OOC: Due to my hectic schedule at the moment, I decided to do this. I'm sorry if it's not what most professors do.
The Phases of the Moon
To explain why the moon appears to be illuminated differently everyday, we need to know that the moon shines by the reflected light of the sun. The moon is
always being half-illuminated by the sun, though what we see depends entirely on where the sun, the earth and the moon are located at a certain moment.
For example, when the moon is near the sun, the side that faces the sun is what we see
(a new moon). A few days after this, the moon stands at 90° away from the sun and we see the
first quarter. When the moon is opposite the sun, the sunlit side faces us
(a full moon). After the full moon, the moon stands again at 90° away from the sun and we see the
third quarter.
There are also phases in between the ones mentioned above. But fist, we must understand the following words:
crescent, gibbous, waxing, and waning.
Crescent: The phases where the moon is less than half illuminated.
Gibbous: The phases where the moon is more than half illuminated.
Waxing: Growing or expanding in illumination
Waning: Shrinking or decreasing in illumination.
After the new moon, the sunlit side is increasing, but less than half, so it is
waxing crescent. After the first quarter, the sunlit side is still increasing, but now it is more than half, so it is
waxing gibbous. After the full moon, the light continually decreases. So the
waning gibbous phase occurs next. Following the third quarter is the
waning crescent, which wanes until the light is completely gone, therefore completing the cycle as it becomes a new moon once again.
The moon takes around
29.5 days to go through the whole cycle of phases, so it must move 360 / 29.5 or about 12° through the sky from day to day. It rises and sets 24 / 29.5 hours or about 50 minutes earlier each day.
How to Tell a New Moon and a Full Moon Apart
A new moon must rise and set with the sun. A full moon must rise with the sunset and set with the sunrise.
What About First and Third Quarters?
A first quarter moon rises around midday and is high in the sky around sunset. A third quarter moon rises around midnight and is high in the sky around sunrise.
For Fun!
See what phase the moon was in on your birthday
here!