Motions of Earth, Moon, and Sun

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Unit 4

• Motions of Earth, Moon, and Sun

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Apparent Motions of Celestial Objects

• Apparent motion is the motion an object appears
  to make. Can be real or an illusion.
• Stars appear to move across the sky from east to
  west, however this is a result of the Earths
  rotation.

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Daily Motion and Stars

• Most celestial objects appear to move across the
  sky from east to west.
• Vast majority are stars.
• The paths of these celestial objects are
  circular, the polar constellations, or an arc.
• All motion is at a constant rate of 15 degrees
  per hour for a total of 360 degrees in 24 hours.

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Apparent Motions of the Planets

• Planets daily motion is similar to that of
  stars, however over a period of weeks or months
  the planets appear to change position with
  respect to the stars.
• This motion is not uniform. The planet will
  undergo the same motion from year to year but in
  different locations in the sky.
• The complicated motion is explained by the fact
  that all the planets rotate and revolve around
  the sun at different rates.

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Apparent Motions of Earths Moon

• Follows daily east to west motion
• Rises 50 minutes later each day
• Shifts eastward each day compared to the
  background of the stars.

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Apparent motions of the Sun

• The sun also seems to move across the sky
• The path is in the shape of an arc
• The path changes with the seasons
• In the summer the days are the longest, the sun
  rises north of east and sets north of west,
  resulting in a longer arc
• In the winter the days are shortest, the sun
  rises south of east and south of west, resulting
  in a shorter arc
• In the spring and fall days are almost equal, the
  sun rises due east and sets due west

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Changes in the Suns Altitude

• Sun is always at its highest position at local
  solar noon
• Altitude depends on time of year and location of
  observer
• Only between latitudes 23 ½ degrees N and S can
  the sun be observed directly overhead

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Models that Explain Apparent Motions

• Geocentric Models- Earth is the center of the
  solar system, Earth is stationary and everything
  revolves around it. (early cultures accepted
  this)
• Explained the motions of the stars, sun, and
  moon, but not the planets.
• Planets had orbits while they orbited
• Heliocentric Models- sun centered model
• Earth rotates on its axis and orbits the sun
• Daily motion of stars is explained by Earths
  rotation
• Motions of the sun are explained by Earths tilt
  and revolution

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Actual Earth Motions

• Moves with the Milky Way as it expands
• Around the center of the galaxy once every 225
  million years
• Rotates and revolves around the sun in an orbit

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Rotation of Earth

• Tilted 23 ½ degrees from a line perpendicular to
  the plane of its orbit to the sun
• Axis remains at the same tilt the entire time it
  orbits the sun.
• Rotates counterclockwise, from west to east
• Satellites have proven Earths rotation

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Other Evidence of Earths Rotation

• Foucault pendulum- if allowed to swing freely the
  path will appear to change in a predictable way.
  If the Earth did not rotate the pendulum would
  maintain its original path.
• Coriolis effect- merry-go-round example

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Evidence of Earths Revolution Around the Sun

• Earths change in distance does NOT cause the
  change in seasons
• If Earth didnt rotate around the sun the same
  part of the Earth would receive the more direct
  rays of sunlight
• See different constellations at different times
  of the year
• Sun appears to change slightly in size
• Slight changes in the Doppler effect

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Earth and Moon Motions and Time

• Throughout time people have used the motions of
  the Earth and moon as a frame of reference for
  time.
• A month is determined by one cycle of the moons
  phases
• One day is determined by the rotation of the
  Earth
• What determines the length of a year?

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Local Time

• Time based on the rotation of Earth as reflected
  in motions of the sun
• All places on the same longitude have the same
  local time
• If you move slightly east or west there will be
  different local times due to Earths rotation

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Local Solar Time

• For any location the sun reaches its highest
  point once each day, solar noon.
• The time it takes the Earth to rotate from solar
  noon to solar noon on two consecutive days is
  known as the solar day.
• This type of time is known as solar time.
• Does a solar day have a constant length?
• A mean solar day has been established, divided
  into exactly 24 hrs.

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Time Zones

• There is no limit to the amount of longitude
  lines that could exist.
• 24 time zones in 15 degree bands have been drawn
• Each 15 degree segment has the same time
• From one band to another is an increase or
  decrease of one hour

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Actual Motions of Earths Moon

• Revolves around the Earth in an elliptical orbit
  with a period of about 27 1/3 days
• Half of the moon receives light at all times from
  the sun.
• Phases of the moon are cyclic. It takes 29 ½
  days from full moon to full moon

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Moon Phases

• New Moon - The Moon's unilluminated side is
  facing the Earth. The Moon is not visible (except
  during a solar eclipse).
• Waxing Crescent - The Moon appears to be partly
  but less than one-half illuminated by direct
  sunlight. The fraction of the Moon's disk that is
  illuminated is increasing.
• First Quarter - One-half of the Moon appears to
  be illuminated by direct sunlight. The fraction
  of the Moon's disk that is illuminated is
  increasing.
• Waxing Gibbous - The Moon appears to be more than
  one-half but not fully illuminated by direct
  sunlight. The fraction of the Moon's disk that is
  illuminated is increasing.
• Full Moon - The Moon's illuminated side is facing
  the Earth. The Moon appears to be completely
  illuminated by direct sunlight.

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More Moon Phases

• Waning Gibbous - The Moon appears to be more than
  one-half but not fully illuminated by direct
  sunlight. The fraction of the Moon's disk that is
  illuminated is decreasing.
• Last Quarter - One-half of the Moon appears to be
  illuminated by direct sunlight. The fraction of
  the Moon's disk that is illuminated is
  decreasing.
• Waning Crescent - The Moon appears to be partly
  but less than one-half illuminated by direct
  sunlight. The fraction of the Moon's disk that is
  illuminated is decreasing.

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Tides

• Cyclic rise and fall of the oceans
• Caused by the gravitation between the Earth,
  moon, and sun.
• There is a bulge of water directly under the
  moons position.
• When the sun, moon, and Earth are in a straight
  line, the sun enhances the tidal effect, and high
  tides are lower and low tides are higher.

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Eclipses

• All opaque objects that do not give off their own
  light cast shadows in space.
• When another celestial object moves into a shadow
  of another object, there is an event called an
  eclipse.

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Lunar Eclipse

• Occurs when the moon revolves into the shadow of
  Earth, at the full moon phase.
• Averages to about 2 total lunar eclipses a year.
• Partial eclipse occurs when only part of the
  Earths shadow covers the moon.

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Solar Eclipse

• Rare
• The new moon can barely block out the sun. When
  this happens a shadow is cast on the Earth. The
  moons shadow can barely reach the Earth.
• A total eclipse can only be seen at any one
  location on Earth for up to 7 ½ minutes and once
  every 200 years.