The moon – proper motion and apparent motion.

If you have been following the moon this week you should have noticed some interesting things, firstly it has “grown” from a very small crescent to a now “half moon”. Hopefully you will have also noticed that at the same time each night the moon was “higher” and further to the west (to your left when looking towards the moon), you may also have spotted that the moon was not in the same position with regards to the background stars – earlier this week the moon was not too far from a very bright star low on the horizon (the bright star was, and still is, in fact Jupiter) and now it’s a long way from Jupiter and much higher in the sky.

So, what’s been happening? As you know, the moon orbits around the earth (i.e. it has “proper” motion), corresponding to a movement relative to the background stars by an amount roughly equal to its own diameter each hour; this movement is from west to east i.e. the moon starts (as a new moon) low in the western sky and moves slowly “upwards” and eastwards. At the same time, due to the fact that the earth is turning on it’s own axis from west to east, the stars and the moon (and of course the sun) appear to move from east to west. It’s a bit like someone “walking up the down escalator” assuming the person is walking upwards slower than the escalator is going down, he or she will eventually arrive at the bottom of the escalator.

If you would like to know more about the motion of the moon, take a look here: http://en.wikipedia.org/wiki/Orbit_of_the_Moon


If you have a pair of binoculars you can take a look at some of the moons “seas”, you should be able to see (1) Sea of Crisis, (2) Sea of Fertility, (3) Sea of Nectar, (4) Sea of Tranquillity, (5) Sea of Serenity and (6) Sea of Cold.

Photo credit: The Universe Today