Why does the Moon change shape?
Understand the changing pattern of moonlight, the monthly cycle, and how to observe it.
STEP 1
Start with sunlight
The Moon makes no light of its own. Moonlight is sunlight reflected from its surface, and the Sun normally lights one half of the Moon at any moment.
STEP 2
Watch the angle change
As the Moon orbits Earth, we see different portions of that lit half. Near new Moon, the lit side faces mostly away from us. At a quarter Moon, about half of the visible face is lit. Near full Moon, almost all of it is.
STEP 3
Count the cycle
From one new Moon to the next takes about 29.5 days on average, so the cycle is not an exact 30-day count.
STEP 4
Sketch it yourself
On three evenings a few days apart, when the Moon is visible, sketch its shape and note the date and time. Compare how much of its face looks lit. This is a simple observation exercise you can try alongside reading a lunar calendar.
Earth's shadow darkens the Moon only during a lunar eclipse, a different alignment from the ordinary monthly phases.