Key Ideas

Ideas preceded by an asterisk (*) are discussed in the Boxes.

Lunar Phases: The phases of the Moon occur because light from the Moon is actually reflected sunlight. As the relative positions of Earth, the Moon, and the Sun change, we see more or less of the illuminated half of the Moon.

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Length of the Month: Two types of months are used in describing the motion of the Moon.

The Moon’s Orbit: The plane of the Moon’s orbit is tilted by about 5° from the plane of Earth’s orbit, or ecliptic.

Conditions for Eclipses: During a lunar eclipse, the Moon passes through Earth’s shadow. During a solar eclipse, Earth passes through the Moon’s shadow.

Umbra and Penumbra: The shadow of an object has two parts: the umbra, within which the light source is completely blocked, and the penumbra, where the light source is only partially blocked.

Lunar Eclipses: Depending on the relative positions of the Sun, Moon, and Earth, lunar eclipses may be total (the Moon passes completely into Earth’s umbra), partial (only part of the Moon passes into Earth’s umbra), or penumbral (the Moon passes only into Earth’s penumbra).

Solar Eclipses: Solar eclipses may be total, partial, or annular.

The Moon and Ancient Astronomers: Ancient astronomers such as Aristarchus and Eratosthenes made great progress in determining the sizes and relative distances of Earth, the Moon, and the Sun.