Astronomy 171
Solar System Astronomy
Prof. Paul Martini

Lecture 7: Phases of the Moon

Key Ideas

The Moon always keeps the same face toward the Earth: Rotation and Revolution are synchronous; Libration
Phases of the Moon:: Fraction of the sunlit side visible to us.; Sidereal and Synodic Periods:
Sidereal Period: 27.3 days
Synodic Period: 29.5 days

Our Nearest Celestial Neighbor

The Moon is a natural satellite of the Earth
The Moon's orbit around the Earth is elliptical: Deviates by about 0.15 percent from circular
The Moon's orbit is tilted by 5 degrees from the Ecliptic

Synchronous Rotation

The Moon rotates at the same rate it revolves: Completes 1 rotation for every 1 orbit around the Earth
Always keeps the same face toward us.: Near side: hemisphere facing towards us; Far side: hemisphere facing away from us
Caused by tides between the Earth and Moon
The Moon and Earth are Tidally Locked
Very small wobbles in the side facing us are called Libration

Phases of the Moon

The Moon produces no visible light of its own: It shines only by reflected sunlight; Surface is very dark, only 7 percent reflective
Each month we see a cycle of Phases: The sunward hemisphere is fully lit; The opposite hemisphere is dark; The Phase of the Moon is the fraction of the sunlit hemisphere visible to us.

New and Full Moon

New Moon: Moon and Sun on the same side of the sky; Near side is in total darkness; Moon and Sun rise together
Full Moon: Moon is opposite the Sun in the sky; The near side is fully illuminated; Moon rises as the Sun set

Quarter Moon

Earth, Moon, and Sun are at right angles: Half of the near side is illuminated; Half of the far side is illuminated
First Quarter:: Between New Moon and Full Moon
Last Quarter:: Between Full Moon and New Moon

Waxing and Waning

Waxing: increasing illumination: Waxing Crescent: just after New Moon; Waxing Gibbous: just before Full Moon
Waning: decreasing illumination: Waning Gibbous: just after Full Moon; Waning Crescent: just before New Moon

Moonrise and Moonset

You don't see all Moon phases at all times: Never see a crescent Moon at midnight; Never see a Last Quarter Moon at sunset; Never see a Full Moon during the day
Times of rising and setting depend on the Earth-Sun-Moon configuration as viewed from the surface of the rotating Earth
Moon rises about 53 minutes later every day

Sidereal Period

The time for the Moon to complete one orbit around the Earth with respect to the stars.: Moon's Sidereal period = 27.3 days; Also called the Sidereal Month
Measure by watching its motions against the background stars:
Tonight: Waning Crescent in Virgo (rises about 2am)
Feb 8 (27.3 days later): Waning Crescent leaving Virgo

Synodic Period

The time between successive New Moons: Moon's Synodic Period = 29.5 days; Also called the Synodic Month; In 27.3 days the Earth has travelled (27.3/365)x360 degrees = 26.9 degrees; The Moon takes (26.9/360)*27.3 days = 2 days to make up for its orbit around the Earth
Moon rises later every day: The Moon takes 27.3 days to travel 360 degrees; 360 degrees/28 days = 13.2 degrees per day; The Earth takes (13.2/360) x 1440 min/day = 52.8 min to rotate 13.2 degrees, therefore the Moon rises about 53 min later every day
The Synodic Period is used by lunar calendars

See A Note about Graphics to learn why some of the graphics shown in the lectures are not reproduced with these notes.