dog stars” on the southeastern horizon early on February evenings  – Sirius and Procyon – and  both are part of what is certainly the brightest, star-spangled  section of our northern night sky – the Winter Hexagon. Here’s how it looked to the camera of Jimmy Westlake who took this gorgeous shot of the Winter Hexagon over Stagecoach, Colorado, USA.  You may not see the faint band of the Milky Way shown here if you live in a light polluted region, but you certainly should be able to pick out the bright stars that outline the Hexagon, as well as the Pleiades star cluster visible in the upper right.

Click on image for much larger view! (Copyright © 2007-2011 JRWjr Astrophotography. All rights reserved.)

Look carefully at that photo, then compare it with this star chart which is what we see from mid-northern latitudes as as we look southeast early on a February evening.

Click image for much larger version. To get the full beauty of this section of sky find an area with a clear horizon to the southeast and go out on a February evening a couple of hours after sunset. The chart shows what you'll see. The link below provides a small black-on-white version you can print and take into the field. (Prepared from a Stellarium screen shot.)

For a printer-friendly version of this chart, click here.

People in the north tend to think that the stars are brighter in winter because the air is so cool and crisp. That certainly could be a factor. But the simple fact is our winter sky is dominated by a whole lot of very bright stars. In fact, there are 22 stars of first magnitude – 16 of which are visible from the northern hemisphere and more than half of these are visible in “prime time” on a February evening. And nearly all these bright stars are jammed into a space taking up less than one-quarter of the February night sky – that’s just one-eighth of the total night sky we can see through the year! Add to that the seven bright stars of the Big Dipper being dragged up the northeastern sky by the Great Bear on a February evening, and it is no wonder that in the dead of a northern winter our skies offer a lively, colorful, star-spangled spectacular.

The Hexagon alone contains seven of the first magnitude stars in our sky and an eighth that is the brightest second magnitude star we see – in other words, that one just misses being first magnitude by a hair. And nearby is Adhara, a star that sits right on the border between second and first magnitude; plus Regulus, another first magnitude star, is rising in the east.

Whew! That’s a lot. Let’s review.  Going  counterclockwise and starting at the bottom, the Hexagon’s corners are marked by:

  • Sirius, the brightest, and at about eight light years one of the closest, stars in our sky – except the Sun, of course.
  • Rigel, the blue giant that marks one of Orion’s feet.
  • Aldebaran, the brilliant orange star that is the eye of Taurus the Bull and dominates the nearest open star cluster, the Hyades.
  • Capella, now high overhead, is really a complex of four stars that we see as one.
  • Castor and Pollux, the twins, one of which (Pollux) is first magnitude, while Castor is the brightest second magnitude star we see.
  • Procyon, the “Little Dog” star, which is dim only in comparison to Sirius, the “Big Dog.”

And . . .

  • Inside the Hexagon is another first magnitude star, Betelgeuse, the red giant that marks Orion’s shoulder, not to mention the three bright stars of Orion’s Belt – all second magnitude.
  • Regulus, the “Little King,” is a first magnitude star that is rising in the east and bringing us the familiar sickle of bright stars that mark the head of the lion. We’ll study it closely next month.
  • Adhara is the western-most star of the distinctive small triangle of stars beneath Sirius. At magnitude 1.5 I call it a first magnitude star, but others consider this second magnitude. So depending on how you count Adhara there are either 21 or 22 first magnitude stars.

Before leaving the Winter Hexagon, I must stress that  this is not simply a northern hemisphere show – if you live  in Sydney, Australia, you could just rename this the “Summer Hexagon.” I see these stars in the southeast – my friends in Sydney see them in the northeast of their sky – and, of course, since they’re “standing on their heads,” they see them a bit differently – something like this!

The "Winter Hexagon" becomes the "Summer Hexagon" in the Southern Hemisphere, but contains all the same bright stars. (Chart prepared from Starry Nights Pro screen shot.)

February Guidepost Stars

Of the stars mentioned so far, the two dog stars, Sirius and Procyon, plus the fence sitter, Adhara, are the guidepost stars to learn this month. They are the ones you can spot near the southeastern horizon, coming into view about 45 minutes to an hour after sunset. (We’ll have more to say about Regulus next month, and the other stars mentioned we’ve met in previous months.) To see the February guidepost stars – and the asterism of the Virgins –  look low in the southeast about 45 minutes to an hour after sunset.  Here’s what you should see.

