What lunar phases are and their cause

Lunar phases are the different shapes of the lit part of the Moon that we see from Earth. These changes happen because of the way sunlight hits the Moon as it moves in its path.

The phases result from the changing positions of the Earth, Sun, and Moon. As the Moon travels around Earth, the angle between these three changes. This affects how much of the Moon's sunlit side we can see. Only the half of the Moon facing the Sun is lit, and we see different amounts of that lit half depending on where the Moon is in its orbit.

The Moon's orbit around Earth and the phase cycle

The Moon orbits Earth in a regular path, completing one full trip about every 29.5 days. This time period is called the lunar month. During this orbit, the Moon's position relative to the Sun and Earth creates a repeating cycle of phases.

This cycle starts with the new moon and goes through growing and shrinking stages before returning to new again. The entire sequence takes about 29.5 days, matching the Moon's orbital period.

The sequence of waxing phases

As the Moon moves in its orbit after the new moon, more of its lit side becomes visible from Earth. This growing stage is called waxing.

The sequence of waxing phases:

1. New moon - No lit part is visible because the Moon is between Earth and the Sun, with its lit side facing away from us.
2. Waxing crescent - A small, curved sliver of light appears as the Moon moves slightly, showing a thin crescent shape.
3. First quarter - Half of the Moon's visible side is lit, looking like a half-circle, as the Moon has completed about one-quarter of its orbit.
4. Waxing gibbous - More than half is lit, creating a bulging shape that grows toward full.

This waxing sequence leads up to the full moon.

The sequence of waning phases

After the full moon, less of the lit side becomes visible as the Moon continues its orbit. This shrinking stage is called waning.

The sequence of waning phases:

1. Full moon - The entire visible side is lit because the Moon is on the opposite side of Earth from the Sun, with its lit side fully facing us.
2. Waning gibbous - The lit area starts to shrink, showing a bulging shape that gets smaller.
3. Last quarter - Half of the visible side is lit again, but now it's the other half compared to the first quarter.
4. Waning crescent - Only a thin sliver remains visible, curving the opposite way from the waxing crescent.

This waning sequence leads back to the new moon, completing the cycle.

Common misconception about Earth's shadow

Many people think lunar phases happen because Earth's shadow falls on the Moon, blocking the light. This is not true. Phases come from the changing view of the Moon's sunlit side as it orbits Earth.

Earth's shadow only affects the Moon during a lunar eclipse, which is a rare event. During an eclipse, the Moon passes into Earth's shadow and can look darkened or reddish. This is different from the regular monthly phases, which do not involve Earth's shadow at all.