Archive for the ‘Sky motions’ Category


Last week’s full moon

August 5, 2012

Ever since the full moon of January 29, 2010, I’ve been wanting to catch another that was perfectly full. Not a day or even half a day early or late, but right on the button. I came pretty close last Wednesday, August 1. Here’s my best shot, taken with the Nikon Coolpix 4500 shooting through my Apex 127 Mak and 32mm Plossl.

It’s hard to say if this moon is really perfectly full or not; in a way it is, and in a way it isn’t. I know that’s enigmatic, and I’ll clarify it at the end of the post. But first, compare last Wednesday’s full moon to the renowned (by me, anyway) January 29, 2010, moon.

Here, let me make that comparison easier for you. As always, click for the big version.

Two things here are worthy of note. First, there is a difference in illumination. On the left, the east side of the moon is better lit, and on the right, the west.

More importantly, these pictures do not show the same stretch of moon! Check out this overlapped composite, with a few prominent landmarks labelled.

All of the offsets are consistently in the southeast-northwest direction, and the two moons are perfectly overlapped at the periphery. The difference between the images is not because the photographs are rotated in two dimensions, but because the moon was differently rotated in three dimensions. This effect is called libration, and because of it we can see almost 60% of the lunar surface from Earth.

Here’s the comparo again, this time with some of the limb features labelled.

On January 29, 2010, the northwest limb of the moon was tipped toward us, allowing a good view of the “shore” of Oceanus Procellarum and some prominent rim craters like von Braun and its equally-spaced outriders Lavoisier A and Harding. Another useful landmark is the bright crater Seleucus, just to the east of the much larger, dark-floored Eddington. On the opposite limb, Mare Marginis and Mare Australe are barely visible, and Mare Smythii is just a dark patch on the limb itself.

Now compare to last week’s fully moon. The northwest limb is rotated so far away that von Braun is completely lost, along with the rest of the shore of Oceanus Procellarum, and Eddington is a barely perceptible dark streak. On the other hand, the southeast limb shows excellent detail in Mare Australe, especially around the very dark-floored craters Lyot and Oken, and farther north we can see all the way across Mare Smythii to the lighter terrain beyond.

Now, as to the “perfection” of the fullness: there is some terminator-like shadow and detail visible in my  photo from last week, but not on the eastern limb where one might expect it. Instead, all of the visibly shadowed craters are around the south pole. This is where the story gets complicated.

There are three widely-discussed causes of libration: (1) the moon leading or lagging, relative to its own rotation, along its eccentric orbital path; (2) the tilt of the  moon’s axis relative to the plane of its orbit; and (3) rotation of the Earth, which from moonrise to moonset carries an Earthbound observer almost 8000 miles from west to east (which is why everything in the sky rises in the east–that’s the direction we’re constantly headed here on the surface). The moon is only 240,000 miles away, so this daily (or nightly) trip equals 1/30 of the distance from Earth to the moon. How much difference does that make? The average interocular distance for a human is 6.5 cm (2.5 inches), so look at something 30 times farther away (195 cm or just over 6 feet) first with one eye and then with the other. You just simulated diurnal libration.

Now, as I noted above, the eastern limb of the moon is darker than the western side in last week’s photo. The Sky & Tel online almanac said max fullness would be at 8:37 PM, PDT. But the moon was just rising then, about four hours before it would cross the local meridian. In other words, at max fullness the moon was dead overhead for people 4000 miles west of me, but the turning Earth wouldn’t carry me directly under the moon for another four hours–and by the time I got there, it wouldn’t be perfectly full anymore. I took the picture I used in this post at about 11:30–three hours too late for a perfectly full moon. I took other pictures at 8:37 and other times in between, but they turned out poorly–seeing near the horizon was rotten and my scope wasn’t properly cooled yet.

So I’m pretty sure that diurnal libration–the effect of the turning Earth–accounts for the less-than-even illumination from east to west in my moon shot. But that doesn’t explain why there are shadows at the south pole. I assume that the alignment of the moon and Earth was such that I was looking up the moon’s skirts, so to speak–so far south, relative to the moon, that I could see past the illuminated area and into the shadowed highlands beyond. If that’s true, then observers in the Southern Hemisphere, being even farther “below” the moon, should have been able to see even farther into the shadowlands.

