The Earth’s Movement

Earth’s Motion is RelativeEarth rotates…

Earth revolves…

Earth precesses…

Earth nutates…

Earth moves with the solar system…

Solar System moves with spiral arm of Milky Way galaxy…

Milky Way rotates with other galaxies in the Local Group…

Local Group moves within the universe…

All this while space is expanding!

Topics

Earth Cycles

Sun’s path and the ecliptic plane

Seasons

Cyclic motions of the Sun and stars in our sky Cyclic motions of the Sun and stars in our sky are due to motions of the Earthare due to motions of the Earth

1.1. ROTATION=the spin of the Earth on its axis. It takes one ROTATION=the spin of the Earth on its axis. It takes one day for the Earth to complete one rotation.day for the Earth to complete one rotation.

2.2. REVOLUTION=the movement of the Earth in orbit around REVOLUTION=the movement of the Earth in orbit around the sun. It takes one year for the Earth to complete one the sun. It takes one year for the Earth to complete one revolution.revolution.

3.3. PRECESSION=the slow conical (top-like) motion of the PRECESSION=the slow conical (top-like) motion of the EarthEarth’’s axis of rotation. It takes 26,000 years for the Earth s axis of rotation. It takes 26,000 years for the Earth to complete one cycle of precession. to complete one cycle of precession.

4.4. NUTATION=a small back and forth wobble in the EarthNUTATION=a small back and forth wobble in the Earth’’s s axis caused by varying effects of the gravitational forces axis caused by varying effects of the gravitational forces from the Moon and the Sun. from the Moon and the Sun.

The Four Motions of the Earth

R = rotation

P = precession

N = nutation

r = revolution

r

Rotation

Not all stars in our sky rise and set, and different latitudes view the Not all stars in our sky rise and set, and different latitudes view the movements of the stars differently.movements of the stars differently.

The stars near the poles of the celestial The stars near the poles of the celestial sphere (shown here) move in trails that sphere (shown here) move in trails that circle the pole and never set. They are circle the pole and never set. They are called called circumpolarcircumpolar. .

The view of the stars movement The view of the stars movement depends on our latitude. (Note: depends on our latitude. (Note: these views represent east or west these views represent east or west orientations)orientations)

NORTH NORTH POLE POLE

EQUATOREQUATOR

MIDDLE MIDDLE LATITUDESLATITUDES

Description of Earth MovementCyclic motions of the Sun and stars in our sky are

due to motions of the Earth

ROTATION=the spin of the Earth on its axis. It takes one day for the Earth to complete one rotation. The daily motion of the Earth is called diurnal motion.

REVOLUTION=the movement of the Earth in orbit around the sun. It takes one year for the Earth to complete one revolution.

PRECESSION=the slow conical (top-like) motion of the Earth’s axis of rotation. It takes about 26,000 years for the Earth to complete one cycle of precession.

NUTATION=an irregular nodding motion caused by tidal forces that occurs as the Earth precesses.

Which way to the stars appear to move?The apparent westward motion of the Sun, Moon, and stars across our sky each day is caused by Earth’s rotation.

We generalize this motion to make statements such as, “The Sun rises in the east and sets in the west.”

The same is true for the Moon, planets and the stars.

At middle latitudes, we see the Sun, Moon, and many of the stars first come into view moving upward, rising at some point along the eastern horizon. Then, they appear to arc across the sky, gaining greatest altitude at the local meridian. Finally, they disappear somewhere along the western horizon.

http://www.opencourse.info/astronomy/introduction/02.motion_stars_sun/

The apparent westward motion of the Sun, The apparent westward motion of the Sun, Moon, and stars across our sky each day is Moon, and stars across our sky each day is caused by Earthcaused by Earth’’s s rotationrotation..

At middle latitudes, we see the Sun, Moon, and many of the stars At middle latitudes, we see the Sun, Moon, and many of the stars first come into view moving upward, rising at some point along the first come into view moving upward, rising at some point along the eastern horizon. Then, they appear to arc across the sky. Finally, eastern horizon. Then, they appear to arc across the sky. Finally, they disappear somewhere along the western horizon.they disappear somewhere along the western horizon.

We generalize this motion to make statements such as, We generalize this motion to make statements such as, ““The Sun The Sun rises in the east and sets in the west.rises in the east and sets in the west.””

The Sun’s Path

Each day we see the Sun’s path across our sky, it is a snapshot of a series of 365 separate paths that occur throughout the year.

Each daily “snapshot” of the Sun is just one position of the Sun along the ecliptic plane (as seen from the Earth).

The seasons we experience are linked to the motion of the Sun along the The seasons we experience are linked to the motion of the Sun along the celestial sphere. celestial sphere.

The point of the Sun’s path farthest north on the celestial sphere is called the summer solstice (JUN 21), while the point of the ecliptic farthest south is called the winter solstice (DEC 21).

The two points on the ecliptic where the Sun crosses the celestial equator are called equinoxes. During the vernal equinox (MAR 21), the Sun is moving north, while during the autumnal equinox (SEPT 21), the Sun is moving south.

Remember that the seasonal names of the equinoxes and the solstices refer to seasons in the NORTHERN hemisphere. The seasons occurring in the SOUTHERN hemisphere are exactly opposite.

The Sun rises at different points along the horizon at different times of the year The Sun rises at different points along the horizon at different times of the year and also peaks at different heights.and also peaks at different heights.

Different parts of the world experience different times of day as the Earth Different parts of the world experience different times of day as the Earth rotates.rotates.

TIME ZONES which can be used to calculate the time of day in any given TIME ZONES which can be used to calculate the time of day in any given part of the world. part of the world.

