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SYFY WIRE Bad Astronomy

It's Just a Phase

By Phil Plait

One of the first things I do whenever I step outsideâand Iâm serious, I do this every single timeâis look up. Itâs a good habit, because I always see wonderful things in the sky.

One of my favorites is the changing face of the Moon. As it swings around the Earth in its monthly orbit, the geometry between us, the Moon, and the Sun changes. The end result is the Moonscape is lit differently by the Sun every day, and we see that as the phases of the Moon. When itâs between us and the Sun, we gaze upon its dark surface, and we say itâs new. A week later itâs lit from the side, and thatâs first quarter. A week after that the Sun is behind us as we look at the Moon, and itâs full.

These phases are entirely predictable, along with other motions of our natural satellite. So predictable, in fact, that NASA put together this incredibly compelling video showing the Moonâs motion over 2013, where each frame of the video is one hour of time! Itâs mesmerizing.

Thereâs a lot to see here! First, the animation uses data from the Clementine lunar mission, a joint NASA and military probe in the 1990s to map the Moon, as well as images and topography measurements from the Lunar Reconnaissance Orbiter. The surface-elevation measurements were used to create the realistic shadows in the animation.

You probably noticed that the Moon appears to rock around, too, tipping and tilting over the course of every month. Thatâs called libration,and is due to the Moonâs orbit being elliptical, as well as the Moon's rotational axis being slightly tilted with respect to its orbit.* We see it from a slightly different angle every day, and thatâs what causes that apparent motion.

The video has lots of bonuses, too, like the scale on the right showing how far the Moon is from the Earth in units of the Earthâs diameter (about 13,000 kilometers, or 8,000 miles). You can watch it oscillate back and forth as it orbits, moving from 28 to 32 times the Earthâs diameter away.

In the lower left it shows the sub-solar pointâthe lunar surface directly under the Sunâwhich means if you were standing there (and not suffocating and/or roasting) the Sun would be exactly overhead. That graphic also shows the equator and prime meridian of the Moon, and you can watch it rock back and forth just like the big animation does.

At the bottom right is just a ton of info that changes with time, including the Moonâs distance, its apparent size (measured in arcseconds, where one degree is 3,600 arcseconds; the full Moon is roughly 1,800 arcseconds, or a half degree across), what percentage of it is illuminated from our viewpoint, and so on.

I added the music. I like Kevin MacLeodâs work, which he keeps on his website Incompetech for use in videos like this. He takes donations, Iâll add.

The NASA website for the animation has a lot more information and details about the video. And bonus: Starting in 2013, the image of the Moon at the top will automatically change every hour to represent the actual view of the real Moon. Thatâll be fun to check! And useful, too. Itâs always nice to know if a thin crescent Moon awaits if I happen to step outside. Thatâs one of my favorite sights of all.

*Correction, Nov. 25: This article originally said that libration is partially due to the tilt of the Moon's orbit with respect to the Earth's equator, but it is in fact partially due to the Moon's rotational axis being tipped with respect to its orbit.

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