The moral of the story is that if you want a good photo of the perfectly full moon, it’s not enough that the moon be visible in the sky at the moment of max fullness–you should also be right underneath it (it should be as high in the sky as it is going to get). Even if you get good enough seeing to get a clean shot of the moon low in the sky, you’ll be several thousand miles to one side or the other, and you won’t be seeing it face-on. On the flip side, if you catch the rising full moon a few hours before max fullness, or the setting full moon a few hours after, you might still get a fully-illuminated disk, because Earth’s rotation will put you along the same line as the incoming light. Sounds a bit hairy, but as Timothy Ferris wrote of making chancy observations, “You can’t catch any fish if you don’t get your line wet.”

Anyway, I had a lot of fun, and got a good look at some southeast limb features that I’d never seen before. I’m anxious to see what libration will bring me next.


Two new Astronomical League observing clubs

February 27, 2012

Award pin for the Binocular Double Star Club, from the AL website

My copy of the Reflector, the Astronomical League’s quarterly magazine, arrived in the mail today, with announcements of two new observing clubs: the Binocular Double Star Club and the Analemma Club.

This is exciting news for me. I’m always looking for new lists of things to look at, especially from home. My Herschel 400 project is chugging along, slowly, as I get dark-sky time, but I can’t get to dark sites all the time and I’m committed to observing from home. I really need structured observing lists or else I spend my driveway observing sessions checking out a handful of old favorites and then wondering what else to wonder at. Binocular lists are good because binocular objects tend to be bright enough that I can see them from Claremont with my 15x70s, and double stars are good because they punch through the light pollution pretty well and many of them are strikingly beautiful. I’ve already finished three binocular observing programs (Binocular Messier, Deep Sky Binocular, and Southern Sky Binocular), and I’m a bit over halfway through the observations for the AL Double Star Club and loving it. So a new club that combines binocular observing and double stars is right up my alley. Update Aug 1 2012: As I often do for AL observing programs I intend to pursue, I made up a blank logbook for the bino double star club. It’s free if you want to use it–you can find a link to the PDF on this page.

Award pin for the Analemma Club, from the AL website

The Analemma Club is a little different. You don’t observe a long list of objects, just one over and over: the sun as it traces out its figure-eight path, or analemma, in the sky over the course of a year. That path is created by Earth’s axial tilt and its elliptical orbit around the sun. A Google Image search for ‘analemma’ will show many composite photos created by amateurs that show the position of the sun in the sky at regular intervals over the course of a year. An analemma can also be recorded by projecting the shadow of a gnomon on the ground, a wall, or a globe–the Wikipedia article on analemmatic sundials has a couple of examples, and there are loads of instructions on how to build these things scattered around the web. Once you have a complete analemma, you can do all kinds of things with it:

  • Calculate your observing latitude and the tilt of the Earth’s axis
  • Sketch or plot the path of the sun on the celestial sphere
  • Calculate the Equation of Time
  • Calculate the eccentricity of the Earth’s orbit

All of this requires math–alegbra and trigonometry. And in the Analemma Club, you have to first generate an analemma and then do that math; the list of four things to be calculated and sketched is taken from the Analemma Club page. Now, I realize that following the sun for a year so you can do math is not everyone’s idea of a good time. But I’ve always been fascinated by sky motions and I’m sufficiently interested in analemmatic sundials to have started a project folder for one at some point, so this club may be the kick in the pants I need to actually, you know, do the work.

So, that’s why I care about these new clubs. Why should you care? Well, if you’re in the US and you’re a member of an astronomy club, you’re almost certainly an AL member already, so if you’re doing any regular observing programs you might as well send in your observations and get some bling.

What if you’re not a member of an astronomy club, or not in the US? Well, if you find the observing programs useful, do ’em anyway. All of the requirements are freely available online, and although the bling is a fun perk, the real benefit is in learning your way around the sky, developing your observing skills, and most importantly, seeing a bunch of awesome stuff.

