Unit 4

Motions of Earth, Moon, and SunMotions of Earth, Moon, and Sun

Apparent Motions of Celestial Objects Apparent motion is the motion an object Apparent motion is the motion an object

appears to make. Can be real or an illusion.appears to make. Can be real or an illusion. Stars Stars appearappear to move across the sky from to move across the sky from

east to west, however this is a result of the east to west, however this is a result of the Earth’s rotation.Earth’s rotation.

Daily Motion and Stars

Most celestial objects appear to move Most celestial objects appear to move across the sky from east to west.across the sky from east to west.

Vast majority are stars.Vast majority are stars. The paths of these celestial objects are The paths of these celestial objects are

circular, the polar constellations, or an arc.circular, the polar constellations, or an arc. All motion is at a constant rate of 15 All motion is at a constant rate of 15

degrees per hour for a total of 360 degrees degrees per hour for a total of 360 degrees in 24 hours.in 24 hours.

Apparent Motions of the Planets

Planet’s daily motion is similar to that of stars, Planet’s daily motion is similar to that of stars, however over a period of weeks or months the however over a period of weeks or months the planets appear to change position with respect to planets appear to change position with respect to the stars.the stars.

This motion is not uniform. The planet will This motion is not uniform. The planet will undergo the same motion from year to year but in undergo the same motion from year to year but in different locations in the sky.different locations in the sky.

The complicated motion is explained by the fact The complicated motion is explained by the fact that all the planets rotate and revolve around the that all the planets rotate and revolve around the sun at different rates.sun at different rates.

Apparent Motions of Earth’s Moon Follows daily east to west motionFollows daily east to west motion Rises 50 minutes later each dayRises 50 minutes later each day Shifts eastward each day compared to the Shifts eastward each day compared to the

background of the stars.background of the stars.

Apparent motions of the Sun The sun also The sun also seemsseems to move across the sky to move across the sky The path is in the shape of an arcThe path is in the shape of an arc The path changes with the seasonsThe path changes with the seasons In the summer the days are the longest, the sun In the summer the days are the longest, the sun

rises north of east and sets north of west, resulting rises north of east and sets north of west, resulting in a longer arcin a longer arc

In the winter the days are shortest, the sun rises In the winter the days are shortest, the sun rises south of east and south of west, resulting in a south of east and south of west, resulting in a shorter arcshorter arc

In the spring and fall days are almost equal, the In the spring and fall days are almost equal, the sun rises due east and sets due westsun rises due east and sets due west

Changes in the Sun’s Altitude

Sun is always at its highest position at local Sun is always at its highest position at local solar noonsolar noon

Altitude depends on time of year and Altitude depends on time of year and location of observerlocation of observer

Only between latitudes 23 ½ degrees N and Only between latitudes 23 ½ degrees N and S can the sun be observed directly overheadS can the sun be observed directly overhead

Models that Explain Apparent Motions

Geocentric ModelsGeocentric Models- Earth is the center of the solar - Earth is the center of the solar system, Earth is stationary and everything system, Earth is stationary and everything revolves around it. (early cultures accepted this)revolves around it. (early cultures accepted this)

- Explained the motions of the stars, sun, and moon, Explained the motions of the stars, sun, and moon, but not the planets.but not the planets.

- Planets had orbits while they orbitedPlanets had orbits while they orbited Heliocentric ModelsHeliocentric Models- sun centered model- sun centered model- Earth rotates on its axis and orbits the sunEarth rotates on its axis and orbits the sun- Daily motion of stars is explained by Earth’s Daily motion of stars is explained by Earth’s

rotationrotation- Motions of the sun are explained by Earth’s tilt Motions of the sun are explained by Earth’s tilt

and revolutionand revolution

Actual Earth Motions

Moves with the Milky Way as it expandsMoves with the Milky Way as it expands Around the center of the galaxy once every Around the center of the galaxy once every

225 million years225 million years Rotates and revolves around the sun in an Rotates and revolves around the sun in an

orbitorbit

Rotation of Earth

Tilted 23 ½ degrees from a line Tilted 23 ½ degrees from a line perpendicular to the plane of its orbit to the perpendicular to the plane of its orbit to the sunsun

Axis remains at the same tilt the entire time Axis remains at the same tilt the entire time it orbits the sun.it orbits the sun.

Rotates counterclockwise, from west to eastRotates counterclockwise, from west to east Satellites have proven Earth’s rotationSatellites have proven Earth’s rotation

Other Evidence of Earth’s Rotation Foucault pendulumFoucault pendulum- if allowed to swing - if allowed to swing

freely the path will appear to change in a freely the path will appear to change in a predictable way. If the Earth did not rotate predictable way. If the Earth did not rotate the pendulum would maintain its original the pendulum would maintain its original path.path.

Coriolis effectCoriolis effect- merry-go-round example…- merry-go-round example…

Evidence of Earth’s Revolution Around the Sun Earth’s change in distance does NOT cause the Earth’s change in distance does NOT cause the

change in seasonschange in seasons If Earth didn’t rotate around the sun the same part If Earth didn’t rotate around the sun the same part

of the Earth would receive the more direct rays of of the Earth would receive the more direct rays of sunlightsunlight

See different constellations at different times of See different constellations at different times of the yearthe year

Sun appears to change slightly in sizeSun appears to change slightly in size Slight changes in the Doppler effectSlight changes in the Doppler effect

Earth and Moon Motions and Time Throughout time people have used the Throughout time people have used the

motions of the Earth and moon as a frame motions of the Earth and moon as a frame of reference for time.of reference for time.

A month is determined by one cycle of the A month is determined by one cycle of the moon’s phasesmoon’s phases

One day is determined by the rotation of the One day is determined by the rotation of the EarthEarth

What determines the length of a year?What determines the length of a year?

