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:
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!
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:
Filed under: 2. Astro Events, b. February | Tagged: Algol, Demon Star, Jupiter, Moon motions, Moon phases, Saturn, Zodaical Light | 1 Comment »