As of this writing, the Astronomical League has 34 different observing programs (and 3 clubs that have no observing requirements), covering everything from Earth orbiting satellites to distant galaxy clusters. Several clubs require only naked-eye observations, several more require binoculars, and the vast majority can be completed with an inexpensive telescope. So whatever your available gear or level of experience, there is probably an AL observing program that would suit you. Go check ’em out.


Don’t miss the moon and planets at sunset this weekend

February 25, 2012

From bottom to top: the moon, Venus, and Jupiter, on Feb. 24, 2012.

Venus and Jupiter are both high in the evening sky at sunset right now. Just look west right when it gets dark and they’ll be the two brightest stars in the west. Venus is the brighter and lower of the two.

For the next couple of nights they’ll be joined by the waxing crescent moon. Tonight the moon was just below Venus, so the three bodies were stacked up the sky from lowest and brightest to highest and dimmest.

Early next week the moon will pull away from the planets as it continues on its monthly eastward trek around the sky, but Venus and Jupiter will still be there and looking good.

A close-up of the moon at the same time as the photo at top.

Venus is slightly gibbous right now (between 4 and 5 in the diagram below). On March 26 it will achieve its greatest eastern elongation from the sun, 46 degrees, meaning that at sunset it will be halfway between the horizon and the zenith. At that point it will be half-lit as seen from Earth (5). From then on into April and May, Venus will get lower and larger as it goes into its crescent phase (6) and gets ready to pass between the Sun and the Earth. Venus makes that passage all the time as it transitions from being the evening star (east of the sun as seen from Earth = above the western horizon at sunset, 6 in the diagram) to the morning star (west of the sun as seen from Earth = above the eastern horizon at sunrise, 1 in the diagram).

Phases of Venus as seen from Earth

Because the orbits of Earth and Venus are not precisely in the same plane, Venus does not usually pass directly between the sun and the Earth but passes above or below the sun as seen from Earth. This time will be different; as happens only a couple of times per century at most, the orbits are lined up just so and Venus will pass across the face of the sun as seen from Earth. That’s the transit of Venus I’ve been so het up about. Stay tuned for more on that, and keep looking up at sunset for the next few weeks to see Jupiter and Venus continue their tango.


Galileo Club, Part 3: Callisto in eclipse

November 8, 2009

There were two chances to see a Galilean moon entering or exiting Jupiter’s shadow from California tonight. At 6:22 PM Europa came out of the planet’s shadow, and at 8:50 Callisto went into it. I missed the first one but caught the second one.

I cheated a little bit; in addition to observing the disappearance of the moon at under 20x as required by the rules, I also photographed it at higher magnification in my 6″ reflector.

Now, when I first started observing, there were four little moons, two on each side of Jupiter, and as I watched, the inner one on the right got dimmer and then disappeared. But that requires a little unpacking. If you punch up Jupiter this evening in Stellarium or Celestia (follow the links on the right to download ’em if you haven’t already–they’re free), you’ll see that Callisto was the inner moon on the left as viewed from Earth. It was the inner moon on the right in the telescope because Newtonian reflectors rotate the image by 180 degrees. No big whoop, but if you watch the un-flipped version in Stellarium, the mechanics of the process are a lot clearer.

Callisto eclipse diagram

If you face south to see Jupiter, the sun is off to your right, having just set. That means the shadow of Jupiter forms a cylinder sticking out into space to the left of the planet as viewed from Earth. Since most stuff in the solar system orbits in a counter-clockwise direction* when viewed from above (Earthly north), Callisto must be on the far side of Jupiter from Earth. Callisto came out from behind the planet (1), was briefly visible from Earth, then entered the planet’s shadow (2), and will re-emerge in a little less than two hours (3).

* Neptune’s largest moon, Triton, is a notable exception, and its backward motion indicates that it is almost certainly a captured Kuiper Belt object and not a “true” moon. In fact, it is possible that all of Neptune’s moons are captured objects.

