Phases for Earthlings

This is fake. A bit. It's a composite of two objects taken on two different days using two different telescopes. The moon shot I took this evening. The tiny crescent is Venus, which I photographed on March 17. Venus is below the horizon now, so if you're looking to replicate that shot you'll have to wait a few weeks and then catch it on the upswing just before sunrise. I put them together because they're both in crescent phase, which tells us something.

Until I got into amateur astronomy about a year and a half ago I never gave the moon a second thought. Occasionally I noted that it was out in the daytime; like most cosmic provincials I did not realize that the moon is "out" during the day every bit as much as it is at night. I also did not understand that Venus is both the morning and the evening star, just not both at once. These things are obvious, though, if you think about them for even a few minutes.

When the moon is full it is opposite the sun in the sky, which means it rises when the sun sets and sets when the sun rises. Conversely, the new moon is "new" because it is between us and the sun and therefore the side facing us is lit only faintly by Earthshine, which is not nearly enough light to make it show up next to the sun. Occasionally the new moon gets precisely between the Earth and the sun and we get a solar eclipse, and occasionally the Earth gets precisely between the sun and the full moon and we have a lunar eclipse. Lunar eclipses may start at any time of day or night but they can only occur when the moon is full, so you can only see one at night.

In between new and full moons the moon is either ahead of the sun or behind the sun in the sky (from our geocentric point of view), and by the end of one cycle the time the moon has spent in the sky during our days will equal the amount of time it has spent in the sky during our nights. If you don't believe me, go outside and look, and report back in a month.

If you see a thin crescent moon in a dark sky it will always be fairly close to the sun, either at sunset (waxing) or sunrise (waning), and the horns of the crescent will face up into the sky and not down toward the horizon. Again, this is obvious after a moment's reflection: if the sky is dark, the sun is below the horizon, and if the moon is above the horizon then the side that is lit is the side that is "down" (horizon-wards) to an Earth-bound observer, so the horns of the crescent have to point up. It can't work any other way.

It should also be obvious that you can only see a crescent phase if an object is closer than the Earth to the sun. Think about Mars, the next planet out. Mars looks completely "full" when it is behind the Sun as seen from Earth (or would, if we could see it in the glare), and when it is opposite the sun in our sky (i.e., at the two planets' closest approach). When it is a quarter-orbit ahead or behind, it looks gibbous, but we will never see a "quarter Mars" or a "crescent Mars" as long as we are on Earth. The only objects we will ever see in those phases are Mercury, Venus, and our own moon. It should have occurred to me sooner, back at the beginning of all of this, that it is impossible to see a crescent Jupiter from Earth; such a view can only be had from a vantage point that is farther from the sun than is Jupiter!

If you like, you should be able to prove all of this to yourself by drawing some orbits as concentric rings, drawing in some planets, and thinking about what phases you would see from various vantage points in different orbits. Better yet, you can go outside, look up, and watch it happen. Best of all is when you can look up and understand what you see, because you've made a little orrery out of paper or brass or pixels or in the theater of your imagination. Now you have a little piece of the cosmos whirling around in your head, and though you may forget some details or have to look up to check the phase of the moon, you will never be entirely lost in the sky again. (You may get very irritated with movies and books that include the moon for ambience, because they almost always get it wrong, and in doing so violate not just physics but also geometry.)

Why this, why now? It's the International Year of Astronomy, and all over the world stargazers are gearing up for 100 Hours of Astronomy. One of the goals is to get more people looking up than ever have in the history of our species--and, more ambitiously, to help them understand what they see, and where they are in the cosmos. It's all in honor of the 400th anniversary of Galileo turning his telescope to the heavens, which brings us back to the crescent Venus, seen at top through the roiling chemical stew of the LA basin.


Galileo was the first to see that Venus goes through a full set of phases like the moon. From this he deduced that Venus must circle the sun (right); in a geocentric cosmos it could only ever be "new" or a crescent (left); to be full in such a cosmos it would have to appear opposite the sun in the sky, like the full moon, which it never does for reasons that the diagram on the right makes clear. Galileo's discovery that four little points of light near Jupiter are in fact its moons and circle Jupiter rather than the Earth or sun often gets more press (immortalized in the name of the biggest Jupiter mission ever, for example), but the phases of Venus are easier to see and more immediately understood, and were arguably more important in sealing the coffin of the geocentric cosmos.

I often think of Galileo, squinting through his telescopes, which were attrocious by modern standards--"plagued by every aberration known to optics"--but nevertheless the first window into the real workings of the cosmos that science ever had, and I feel very humbled, and a little lost. Humbled because I am fantastically spoiled--the first telescope I ever owned is better by far than any telescope built in the first 200 years that telescopes existed--and I doubt if I would have had Galileo's perseverance, to just keep looking. Lost because I grew up in America in the late 20th century, with more information at my disposal than all previous generations of humans combined, and it still took me more than three decades to stop, look up, and notice the very basic cycles--phases of the moon, for example--that govern the rhythms of the living world, that have informed the calendars of every human society as long as societies have existed, and that formed the first really solid step in our species' long climb into enlightenment.

So, stop. Just stop. Look up. Keep track of the moon for a month. Draw some orbits and think about what phases mean. Discover, or rediscover, the universe.

