futsanglung Post time: 2014-7-14 10:34
But surly the diameter of the moon is constant, how can it expand by 14%? Gas? Mouldy cheese?
I'll leave out all of the complicated orbital stuff and give you a really simplified version.
The Moon's orbit is an elliptical in shape, it's not a circle.
The time to do once complete circuit is a bit over a day longer than the period between full moons (or new moons etc), this means that the location on the orbital "path" varies from one month to the next.
The movement of the location of a full moon on the orbital path means that you can have a full moon at perigee, which is the point when the moon is at it's perigee-zyzgy and it's most distant point which is called apogee. This distances varies between 356,400 at perigee-zyzgy 356,400km and at apogee it's 406,700km.
This variation causes the apparent difference in diameter and brightness.