So how did the pictures turn out? Well…the listed time of 8:50 PM turned out not to be the midpoint of the entrance of the moon into the shadow, but the  completion. Which I guess makes sense, but I was prepared to start my observations 10 minutes early and run 10 minutes after. In fact, by 8:40 Callisto was already noticeably dimmer than the other moons, and at 8:50 the show was over. “Noticeably dimmer” in the eyepiece means “almost impossible to photograph”, at least with my setup (Orion XT6 telescope, 25mm Sirius Plossl eyepiece, and Nikon Coolpix 4500 camera). I thought I had missed it completely. My only shot of Callisto this evening:

Callisto 1 - nothing to see

Can’t see it? Neither could I. But I blew out the contrast and look who showed up, if only just:

Callisto 2 - ugly but there

By using that image as a template, I was able to copy, paste, and lightly sharpen my best Jupiter image of the evening, and replace the moon blobs with tiny little brush-dots that approximate their actual size and brightness, to create this much prettier and more representative, but less real, image:

Callisto 3 - too pretty to be real

Jupiter looks lousy compared to previous efforts because I was looking through most of the LA light dome and attendant haze, but still: eat yer heart out, Galileo. One required task down, ten to go!

(If you haven’t bagged a moon eclipse yet, there are still several opportunities this week.)


Moon and moon rocket

October 26, 2009

2009-10-26 moon and Jupiter 009

The solar system is a big disk, and most everything in it orbits in or nearly in the same plane, called the ecliptic. Projected on the sky, the ecliptic forms the track along which the sun, moon, and planets all appear to move. One nice consequence of this is that we get pretty alignments of the moon and planets fairly regularly. The moon has to pass each of the planets every month, but the passings happen in the daytime about half the time (as you’d expect), and sometimes after the moon rises or sets from a particular spot. Still, these conjunctions come along several times a year and they’re always worth watching for.

Right now as I type the moon is getting close to Jupiter; the two are separated by only about three degrees. Unfortunately, the moon is also getting close to the horizon, and the closest approach of the two bodies as seen from Earth will be visible to some lucky folks about halfway around the planet, I reckon. Still, I got out this evening with my Astroscan (about which more later) and digital camera and took a stab at capturing the event in pixels. Moon and Jupiter at top, moon by itself right here.

2009-10-26 moon and Jupiter 018Tip o’ the hat to Doug at Revolving Rock for reminding me about the conjunction!

In other news, the folks at NASA finally got a photo up showing Atlantis on pad 39A with Ares 1-X on pad 39B. If the weather is good, Ares 1-x will be blasting off in just a few hours. Between the possible weather and NASA being, well, NASA, there will probably be a delay or two, so keep an eye on NASA’s Ares 1-X page this morning.

389937main_2009-5728Finally, this 360-degree panorama of the Ares 1-X in Highbay 3 of the Vehicle Assembly Building may take a while to load, but it is definitely worth the wait. If you ever wondered what it would be like to stand in a 500-foot-tall room in front of a 327-foot-tall rocket, here’s your chance to find out.

And yes, I know that the Ares 1-X is not a moon rocket in the sense of a vehicle that will actually send people to the moon. But Ares 1 will be part of the system that does…hopefully.

I want to go there. Don’t you?


Extended Mission: Follow the Moon

August 26, 2009
The moon from downtown Claremont this evening, taken with a Nikon Coolpix 4500 digital camera held up to the eyepiece of my 3.5 inch telescope.

The moon from downtown Claremont yesterday evening, taken with a Nikon Coolpix 4500 digital camera held up to the eyepiece of my 3.5 inch telescope.

Mission Objectives: The Moon, Sky Motions

Equipment: Naked eye

Required Time: 1 minute, every few nights, for one month

Instructions: Follow the moon through one complete cycle of phases. Should be easy enough, right–you’ve seen it, what, about a million times in your life? But do you know where it is right this minute?

The moon orbits the Earth in the same direction that the Earth spins, west to east (counterclockwise if you’re looking down on the North Pole). The west-to-east rotation of the Earth means that when you’re on Earth, stuff in the sky appears to move from east to west–most notably the sun, but also the stars and the moon. The stars might as well be bolted to the dome of heaven for all the motion they reveal to the casual stargazer (beyond the basic rising and setting). The apparent motion of the sun is different–as the Earth swings around it in orbit, the sun appears to make one complete circuit of the sky each year, at a rate of a little more than 1 degree per day (360 degrees/365 days). The apparent motion of the moon is also different; the moon is orbiting the Earth every 27.3 days, so it makes one complete circuit of the sky (relative to the background stars) in that time, at about 13 degrees per day.

