Day 0 = new Moon. ~ means about. Move the slider or choose a phase. The buttons show example days.
Side view of the orbit · sunlight from the left
Not to scale. Orbital tilt is not shown.
What we see from the Northern Hemisphere
New moon
0% lit
The sunlit half faces away from Earth.
Day 0: the cycle begins
Day labels are rounded to whole numbers, except 29.5 at the end. Timing and percentages are approximate.
Learn more about the science
Why does the shape look different?
The Sun lights about half the Moon. As the Moon orbits Earth, we see different amounts of its sunlit half. The Moon itself does not grow or shrink.
Earth's shadow causes a lunar eclipse, not the usual phases. The Moon's tilted orbit usually carries it above or below Earth's shadow.
What do the names tell us?
Waxing: the lit part we see is increasing.
Waning: the lit part we see is decreasing.
Crescent: less than half the face we see is lit.
Gibbous: more than half is lit, but it is not full.
Each crescent or gibbous phase lasts about a week.
Does a quarter phase last a whole day?
First quarter and third quarter each mark a particular moment in the cycle, when the Moon appears about half lit. It can look nearly half lit before and after that moment.
Quarter describes the Moon's progress through its phase cycle. At first quarter it is waxing; at third quarter it is waning.
How should I read the days?
One phase cycle averages about 29.5 days. Day 0 is new Moon, not the first day of a calendar month.
This simplified model places first quarter near Day 7, full Moon near Day 15, and third quarter near Day 22. These are approximate points, not whole-day phases. Real timing varies between cycles.
Science references: NASA and the U.S. Naval Observatory.
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