Over the next two weeks, we see progressily less of the illuminated surface as the Moon traverses its orbit. In fact following the full moon the phases are reversed. The moon passes through a phase called waning gibbous, last quarter moon (somtimes called third quarter because the Moon is 3/4 of the way around its orbit), and waning crescent moon, and lastly new moon.

Determining when the Moon rises and sets is often tricky for students. We have discussed it some, but lets try again. Lets try to determine the time the third quarter moon rises. The situation is not much different than the situation we encountered for the first quarter moon. The angle between the Earth, Sun and Moon is once again 90o. The difference arises in that the Moon is approaching the region of the sky where the Sun is, therefore the third quarter Moon rises at Midnight and sets again at noon.

Then after 29.5 days after the previous new moon the Moon begins its sequence again. But wait! Didn't we say earlier that the Moon took 27.3 days to orbit the Earth, but now we say 29.5 days! This brings up the a distinction between two different types of months that Astronomers go to great lengths to distinguish. The unique thing is that neither of two ways of determining a month corresponds exactly to the months that appear on our calenders.