22.2 The Earth-MoonSun System
Motions of Earth
 Rotation – turning or spinning of a body on its
axis
 Day & Night
 24 hr. time frame
 Solar Day or Sidereal Day
 Revolution – motion of a body along a path
around some point in space
 Perihelion – closest to sun (Jan. 3)
 Aphelion – Farthest from sun (July 4)
Earth’s Axis & Seasons
 Axis tilted about 23.5 degrees
 Tilted axis = yearly cycle of seasons
 Spring Equinox (March 20-21)
 Autumn Equinox (September 22-23)
 Summer Solstice (June 21-22)
 Winter Solstice (December 21-22)
Precession
 Slow
 Axis traces a circle on the sky
 Similar to the wobble of a spinning top
 Minor effect on seasons b/c the angle of tilt
changes only slightly
Earth-Sun Motion
 Sun revolves around the galaxy
 Trip takes 230 million years to traverse at
speeds approaching 250 km per second
 Galaxies themselves are also in motion
Motions of the EarthMoon System
 Moon orbits Earth in about a month’s time
 Perigee – moon is closest to Earth
 Apogee – Moon is farthest from Earth
 Constantly change the relative positions of
the sun, Earth, and Moon
Phases of the Moon
 Monthly
 Crescent Phase
 Waxing
 First-quarter phase
 Full Moon phase
 Waning
 New-moon phase
 Lunar phases are a result of the motion of the moon &
the sunlight that is reflected from its surface
 pg. 626 Fig. 15 A & B
Lunar Motions
 Cycle requires 29 ½ days = moon cycle =
synodic month
 Basis for the Roman calendar
 Moon’s revolution around Earth & not the
true period which only takes 27 1/3 days =
sidereal month
 Moon’s period of rotation on its axis & its
revolution around Earth are the same
Eclipses
 Shadow effects
 When the moon moves in a line directly btw
Earth & Sun it casts a dark shadow on Earth =
Solar Eclipse
 Moon is eclipsed when it moves within Earth’s
shadow = Lunar Eclipse
 During a new-moon or full-moon phase, the
moon’s orbit must cross the plane of the ecliptic
for an eclipse to take place
 Usually 4 eclipses (in sets)
REVIEW REVIEW REVIEW
 Why don’t eclipses occur during every full-
moon or new-moon phase?
 In what ways does Earth move?
 What causes the phases of the moon?
 Describe the locations of the sun, moon, &
Earth during a solar eclipse & during a lunar
eclipse.
22.3 Earth’s Moon
The Lunar Surface
 Moon has no atmosphere or water
 Erosion occurs b/c the lack of a protective
atmosphere
 Most obvious structures are craters (round
depressions in the surface)
 Produced mainly by the impact of rapidly
moving debris
 The heat generated by the impact is enough
to melt rock
Continued…
 Most of lunar surface is made up of densely
pitted, light-colored areas = Highlands
 Within the highland regions are mountain
ranges
 Dark relatively smooth area on the moon’s
surface = Mare
 Maria, ancient beds of basaltic lava, originated
when asteroids punctured the lunar surface,
letting magma bleed out.
 Long channels that are associated with maria =
Rilles
Continued…
 All lunar terrains are mantled with a layer of
gray debris derived from a few billion years
of bombardment from meteorites = lunar
regolith
 Composed of igneous rocks, glass beads, &
fine lunar dust
Lunar History
 Moon is our nearest planetary neighbor
 Most widely accepted model for the origin of
the moon is that when the solar system was
forming, a body the size of Mars impacted
Earth
 Impact would have liquefied Earth’s surface
& ejected huge amounts of crustal & mantle
rock from an infant Earth
 A portion of this ejected debris would have
entered an orbit around Earth where it
combined to form the moon
REVIEW REVIEW REVIEW
 How do craters form?
 How did maria originate?
 What are the stages that formed the moon?