Chapter 22
Origin of Modern
Astronomy
Section 2
The Earth-Moon-Sun System
Notes 22-2
Vocabulary
• Rotation
• Revolution
• Precession
• Perihelion
• Aphelion
• Perigee
• Apogee
• Phases of the
moon
• Solar eclipse
• Lunar eclipse
Motions of Earth
• Rotation: the turning or spinning of a body on its
axis
o Main result is day and night
o Standard for telling time (24 hours in one
rotation)
o Sidereal day: time it takes for Earth to make
one complete rotation (360 degrees) with
respect to a star other than our Sun
• Measured by the time it takes a star to
reappear in the same position it was the day
before
• Occurs 4 minutes earlier each day
o This would cause noon(12pm) to be
midnight(12am) after 6 months of time
had gone by
Motions of Earth
• Revolution: motion of a body on its orbit around a
point in space
• Earth revolves around 107,000 km/hr
o Avg distance from the Sun is 150 million km
o Perihelion: closest to sun
• 147 million km
• January 3
o Aphelion: farthest from sun
• 152 million km
• July 4
• Also appears to move the constellations in the
night sky
o Ecliptic: the apparent path of the sun against
the celestial sphere
Motions of Earth
• Ecliptic: the imaginary plane that connects
the Earth’s orbital path with the celestial
sphere
o Placement of the Equator is the celestial equator
• Earth is tilted 23.5 degrees
This gives the earth its seasons
Spring Equinox (March 20 or 21) – Sun appears at Equator
Autumnal Equinox (Sept 22 or 23) – Sun appears at Equator
Summer Solstice (June 21 or 22) – Sun appears at Tropic of
Cancer
o Winter Solstice (Dec 21 -22) – Sun appears at Tropic of Capricorn
o
o
o
o
Motions of Earth
• Precession: wobble of Earth on its axis every
26,000 years
o Varies in tilt between 21.5 and 24.5 degrees
o Affects climate changes; can cause ice ages and warm ups
o North Star is currently Polaris
• In 13,000 years, precession will cause Vega to the North Star
• Earth-Sun motion
o The whole solar system is moving around the center of our
galaxy (Milky Way) at 250 km/s
o The galaxy is approaching our neighboring galaxy, Andromeda
Motions of Earth-Moon
System
• Moon orbits the Earth in a counterclockwise
direction when observed from the North Pole.
• The path of the moon is elliptical
o Perigee: the moon is closest to the Earth
o Apogee: the moon is farthest from the Earth
• Phases of Moon
o
o
o
o
Changes in the amount of the moon that appears lit
Caused by how much of the sun lit side is facing Earth
Waxing phases: causes more and more of the moon to appear
Waning phases: causes less and less of the moon to appear
• http://www.moonconnection.com/moon_phas
es_calendar.phtml
Motions of Earth-Moon
System
• Lunar Motions
o Synodic Month: (29.5 days) cycle of the moon through its phases
• Only an apparent time period of the moon around the Earth
o Sidereal Month: (27 1/3 days) true time period for the moon to
revolve around the Earth
o Moon’s rotation and revolution around Earth are the same
• Same side of the moon always faces the Earth
• This is where the dark side of the moon comes from
o Only satellites and astronauts have seen the other side of
the moon
o More cratered than the side facing the Earth
• Same side of the moon faces the sun for about two weeks
o Causes a very high temp on the sunny side of the moon –
127°C
o Dark side temp – (-173°C)
Motions of Earth-Moon
System
• Eclipses
o Solar Eclipse: when the moon moves in a line directly between
the Earth and the sun…this casts a dark shadow on the Earth
o Lunar Eclipse: when the Earth is between the sun and the
moon…this casts a shadow on the moon
o http://eclipse.gsfc.nasa.gov/eclipse.html - NASA’s Official Eclipse
Website
o http://www.space.com/15584-solar-eclipses.html - Space.com
Eclipse
• Why isn’t there a solar eclipse every new
moon and a lunar eclipse every full moon?
• Moon’s orbit is inclined about 5 degrees to the
plane of the Earth and the sun’s orbit
• Most of the time the shadow created during a new
moon and full moon miss the Earth.
• The moon’s orbit must cross the plane of the ecliptic
for an eclipse to take place
Eclipse
• Lunar Eclipse:
o Can see the shadow of the Earth moving across the moon surface
o At completion, the moon is visible as a copper colored disk
• This is because the Earth’s atmosphere bends and transmits longwavelength light (red) into the shadow
o Can last up to 4 hrs and is visible to anyone on the side of the Earth facing
the moon
• Solar Eclipse:
o Moon moves between the sun and Earth.
o Can see the sky darken while in the shadow of the moon
• Only the outer edge of the sun (corona and chromosphere) can
been seen
o Total eclipse happens when the viewer is in the umbra
• Partial eclipse happens when the viewer is in the penumbra
o Next Total eclipse in the US is August 21, 2017
• From Oregon to South Carolina
Lunar Eclipse
Solar Eclipse