BUT the Earth is still spinning west to east, and going much faster than the sun or the moon are relative to the background stars; from Earth, the background stars themselves appear to turn 360 degrees in 24 hours, at about 15 degrees per hour. So the predominant motion of the moon in the sky is still the east-to-west progression that we expect from the sun, the stars, and everything else up there. But the moon is moving around us west-to-east, so it appears to go more slowly than the sun. Each day the moon is a bit more than 13 degrees farther from the sun. You may also think of it like this: the moon completes an orbit in 27.3 days, and there are 24 hours in a day, so each day the moon rises and sets about an hour later, until it has come back around to where it started, one month hence.

If all that makes your brain hurt–and it sometimes does mine–let’s try it with pictures. Imagine that you are standing in an open field at sunset, facing south (better yet, arrange to be in an open field at sunset, facing south!). If it  just a couple of days after the new moon, this is what you’ll see:

1 - crescent moon

Of course you know that the moon is tidally locked to the Earth and always shows us the same face; all that changes from our perspective is how much light falls on the near face. There is no “dark side of the moon”, just a near side and a far side, and over the course of a month they receive equal amounts of sunlight and darkness.

In the diagram above, showing a young crescent moon, the moon has not yet moved very far from the sun; in fact, is still very far sun-ward of the Earth. Most of the sunlight is falling on the far side of the moon, but the moon is enough off to one side of the Earth-Sun line that a little light spills over to the near side.

2 - first quarter moon

The next night the moon will rise about an hour later and be about 13 degrees farther from the sun in the sky. From being in between the Earth and Sun, the moon is moving around to be beside the Earth, relative to the sun. When it gets there, then it will be halfway up the sky at sunset, and halfway illuminated on the near side. This is the first quarter moon, and it’s absolutely the best time to haul out binoculars or a telescope and see the play of light and shadow over the craters, mountains, and valleys of la Lune.

3 - waxing gibbous moon

Once past first quarter, the moon continues to rise later and is consequently less far up the sky when the sun sets. But now the near face is turned more squarely to the sun and appears more fully lit. This more-than-half-lit condition is gibbous, and since the moon is getting fuller each night, it is a waxing gibbous moon.

4 - full moon

Eventually, about two weeks after new moon and one week after first quarter moon, the moon rises at the same time that the sun sets. The moon and sun are now precisely opposite each other in the sky, so the near face is entirely lit and the moon is full. Once in a while things line up so that the Earth is exactly between the sun and moon, and the shadow of the Earth on the moon creates a lunar eclipse. It should be obvious that a lunar eclipse can only occur at full moon.

That doesn’t mean that a lunar eclipse can only occur at sunset; the moon may become maximally full when it is halfway across the sky (and the sun is halfway between rising and setting), or during the day, or at any other time. Another way to think of it: whenever a lunar eclipse occurs, it will be at sunset for somebody, somewhere–and sunrise for someone else, and midnight for someone else, and noon for someone else.

Most months there is no eclipse, because the moon’s orbit describes a slightly different path in the sky than the Sun,  and the moon passes over or under the sun from our vantage point. If everything was in perfect alignment, we’d have a lunar eclipse every full moon, and a solar eclipse every new moon.

5 - waning gibbous moon

What happens after full moon? The moon continues to rise an hour later each night, but it has now gone past the point where it was opposite the sun in the sky (full moon), and starts to approach the sun from the other side. From our standpoint, it looks like the sun is catching up to the moon. From full moon to new moon the same phases pass–gibbous (mostly lit), quarter (half lit), crescent (less than half lit)–but in reverse order, and you have to stay up later and later to see them against a dark sky.

Eventually as the moon rises later and later, there comes a day when it rises at the same time as the Sun. In other words, the moon is now squarely between the Earth and Sun, the far side is entirely lit, the near side is entirely dark, and we can’t see the moon in the sky at all. This is the new moon, and in nights to come the moon will rise a bit later, trail the sun across the sky, and first be visible as a thin crescent low in the west at sunset, as in the first diagram up top. It should be obvious that a solar eclipse–when the moon gets squarely between the Earth and  Sun, and the shadow of the moon falls on the Earth–can only happen at new moon.