Click image for larger version. This chart shows the three guidepost stars for February as they appear about an hour after sunset in the southeast. Sirius is the brightest star we see and Procyon is not far behind, but Adhara is not much brighter than its companions, which form a distinctive, small triangle the ancient Arabs knew simply as "the Virgins." (Prepared from Starry Nights Pro screen shot.)

For a printer-friendly version of this chart, click here.

Procyon, the seventh brightest star we see, is first up in our sky, and thus the highest, of these three. To the southeast and a tad lower is brilliant Sirius, brightest star in our sky, and next to the North Star, Polaris, probably the best known star in the world. Adhara is the brightest star in the  “Virgins,” a simple,  distinctive  triangle asterism.

But, of course, Sirius is dominant – far brighter than any other star we see in our night sky. I always think of Sirius as the eye of the great dog and as he sits, the triangle seems to be his rear haunches. From our perspective Adhara may be just another bright star, but of these three it is really the brightest by far – it’s just much farther away than the other two.  If we compared them side by side we would find that Procyon shines with the light of seven Suns, Sirius 23, and Adhara has a luminosity to the eye of 3,700 Suns! Now that’s bright.  And in another way, Adhara reveals our human bias, for if we had ultraviolet vision Adhara would be the brightest star in our sky, not Sirius.

But again – that not the way we see it. From our perspective Sirius and Procyon are very bright because they are very close to Earth. Sirius, at a little more than eight light years is the closest star that we in the mid-northern latitudes see in our night sky. Procyon, at about 11 light years, is fourteenth on the list of nearest stars.  Most of the stars that are nearer than Procyon are also much fainter – in fact, too faint to see with the naked eye. If we count just those stars bright enough to see with the naked eye, Procyon is the sixth closest and Sirius is the second closest.  (The closest star, Alpha Centauri, is visible only to those in, or near, the Southern Hemisphere.) But Adhara? Adhara is 405  light years away – about the same distance as the North Star, Polaris.

Sirius will frequently seem to be changing colors, but that’s just the effect of our atmosphere. Just as our atmosphere makes our Sun look red when it is rising or setting, it makes any bright star near the horizon appear to dance and change colors rapidly.

The Big Dog as Johannes Bayer depicted him in 1603. ((Image courtesy of Linda Hall library of Science, Engineering and Technology.)

The Little Dog as shown in the 1603 Uranometria chart. (Image courtesy of Linda Hall library of Science, Engineering and Technology.)

Sirius is known as a “dog star” because it is the brightest star of the classic constellation, Canis Major – the Big Dog. Procyon is the brightest star in the constellation Canis Minor, the Little Dog. When you look at these constellations as depicted in early star charts, it’s hard to see how connecting the dots makes the stars take the forms the constellation’s name implies, but the images are still useful memory joggers.

Modern science, though, gives us even more reason to remember these two stars, or rather the faint companion stars that orbit them. These are designated Procyon B and Sirius B and they defy our ability to even imagine because there’s just nothing in our down-to-earth experiences that compare with these tiny stars.  One of these “pups”  – the one belong to Procyon – is impossible to see with a backyard telescope and the other an extreme challenge.  The reason is they are quite dim and being very close to the bright stars, get lost in the glare.

But the mystery of these two fainter stars is that they are both white, indicating they are among the hottest of stars. So how could something be that hot, that close to us, and yet so dim? And the answer is more mind-boggling than the question – they are both white dwarfs, and white dwarfs are a class of stars far denser than anything we encounter on Earth. Now I always find talk of the density of stars counter-intuitive because it gets drilled into our heads that stars are gas and the gas we encounter in our daily lives is anything but dense!  In fact, it’s quite – well – gaseous!  To appreciate this, let’s take a close look at our own Sun.