Local Time

Time based on the rotation of Earth as Time based on the rotation of Earth as reflected in motions of the sunreflected in motions of the sun

All places on the same longitude have the All places on the same longitude have the same local timesame local time

If you move slightly east or west there will If you move slightly east or west there will be different local times due to Earth’s be different local times due to Earth’s rotationrotation

Local Solar Time

For any location the sun reaches its highest point For any location the sun reaches its highest point once each day, solar noon.once each day, solar noon.

The time it takes the Earth to rotate from solar The time it takes the Earth to rotate from solar noon to solar noon on two consecutive days is noon to solar noon on two consecutive days is known as the known as the solar day.solar day.

This type of time is known as This type of time is known as solar time.solar time. Does a solar day have a constant length?Does a solar day have a constant length? A mean solar day has been established, divided A mean solar day has been established, divided

into exactly 24 hrs. into exactly 24 hrs.

Time Zones

There is no limit to the amount of longitude There is no limit to the amount of longitude lines that could exist.lines that could exist.

24 time zones in 15 degree bands have been 24 time zones in 15 degree bands have been drawndrawn

Each 15 degree segment has the same timeEach 15 degree segment has the same time From one band to another is an increase or From one band to another is an increase or

decrease of one hourdecrease of one hour

Actual Motions of Earth’s Moon

Revolves around the Earth in an elliptical Revolves around the Earth in an elliptical orbit with a period of about 27 1/3 daysorbit with a period of about 27 1/3 days

Half of the moon receives light at all times Half of the moon receives light at all times from the sun.from the sun.

Phases of the moon are cyclic. It takes 29 Phases of the moon are cyclic. It takes 29 ½ days from full moon to full moon½ days from full moon to full moon

Moon Phases New MoonNew Moon - The Moon's unilluminated side is facing the - The Moon's unilluminated side is facing the

Earth. The Moon is not visible (except during a solar Earth. The Moon is not visible (except during a solar eclipse).eclipse).

Waxing CrescentWaxing Crescent - The Moon appears to be partly but - The Moon appears to be partly but less than one-half illuminated by direct sunlight. The less than one-half illuminated by direct sunlight. The fraction of the Moon's disk that is illuminated is increasing.fraction of the Moon's disk that is illuminated is increasing.

First QuarterFirst Quarter - One-half of the Moon appears to be - One-half of the Moon appears to be illuminated by direct sunlight. The fraction of the Moon's illuminated by direct sunlight. The fraction of the Moon's disk that is illuminated is increasing.disk that is illuminated is increasing.

Waxing GibbousWaxing Gibbous - The Moon appears to be more than - The Moon appears to be more than one-half but not fully illuminated by direct sunlight. The one-half but not fully illuminated by direct sunlight. The fraction of the Moon's disk that is illuminated is increasing.fraction of the Moon's disk that is illuminated is increasing.

Full MoonFull Moon - The Moon's illuminated side is facing the - The Moon's illuminated side is facing the Earth. The Moon appears to be completely illuminated by Earth. The Moon appears to be completely illuminated by direct sunlight.direct sunlight.

More Moon Phases

Waning GibbousWaning Gibbous - The Moon appears to be more than - The Moon appears to be more than one-half but not fully illuminated by direct sunlight. The one-half but not fully illuminated by direct sunlight. The fraction of the Moon's disk that is illuminated is fraction of the Moon's disk that is illuminated is decreasing.decreasing.

Last QuarterLast Quarter - One-half of the Moon appears to be - One-half of the Moon appears to be illuminated by direct sunlight. The fraction of the Moon's illuminated by direct sunlight. The fraction of the Moon's disk that is illuminated is decreasing.disk that is illuminated is decreasing.

Waning CrescentWaning Crescent - The Moon appears to be partly but - The Moon appears to be partly but less than one-half illuminated by direct sunlight. The less than one-half illuminated by direct sunlight. The fraction of the Moon's disk that is illuminated is fraction of the Moon's disk that is illuminated is decreasing.decreasing.

http://aa.usno.navy.mil/graphics/Moon_movie.gif

Tides

Cyclic rise and fall of the oceansCyclic rise and fall of the oceans Caused by the gravitation between the Earth, Caused by the gravitation between the Earth,

moon, and sun.moon, and sun. There is a bulge of water directly under the There is a bulge of water directly under the

moon’s position.moon’s position. When the sun, moon, and Earth are in a straight When the sun, moon, and Earth are in a straight

line, the sun enhances the tidal effect, and high line, the sun enhances the tidal effect, and high tides are lower and low tides are higher.tides are lower and low tides are higher.

Eclipses

All opaque objects that do not give off their All opaque objects that do not give off their own light cast shadows in space.own light cast shadows in space.

When another celestial object moves into a When another celestial object moves into a shadow of another object, there is an event shadow of another object, there is an event called an eclipse.called an eclipse.

Lunar Eclipse

Occurs when the moon revolves into the Occurs when the moon revolves into the shadow of Earth, at the full moon phase.shadow of Earth, at the full moon phase.

Averages to about 2 total lunar eclipses a Averages to about 2 total lunar eclipses a year.year.

Partial eclipsePartial eclipse occurs when only part of the occurs when only part of the Earth’s shadow covers the moon.Earth’s shadow covers the moon.

Solar Eclipse

RareRare The new moon can barely block out the sun. The new moon can barely block out the sun.

When this happens a shadow is cast on the When this happens a shadow is cast on the Earth. The moon’s shadow can barely Earth. The moon’s shadow can barely reach the Earth.reach the Earth.

A total eclipse can only be seen at any one A total eclipse can only be seen at any one location on Earth for up to 7 ½ minutes and location on Earth for up to 7 ½ minutes and once every 200 years.once every 200 years.