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.