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.

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

• With respect to the stars, the Moon completes one orbit around Earth in a sidereal month, averaging 27.32 days.
• The Moon completes one cycle of phases (one orbit around Earth with respect to the Sun) in a synodic month, averaging 29.53 days.

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.

• The line of nodes is the line where the planes of the Moon’s orbit and Earth’s orbit intersect. The gravitational pull of the Sun gradually shifts the orientation of the line of nodes with respect to the stars.

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.

• Lunar eclipses occur at full moon, while solar eclipses occur at new moon.
• Either type of eclipse can occur only when the Sun and Moon are both on or very near the line of nodes. If this condition is not met, Earth’s shadow cannot fall on the Moon and the Moon’s shadow cannot fall on Earth.

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.

• During a total solar eclipse, the Moon’s umbra traces out an eclipse path over Earth’s surface as Earth rotates. Observers outside the eclipse path but within the penumbra see only a partial solar eclipse.
• During an annular eclipse, the umbra falls short of Earth, and the outer edge of the Sun’s disk is visible around the Moon at mideclipse.

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.