You don’t always have to stay up late (or get up early) to see the waning phases. In a month, the moon spends just as much time in the daytime sky as it does in the nighttime sky. Think about it–at first quarter, the moon is at it highest point in the sky at sunset. Therefore it must have risen six hours earlier (1/4 of the way around the sky x 24 hours of sky rotation in a day = six hours), and been visible for most of the afternoon and early evening. Similarly, at last quarter, the moon is at the same point at sunrise; it rose in the middle of the night and won’t set until the middle of the day. The gibbous moons on either side of full are easiest to observe during daytime, because they’re bright enough to see easily. The crescent moons must necessarily be very close to the sun in the sky, and so they are up almost all day, having risen either just before the sun (waning crescent) or just after (waxing, as shown in the first diagram up top). But they are almost impossible to spot because they are so poorly lit (from our point of view); their feeble light is lost in the glare of the sun.

Why make a big deal out of that? For roughly three decades I thought it was unusual to see the moon in the daytime. Then I picked up an intro astronomy book and learned that the moon is out in the daytime just as much as it is at night, and then I felt quite foolish. Because when you think about it, it can’t be any other way.

I’m posting this now because we’re almost to first quarter moon (Wednesday night to Thursday morning), and because this is a blog for busy people. At first quarter the moon is as high as it is going to get right at sunset, and it looks great all evening, and it shows a maximum amount of detail in binoculars and telescopes. So if you want to start observing the moon, with the naked eye or anything else, this is the most convenient time and the time when the moon looks her best.

After this you don’t have to observe the moon every night (although it’s not a bad idea if you can swing it), just check in it every two or three nights until it’s rising late enough (past full moon) that you don’t feel like staying up for it anymore. After that you can mostly forget about observing the moon for a couple of weeks, unless you want to get up in the middle of the night, or you’re up early before the sky gets light, or you remember to see the moon high in the sky in the middle of the day near last quarter (about a week after full moon). I’ll give a heads up in a few weeks about the coming new moon, and you can start looking for the new crescent moon in the evenings right after. We’ll be back to first quarter in 29.5 days.

Hold the freakin’ phone! If the moon orbits the Earth in 27.3 days, why does it take 29.5 days to complete one cycle of phases? The answer is that in the not-quite-four-weeks it takes the moon to orbit the Earth, the Earth has moved on in its orbit around the sun. Moved on a lot–1/13 of our way around the sun (4 weeks/52 weeks = 1/13). So the moon has to move past the point where its orbit is complete (and where it was relative to the background stars–its sidereal period) to get to the point where it is lit the same (= same phase–its synodic period). This takes a little over two days, hence the difference. If that doesn’t make any sense, check out the diagram at the bottom of this page (and read the rest while you’re at it). H.A. Rey’s book The Stars is also just outstanding at explaining all of this, and has the best diagrams I have ever seen anywhere, bar none (plus it’s under ten bucks at Amazon).

You can also verify this for yourself once you know some of the background stars, or even just one (it should be a bright one, so you can see it when the moon is out). The harder way is to pick an unmistakable phase, one you’ll be able to tell apart from the nights on either side (first quarter is perfect), note the proximity of the moon to your reference stars, and then do the same thing when that exact phase comes around next month. This is the hard way because you have to get your phases exactly right; one night off is enough to blow the whole deal. The slightly easier way is to pick a time when the moon is moving past a reference star, note the phase (preferably with a drawing or photograph), wait 27.3 days until the moon is moving past the same reference star, and compare the phase to your record from four weeks earlier.

The moon is probably my favorite astronomical object. I like the fact that it’s close enough and detailed enough to look great through binoculars and phenomenal through even the most modest telescope. I like watching the phases change and being able to understand how and why it happens. I like knowing that as long as I can see the night sky, I can figure out what direction I’m facing and roughly what time it is, and what season. I want you to have the same easy familiarity with the moon, but to still let it tickle your sense of wonder. Your relationship with the moon starts whenever you go outside and look up. So why not tonight?