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Chapter 22.2 The Earth-
Moon-Sun System
Chapter 22.3: Earth’s Moon
Chapter 22.2 The Earth-Moon-Sun System
Motions of the Earth
� The two main motions of the Earth
are rotation and revolution
� Rotation is the turning or spinning
of a body on its axis
� Revolution is the motion of a body,
such as a planet or moon, along a
path around some point in space.
Motions of the Earth� Precession—the slight movement of the
Earth’s axis over a period of 26,000 years.
Rotation� Produces day and night
� The “mean solar day” is the time from one noon until the next ~ 24 hours
� Noon occurs when the sun reaches the zenith—highest point in the sky
� Not the same as the “sidereal day”
Revolution
� The Earth revolves at a speed of 107,000 kilometers per hour (66,000mph)
� Perihelion—when Earth is closest to the sun
� Aphelion—when Earth is farthest from the sun
� Average distance ~ 93 million miles
Revolution
� The apparent annual path of the sun against the backdrop of the celestial sphere is called the ecliptic
Earth’s Axis and Seasons� The Earth is tilted 23.5 degrees—this is why
we have the seasons
� When is Summer?� June 21 or 22
� When is Winter?� December 21 or 22
� When is Spring?� March 20 or 21
� When is Fall?� September 22 or 23
Precession� The direction in which the axis of the
Earth points changes direction very slowly.
� It traces out a circle
� The period of precession is 26,000 years.
Earth-Sun Motion� The entire solar system is moving at 20
kilometers per second toward Vega
� The sun and solar system is revolving around the center of the galaxy at 250 kilometers per second
� The entire galaxy is also moving toward the Great Galaxy in Andromeda
Motions of the Earth/Moon� The moon rotates counter-clockwise
when viewed from above the North Pole
� The average distance from the earth to the moon is 384,401 Km (238,867miles)
� The moon is closest to the earth at a point called perigee
� The moon is farthest from the earth at a point called apogee
Phases of the Moon� Lunar phases are a result of the
motions of the moon and the sunlight that is reflected from its surface.
� Half of the moon is illuminated at all times.
Lunar Motions� The cycle of the moon (apparent
motion) through its phases requires 29.5 days, a time span called the synodic month
� The actual, true period of revolution for the moon around the earth is 27.3 days, a time span called the sidereal month.
Temperature on the Moon� The lit side has a temperature of
approximately 127 degrees Celsius (260 degrees Fahrenheit)
� The dark side has a temperature of approximately –173 degrees Celsius(-279 degrees Fahrenheit)
Eclipses� Eclipses are shadow effects
� Usually four each year, occasionally ,up to seven
� Solar Eclipse—the moon moves in a direct line between the Earth and the sun
� Lunar Eclipse—the earth moves in a direct line between the Sun and the Moon
Lunar Eclipse� Occurs during the Full Moon phase
� Moon looks like a copper color
� Lasts up to four hours
� Can be seen by everyone on the side of Earth facing the moon
Solar Eclipse� Occurs during the New Moon phase
� The shadow casts on the Earth is about the size of South Carolina
� Can last up to seven minutes
� If in the umbra, you see totality
� If in the penumbra, you see a partial solar eclipse
Annular Solar Eclipse� Occur when a region on the Earth's
surface is in line with the umbra, but the distances are such that the tip of the umbra does not reach the Earth's surface.
22.3 Earth’s Moon
� The moon is about ¼ the size of
the Earth
� The moon’s gravity is 1/6th that of
the Earth
The Lunar Surface
� Dark regions are called Maria
� Most obvious feature are the
craters
� Splash marks that radiate outwards
from craters are called rays
� The light colored regions on the
moon are highlands
The Lunar Surface
� Large channels associated with
maria are called rilles, which are
probably the remnants of ancient
lava flows
� The moon has a soil-like layer
called lunar regolith
Lunar History
� The 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.
� The debris from the impact formed
the moon.