If I was emperor...

Here's what I'd do first. I'd come to your home, right after dinner, take you by the hand, drag you outside, and make you look at Saturn through a telescope, so you can see the rings.

Partly because I know you'll really dig it. I have to yet to meet anyone who didn't.

But mostly because I love watching people's reactions when they see it for the first time. About three weeks ago I took some of my ecology students on a field trip to Yosemite, and I took my little Edmund Scientific Astroscan (love it!). About 9:30 I set it up on the hood of an SUV (on a towel, so it wouldn't scratch the paint) and gave the interested a brief tour of the sky. We started and ended at Saturn. One of the guys had never looked through a telescope before. I'm not saying that to knock on him--most people haven't. I'm saying it because it was freakin' awesome to get to be the one to show him this stuff for the first time, and especially freakin' awesome to start with Saturn.

Then a couple of weeks ago we had some friends over and as we were walking them to the car at the end of the evening we all stopped to have a naked-eye gander at the moon. Somebody asked if any planets were up so I pointed out Saturn right by Regulus, and then I said, "Look, just wait two minutes. You've got to see this." I keep Shaft, my Orion XT6 Dobsonian reflector, parked against the wall in our pointlessly immense* entryway for just this purpose, and about 90 seconds later one of our friends was getting her first ever look at Saturn. She literally squealed with delight.

* I'm convinced that the houses in this addition have big entryways just to stick it to people from the coast. It's about a third the area of our entire apartment in Berkeley and it serves no purpose other than to ostentatiously show off the fact we have tons and tons of space.

Look, seriously, if you haven't seen it you just have to. It's mandatory. Most space stuff you can see pretty well with binoculars, like the Orion Nebula and the moons of Jupiter, but you'll need higher magnification to grab Saturn's rings and that means a telescope. In the Bad Old Days decent telescopes were usually prohibitively expensive, but not anymore. Orion's StarBlast has gotten uniformly great reviews (here and here for starters; Sky & Telescope and Astronomy both loved it but their reviews are behind paywalls) and it is under $180, so unless you're reading this from a library computer you've got the juice. A new Edmund Astroscan is just under $200, damn near indestructible, and will last forever.


It works well for kids, too, and it's tough enough you won't freak about that. If you need something bigger--something you might need more than one hand to carry outside, say--I got Shaft on sale for under $250 and it looks like a freakin' howitzer.

Back to task: Saturn. Look now, and save me some work in the future.

Got binoculars? Make some science.

This Saturday night a bright star in Taurus will be occulted (eclipsed) by the moons of the asteroid Eugenia. Yeah, that's right, the moons of an asteroid. If you didn't know asteroids had moons, check this out. Anyway, observers in the southern US and Mexico can help astronomers determine the positions of the moons by--get this--watching the star go off and on and noting the times. For this, even the largest telescopes on Earth are less useful than you are, as long as you have binoculars, a timekeeping device, and some idea of where you are. Details here, instructions and finder charts here (scroll down to the hideous yellow part).

The photo above shows the 53-km-long asteroid 243 Ida and its tiny, 1.4-km moon Dactyl. Stolen from Wikipedia.

Dr. Vector visits the other Tethys

Usually when paleontologists talk about Tethys, they mean the ocean. Which is gone, or at least busted up pretty good. But tonight I explored the other Tethys for the first time, if "explored" is not too grand a word for sighting a wee dot of light at the edge of visibility at 125x in my telescope. Tethys is the fifth-largest of Saturn's 60-odd moons. It's about a thousand kilometers in diameter, slightly less than the distance between Berkeley and Salt Lake City on I-80 (a drive that figures prominently in John McPhee's Basin and Range and Assembling California, and in my own personal history).

I also saw Titan--biggest moon in the solar system, larger than the planet Mercury--and Iapetus. Dione and Rhea were too dim and too close to the planet to make out. (I don't know which moons those are in the photo above, I just liked it and didn't feel bad about ripping it from the horoscope site where Google Image Search found it.)

Actually I had no idea which moons I was seeing at the time; I sketched Saturn and the three I could see and did the IDing later, using Stellarium (still free, still awesome). Here's a screencap:


Seriously: if you haven't seen the rings of Saturn, or the moons of Jupiter, or the Orion Nebula, or the Double Cluster, with your own eyes, you owe it yourself to start figuring out how to make that happen. If you don't have a telescope, find a friend who does. Or buy one; Orion has some good 'uns for less than you think (my six-inch Dob*, which looks like a freakin' cannon, was under $300 nicely equipped). Or if you're not too far, invite yourself over and look through mine. The views don't suck.

* One of the ironies of amateur astronomy is that, thanks the success of even larger Dobs, a telescope four feet long, seven inches in diameter, and weighing 35 lbs is considered small.

Yes, it's true, you can Google up some Hubble images that will smoke anything you can see through a backyard telescope. Nothing wrong with that--I've got a big fat book on the shelf next to me with unreasonably gorgeous views of everything from the moon on out to the galaxy swarm in the Hubble Ultra Deep Field. But I promise you this: your first eyeball view of Saturn or Jupiter or whatever gem the scope is pointed at will sock you in the brainpan in a whole different way. Try it and see.