Why the Moon changes appearance
The Sun illuminates half of the Moon, and our changing viewing geometry determines how much of that sunlit half is visible from Earth. Ordinary lunar phases are not caused by Earth’s shadow; that shadow is involved in a lunar eclipse.
The growing half of the cycle
New Moon begins the cycle near the Sun’s direction in the sky. Waxing crescent follows, then first quarter, then waxing gibbous. “Waxing” means the visible illuminated fraction is increasing. At first quarter, about half the disk appears lit even though one quarter of the phase cycle has passed.
The waning half of the cycle
Full Moon occurs near opposition to the Sun. The sequence continues through waning gibbous, last quarter, and waning crescent. “Waning” means illumination is decreasing. The complete phase cycle averages about 29.5 days.
An instant versus a label
New Moon, first quarter, full Moon, and last quarter each have a calculated instant. An eight-phase label describes a broader interval around the cycle. A calendar can therefore show “full Moon” as a phase label even when the exact full-Moon instant is hours away.
A phase is not a sign
The Moon’s sign records its zodiacal longitude. Its phase records its angular relationship with the Sun. The two measurements answer different questions. Two Moons in the same sign can have very different phases.
Using the cycle reflectively
Some people use new Moon for setting intentions and full Moon for reviewing them. That is a chosen reflective practice, not a demonstrated effect on personal outcomes. Let the cycle provide a reminder to pause without treating its symbolism as a demand.
Explore the sources
This guide describes symbolic conventions and traditions. Astrological interpretations are not scientifically established predictions about personality or events. Our sources and editorial approach.