What Causes the Moon’s Phases

The relative positions of the Earth, Sun, and Moon as the Moon orbits Earth cause the phases of the Moon.

Every month as the Moon cycles through its phases it appears to change shape. The Moon always has the same spherical shape, but stargazers on Earth see different fractions of the dark and illuminated portions of the Moon as the Moon orbits Earth. The common urban myth that Earth’s shadow falling on the Moon causes lunar phases is incorrect.

Phases of the Moon

Every month as the Moon cycles through its phases it appears to change its shape. The Moon of course always has the same spherical shape. There is a common urban myth that Earth’s shadow falling on the Moon causes lunar phases. As logical as it seems, this myth is incorrect.

A few simple observations of the Moon’s position in the sky relative to the Sun demonstrate that Earth’s shadow can not cause the lunar phases.

The Moon starts its cycle of phases with the new Moon. As the Moon orbits Earth, a few days after the new Moon a waxing crescent Moon is visible in the western sky just after sunset. Each night the crescent Moon is a little larger and higher in the sky.

About a week after the new Moon, the Moon appears as a half-circle. This is the first quarter, Moon. Many people call this phase the half Moon, but half Moon is not the correct astronomical terminology.

As the Moon continues to wax, it appears as more than a half but less than a full circle. This phase is the waxing gibbous Moon.

About two weeks after the new Moon, stargazers on Earth see the full Moon. The full Moon appears as a full circle in the sky. then the waning lunar phases begin.

Full Moon

Observe the full Moon’s location in the sky. The full moon rises in the east as the Sun sets in the west. It also sets in the west as the Sun rises in the east. As seen from Earth, the full Moon is directly opposite from the Sun in the sky during the full Moon.

When Earth’s shadow falls on the Moon, however, Earth is between the Sun and Moon so that the Moon is directly opposite the Sun in the sky, as seen from Earth. If the Earth’s shadow falling on the Moon caused the lunar phases, then when the Moon was directly opposite the Sun in the sky, it would be a dark new moon or very thin crescent rather than a bright full Moon.

Observations of the Full Moon’s position in the sky are not consistent with the idea that Earth’s shadow causes phases of the Moon.

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As the Moon wanes, it goes through the waning gibbous phase. Then earthbound stargazers see the third quarter Moon about a week after the full Moon. As the Moon continues to wane, it goes through the waning crescent phase. The waning crescent gradually gets smaller and closer to the Sun in the predawn sky. About 29.5 days after the previous new Moon, the Moon is again new. The lunar cycle repeats.

Crescent Moon

Observing a crescent Moon also shows that Earth’s shadow can not cause lunar phases. If Earth’s shadow causes lunar phases, then the crescent Moon should be in Earth’s shadow because most of the Moon is dark during the crescent phase. Can the crescent Moon be in Earth’s shadow?

The waxing crescent moon, just after the new Moon, is visible low in the western sky just after sunset. The waning crescent Moon, just before the new Moon, is visible low in the eastern sky just before sunrise. As seen from Earth, the crescent Moon is always fairly close to the Sun’s position in the sky. With this configuration, it is impossible for Earth’s shadow to fall on the Moon.

If Earth’s shadow caused the lunar phases, the crescent Moon would have to be mostly in Earth’s shadow. Hence the crescent Moon would have to be nearly opposite the Sun in the sky. Observing the crescent Moon’s position in the sky relative to the Sun shows that the crescent Moon can not be caused by Earth’s shadow falling on the Moon.

Analyzing simple observations of the Moon’s position in the sky during lunar phases shows that Earth’s shadow can not cause phases of the Moon.

The Moon’s phases instead result from the relative positions of the Earth, Sun, and Moon and from the fractions of the night and day side of the Moon visible from Earth.

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Cause of Moon’s Phases

At any given time, half of the Moon is illuminated by the Sun and the other half is dark. The lunar phase seen from Earth depends on what fraction of the illuminated and dark portion of the Moon is visible from Earth.

During the new Moon phase, the Moon is between the Sun and Earth. The dark portion of the Moon faces Earth, so the Moon is not visible during the new Moon. Usually, the Moon is either north or south of the Sun, but occasionally the Moon lines up directly between the Earth and Sun. Then Earth has a solar eclipse, which can only occur during the new Moon.

As the Moon continues its monthly journey around Earth, a small portion of the illuminated side of the Moon begins to face Earth. Earthbound stargazers see a waxing crescent Moon low in the west just after dark.

The Moon’s orbit continues. When the Moon is one-quarter of the way around its orbit, it is 90 degrees from the Sun as seen from Earth. Half of the side of the Moon facing Earth is illuminated by the Sun. The other half is dark. Observers on Earth see the first quarter Moon. A few days later, observers see a waxing gibbous Moon because more than half of the side of the Moon facing Earth is illuminated by the Sun.

When the Moon is directly opposite the Sun in the sky, observers on Earth see only the illuminated portion of the Moon. The Moon is full. If the Moon lines up so that Earth is directly between the Sun and Moon, then Earth’s shadow can fall on the Moon causing a lunar eclipse.

As the Moon continues orbiting Earth, it goes through the waning gibbous, third quarter, and waning crescent phases. The waning phases are similar to the corresponding waxing phases, but because they are on the opposite side of the Earth, observers see the opposite side of the Moon illuminated.

As the Moon orbits Earth every month, it cycles through the lunar phases because different fractions of the dark and illuminated portions of the Moon face Earth.

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