Download Motions of the Earth, moon, and sun

Unit 4
Motions of Earth, Moon, and Sun
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
Earth’s 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
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Planet’s 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.
Apparent Motions of Earth’s
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
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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
Changes in the Sun’s 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 Earth’s
rotation
Motions of the sun are explained by Earth’s tilt
and revolution
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 Earth’s rotation
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Other Evidence of Earth’s
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 Earth’s Revolution
Around the Sun
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Earth’s change in distance does NOT cause the
change in seasons
If Earth didn’t 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
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
moon’s 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 Earth’s
rotation
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Local Solar Time
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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.
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 Earth’s 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
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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.
More Moon Phases
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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.
http://aa.usno.navy.mil/graphics/
Moon_movie.gif
Tides
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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
moon’s 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.
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
Earth’s 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 moon’s 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.
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