Click image for larger view.Sirius – with Sirius B at lower left. Credit: NASA, H.E. Bond and E. Nelan (Space Telescope Science Institute, Baltimore, Md.); M. Barstow and M. Burleigh (University of Leicester, U.K.); and J.B. Holberg (University of Arizona)

The Sun is a ball of gas which reaches densities that near the center are sixteen times that of lead!  That alone should stretch your mind. But now imagine the white dwarf. The stuff that makes up a white dwarf is about one million times denser than the stuff in the Sun. Jim Kaler writes that if you had a billiard ball made up of the stuff of one of these white dwarfs it would weigh about 70 tons – roughly the weight of an M1 Abrams tank. (Think of what that would do to your pool table, not to mention your foot if it fell on it!)  We know this because we can calculate the mass of the stars by their orbit around their bigger, brighter companions. The result is, we end up with a mass roughly that of the Sun but a size roughly that of the Earth. You can fit one million Earths – and therefore one million white dwarfs – inside the Sun. (See why a white dwarf is one million times as dense as the Sun?)

How do you take all that mass and squeeze it down to such a small size? The physics of how that’s done goes way beyond me, but if you want to put a name to it, a white dwarf consists of “degenerate matter.” Unlike other stars, white dwarfs no longer burn with nuclear fires. In fact, they are no longer burning at all. They are the dying embers of stars – and in the case of the “pups,” the embers are being seen while still white hot. But they will eventually cool.  The name white dwarfs is given to this class of stars, but in truth not all white dwarfs are white – some can even be red. To make sense of this contradiction of terms, just think about an ordinary dying ember and how its color will change as it cools. So it is with these dying stars. Unable to generate any heat, what they radiate they lose.

This is also the ultimate fate scientists expect for our Sun.  As it eventually exhausts its nuclear energy, it will turn into a bloated red giant like Betelgeuse in Orion.  Later still it will blow off its outer shell of gases, turning into a planetary nebula, such as the Ring Nebula (M57) in Lyra.  And at the core of that nebula will be the dying ember we know as a white dwarf.

I’ve never seen the white dwarf that revolves around Sirius, but perhaps this season I will. Orbits are not circles, but ellipses. This means that sometimes there’s more distance between Sirius and its “pup” than at other times – and we happen to be in a period of several years when that distance will be growing, and so it will become easier to see the pup in a good, backyard telescope. (Sirius B completes an orbit around Sirius A in 50.2 years. Procyon B, while visible to professionals, is just simply too difficult a target for most backyard telescopes.) I also plan to take a close look at Adhara with a telescope, for it has a 7.5 magnitude companion just 7 arcseconds away. This should be a challenge – because of the difference in brightness of the two –  but not nearly the challenge that seeing the companion of Sirius.

For those with binoculars and small telescopes, some of the most fascinating objects are in this general area of sky, near, or inside the Winter Hexagon, including the Pleiades, the great Orion Nebula, and the spectacular telescopic open clusters in Gemini and Auriga, M35, M36, M37, and M38. All that star light certainly can make for bright nights during the dark  of a northern winter.

Vital Stats for the Guidepost Stars

For Procyon:

  • Brilliance: Magnitude 0.38, the 7th brightest star in our sky. Shines with the luminosity of about 7 Suns.
  • Distance: 11.4 light years
  • Spectral Type: F
  • Position: 07h:39m:18s, +5°:13′:29″
  • Procyon B is magnitude 10.7 and orbits Procyon in 40.8 years.  It can be as close as 9 AU to Procyon (1 AU is the distance between the Earth and Sun), or as far as  21 AU.

For Sirius:

  • Brilliance: Magnitude -1.5,  the brightest star in our sky.  Shines with the luminosity of about 23 Suns.
  • Distance: 8.6 light years
  • Spectral Type: A
  • Position: 06h:45m:09s, -16°:42′:58″
  • Sirius B is magnitude 8.3 and orbits Sirius in 50.2 years. It can be as close as 8.1 AU to Sirius, or as far as 31.5 AU. (It will reach this greatest separation in 2019.)

For Adhara:

  • Brilliance: Magnitude 1.5, it has a luminosity to the eye of 3500 times that of the Sun! (In other words, much brighter, really, than Procyon or Sirius.)
  • Distance: 405 light years
  • Spectral Type: B2
  • Position: 06h:59m, -28°:59′:18″
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