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SYFY WIRE Bad Astronomy

Silhouettes and Moonrise in Real Time

By Phil Plait

This is simply magnificent: a video of the Moon rising, silhouetting people in the foreground. Mind you, this is filmed in real time. Make sure itâs in HD, make it full screen, sit back, and enjoy the dance of orbit and light.

The video was taken on Jan. 28, 2013 by photographer Mark Gee, who made the attempt several times before finally capturing the event. This isnât an example of the famous Moon Illusion, where the Moon looks huge on the horizon, though. He wanted to get the silhouetted people walking around, so he camped over two kilometers away from the Mount Victoria lookout in Wellington, New Zealand. Using a telephoto and exquisite pre-planning (not to mention aim), he was able to show the people who, at that distance, were barely able to be seen by the naked eye. Itâs the magnification of the lens that makes the moon look so big.

If the motion of the people doesnât convince you this is not a time-lapse video, the Moon itself should. It circles the sky roughly once per day, moving the 360° around the sky in just under 25 hours (the stars appear to go around once every 24 hours, but the Moonâs orbital motion takes it a few degrees east every day, adding to the amount of time it takes to apparently go around the sky). So it moves 360°/25 hours = 14° per hour. The Moon itself is half a degree across, so it should move through its own diameter very roughly 30 times per hour, or once every two minutes.

In other words, it should take about two minutes to completely rise once its upper limb starts to peek over the horizon. A big factor is also the latitude of the observer; that changes the angle the Moon rises, which can add significantly to the time it takes to clear the horizon. Go ahead and check the video to see how long it takes the Moon to rise, and note the angle it moves.

In fact, I have to point out that in the northern hemisphere, the Moon rises left to right. Here itâs right to left, showing you the video was taken in the southern hemisphere. And Iâll mention a trick used in movies: When they show the Sun or Moon rising, a lot of the time they film those at sunset/moonset and run the video backwards. Itâs easy to point your camera to where an object is setting, because you can see where itâs heading beforehand. But aiming it at the right spot before it even appears is a little tougher. Gee nailed it, so good on him. And also good on him for making such a beautiful, haunting video for all of us to enjoyâ¦and maybe also get a little insight into the way the sky works.

Tip oâ the lens cap to APOD.

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