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.