By Lauren Vork | Updated Mar 24, 2022

Kevork Djansezian/Getty Images Sport/Getty Images

While the Moon’s changing appearance has inspired myths for centuries, the science behind its phases is straightforward. A lunar phase refers to the portion of the Moon’s surface that is illuminated by the Sun and visible from Earth during its ~29.5‑day orbit.

What Is a Lunar Phase?

A lunar phase is the visible, illuminated fraction of the Moon that we see from Earth. Because the Moon is always half‑illuminated, the changing view results from the relative positions of the Sun, Moon, and Earth.

Myth vs. Fact

Many people think the Moon’s phases are caused by the Earth casting its shadow on the Moon. That happens only during a lunar eclipse, which is a rare event. In reality, the phases are produced by the Moon’s orbit and the angle at which sunlight falls on it.

The Key Phases

• New Moon – The Moon is positioned between Earth and Sun. The illuminated side faces away from us, making the Moon nearly invisible.
• Waxing Crescent – After a new moon, a sliver of light begins to appear. The illuminated portion grows each night.
• First Quarter – Half of the Moon’s face is illuminated. The Moon is roughly one quarter of the way through its orbit.
• Waxing Gibbous – More than half the Moon is lit, but it is not yet full.
• Full Moon – The Sun, Earth, and Moon are in a straight line with Earth between the Sun and Moon. The entire illuminated half faces us.
• Waning Gibbous – The illuminated portion starts to shrink.
• Last Quarter – Again, half of the Moon is lit, but it is now moving toward the new moon.
• Waning Crescent – Only a thin crescent remains visible before the cycle repeats.

Why the Moon Looks Different

The apparent change in shape is purely geometric. The Sun always illuminates the same side of the Moon, but our line of sight from Earth changes as the Moon orbits. This consistent cycle—lasting about 29.5 days—provides the rhythm for calendars, folklore, and navigation.

For more detailed explanations, NASA’s website offers interactive diagrams and animations.