Archive for the ‘Astronomy’ Category

The moon is tumbling down

Wednesday, November 30th, 2011

Our balcony faces due south, and since we don’t smoke in the flat, I’ve been watching the moon quite often over the last weeks, even months. And I noticed something that appeared rather strange at first. Each evening/night the moon wanders from the left (east) side of the sky to the right (west) side — which is to be expected, if you remember that this apparent motion is the same as the motion of the sun (both are due to the earth’s rotation). But when the moon is only a half moon, it seems to “tumble” while it moves from left to right. Here’s what it looks like, roughly, at three times during one evening:

The moon is tumbling down

At first I wondered why this should happen, till I connected the three moon shapes in this picture with a circular arc and realised that the moon isn’t tumbling at all; it’s just following a circular (or near-circular) path along the sky and keeping the angle of its “crescent” perpendicular to this path all the time.

Problem solved, but then I ran straight into the next one, which I couldn’t solve just with pencil and paper (and a graphics program) — what would the motion in the picture above look like if I’d observed it from the southern hemisphere, say from Australia?

Since I have some online friends in Oz and NZ, I wondered whether I should ask them whether it’s the same there. But then I worked it out on my own. Can you?