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← Understanding Lunar Cycles

Lunar Cycles • A visual guide

Lunar Phases

Why the Moon’s appearance changes

A phase describes how much of the Moon’s sunlit side we can see from Earth. Follow the changing view through a lunar cycle.

Four principal lunar phases as seen from EarthNew moon is dark, first quarter has its right half illuminated, full moon is illuminated, and last quarter has its left half illuminated. This is one illustrative orientation.Views from Earth • in cycle orderNewFirst quarterFullLast quarterWaxing toward full → waning toward new

Scroll the diagram sideways to explore its labels.

Four principal phases, shown as idealized views from Earth. Orientation is illustrative; the Moon’s orientation in your sky varies. These are examples, not the current phase. Original LuNAC illustration; conceptual basis: NASA Science — Moon Phases.

Sunlight and our changing viewpoint

The Moon reflects sunlight. As it travels around Earth, we see different amounts of its illuminated hemisphere. Ordinary phases come from this changing viewpoint; Earth’s shadow is involved in a lunar eclipse.

Source: NASA Science — Moon Phases

Reading the cycle

At new moon, the sunlit side faces mostly away from Earth. First quarter shows half of the visible disk illuminated. At full moon, the sunlit side faces us. Last quarter shows the other half. Between these markers are crescent and gibbous phases. Waxing means the illuminated portion we see is growing; waning means it is shrinking.

Source: NASA Science — Moon Phases

What to notice

A quarter moon is named for its place in the cycle, even though its visible disk looks half lit. The phase sequence repeats roughly every 29.5 days. That is a cycle average, not a schedule for calculating the next phase.

Source: NASA Science — Moon Phases