The moon is one of the coolest things in the sky. In fact, you don’t need a telescope to get a good look at it, and you don’t need to be in a particularly dark location, either. Really! Go outside right now and check out that moon!

What’s that? Maybe you can’t find the moon in the sky at this very moment? Well, the moon can change its appearance. Sometimes you won’t see it for days at a time, or you can only see a small sliver of it.

I’m a little embarrassed to admit that I didn’t really understand why this happened until I went to graduate school. Sure, I knew the earth goes around the sun, and the moon goes around the earth, and the moon reflects light from the sun, and everything rises in the east and sets in the west, and we only see part of the moon sometimes… because, you know, geometry and angles and stuff.

You probably know this too: some evenings you look up and see a crescent moon suspended in the sky. One morning, you might see a not-quite-full moon in the daytime sky. Another night, you could witness a glorious full moon rising. And sometimes the moon is nowhere to be seen.

But, for one reason or another, the beauty of how this cosmic dance works didn’t make it into the deepest parts of my brain for years. The best way to discover and understand the phases of the moon is to observe them – truly, intentionally observe the moon, every day or two for at least two weeks, by taking notes and drawing pictures. The best time to start is two or three days after the “new moon” phase.

Let’s speed these observations up a bit…

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What an interesting pattern. We’ve observed for just shy of two weeks, which is how long it takes for the moon to go from New (completely invisible) to Full (completely bright). But it’s not just the phase, or part of the moon that is illuminated, that gradually changes. The timing of when it rises and sets also does.

If we kept observing, we would see the process happen in reverse. Instead of appearing more illuminated each night, or “waxing,” the moon’s illumination would start getting smaller and dimmer, which is called “waning.” After another two weeks or so, we wouldn’t be able to see it at all.

The pattern of the moon’s waxing and waning is actually pretty easy to predict. If I ask you, “When could I see the First Quarter moon rise?” The answer is always the same – around noon – because the moon always spends half a day (12 hours) in the sky. I remember this because the Full Moon is directly opposite from the Sun in the sky, which means it rises at sunset and sets and sunrise. From there, you can work backwards or forward to any other phase.

If you’re still confused about the phases of the moon, don’t despair. What really made it “click” for me, even after lots of observations and even teaching it to others, was this online tool. Play around with it. See if you can predict what will happen. Challenge yourself to predict which moon phase will always appear highest overhead at, say, 3am.

Even without knowing how the moon’s phases work, it is one of the easiest astronomical objects to share and enjoy. You can see the bright Full Moon at nighttime every 27 days from pretty much anywhere on the planet, so long as it’s not too cloudy.

A “Full Moon Night” at White Sands National Monument, October, 2013. The dunes are lit by the light of the full moon (with some help from headlights!). You can clearly see the planet Venus in the western sky and the constellation Sagittarius to the left, which looks a bit like a sideways teapot.

Slideshow image credits: Day 1, Day 3, Day 5, Day 7, Day 9, Day 11.


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