Earth RevolvesAs the Earth revolves, it changes position relative to Sun. This causes two effects:

• Zodiac constellations appear to shift along the ecliptic slightly less than 1 degree per day. Eventually, constellations visible at night become invisible during the day.

• The altitude of the Sun in the sky at local meridian changes daily. This alters the Sun’s path across the sky.

Variations in the Earth’s rotation

Earth averages one rotation every 24 hours, which means that some days are slightly longer or shorter than the average.

Earth’s rotational speed throughout geologic time has slowed due to it’s gravitational (tidal) interaction with the Moon.

Variations in The Earth’s Revolution

The Earth’s orbital speed varies because its orbital path is not a perfect circle; it is elliptical.

The Earth orbits fastest when it is furthest away from the Sun (at aphelion) and slowest when it is closest (at perihelion).

http://www.mhhe.com/earthsci/geology/mcconnell/earths_climate/intro.htm

http://www.windows.ucar.edu/tour/link=/physical_science/physics/mechanics/orbit/perihelion_aphelion.html

Reason for the SeasonsReason for the Seasons

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• The hemisphere tilted toward the Sun receives sunlight at higher angles than the hemisphere tilted away from the Sun.

• Higher insolation angle means greater intensity of radiant energy (the sun shines more directly on a smaller area).

Changing Angle of Sunlight

Why does the equator receive more energy from the Sun throughout the year than Fort Thomas?

The angle of the Sun above the horizon at the equator is never lower than 66.5 degrees during the year.

Since the angle of the Sun at the equator is higher above the horizon than at Fort Thomas, more solar radiation is received at the Equator due to the higher insolation angle.

The angle of the Sun above the horizon at Fort Thomas varies from 74 to 27 degrees during the year.

Season _________

Equinox or Solstice

Season _________

Equinox or Solstice

Season ________

Equinox or Solstice

Season _________

Equinox or Solstice

Identify the Seasons and the Equinox or Solstice, and comment about the number of daylight vs. night hours

_________

Seasons are caused because the EarthSeasons are caused because the Earth’’s axis is tilted and as the Earth s axis is tilted and as the Earth revolves around the Sun, different parts of the Earth receive more direct revolves around the Sun, different parts of the Earth receive more direct sunlight (summer), while other parts of the Earth receive sunlight which is sunlight (summer), while other parts of the Earth receive sunlight which is more spread out (winter).more spread out (winter).

Time and SeasonsTime and Seasons

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• Factors that affect season change are tilt of axis, revolution and rotation.

• The reason that the seasons change on Earth is because the tilt of the Earth at 23.5° as it revolves around the Sun.

• This causes two effects:

• 1) sunlight strikes the Earth’s surface at different angles during different times of the revolution year.

• 2) the number of hours of daylight each rotational day varies because the Sun is up for different numbers of hours per day.

Why do seasons change?

The Ecliptic

It is the flat plane of the orbit of the Earth around the Sun. (Earth orbits at an angle)

The apparent path of the Sun across the sky. (Sun takes a slightly different path across the sky each day)

The angle between the ecliptic plane and the celestial equator is 23.5o.

The Earth also The Earth also revolvesrevolves around the Sun, which changes our view of the stars. around the Sun, which changes our view of the stars.

From our perspective, the Sun From our perspective, the Sun appears to move through the appears to move through the stars along a special path stars along a special path called the ecliptic.called the ecliptic.

From an outside view, we see the From an outside view, we see the Earth revolve around the Sun. We Earth revolve around the Sun. We define the plane of the Earthdefine the plane of the Earth’’s orbit s orbit as the ecliptic plane.as the ecliptic plane.

Tilted axis and rotation of Earth combined with revolution around Sun causes the seasons.

1) Tilted axis (23.5 degrees), rotation and revolution cause the northern hemisphere to receive more direct sunlight for more hours than the southern hemisphere during the summer and less direct sunlight for less hours during the winter.

2) So, as one hemisphere is warmer, the other is colder. This condition reverses every 6 months. In other words, this condition is seasonal.

3) The fall and spring represent 3-month transitions from summer to winter and from winter to summer.

Seasons

Precession

Precession of the Equinoxes

Sunrise during the vernal equinox will occur in a different zodiac constellation every 2000-3000 years or so. This is due to the precession of the equinoxes.

This precession causes the position of the North Celestial Pole to slowly This precession causes the position of the North Celestial Pole to slowly change over time. Today, the North Celestial Pole is near the star change over time. Today, the North Celestial Pole is near the star Polaris, which we call the Polaris, which we call the ““North Star.North Star.”” However, in 3000 BC, Thuban However, in 3000 BC, Thuban was close to the North Celestial Pole and in 14,000AD, Vega will be in was close to the North Celestial Pole and in 14,000AD, Vega will be in this location. this location.

Gravitational forces of the Sun and the Moon pulling on Gravitational forces of the Sun and the Moon pulling on the Earth as it rotates causes the Earth to undergo a the Earth as it rotates causes the Earth to undergo a top-like motion called top-like motion called precessionprecession. Over a period of . Over a period of 26,000 years, the Earth26,000 years, the Earth’’s rotation axis slowly moves in s rotation axis slowly moves in a circular motion.a circular motion.

North Star isn’t always Polaris!

Latitude vs. Longitude

latitude longitude+ =

Earth’s Orbit Around the SunThe Earth revolves around the Sun, which changes our view of the stars.

Seasons are caused because the Earth’s axis is tilted and as the Earth revolves around the Sun.

As the Earth revolves, different parts of the Earth receive sunlight at different angles.

The more direct the sunlight, the more energy delivered.

The angle of the Sun’s path across the sky determines the daylight hours.

Daylight hours + the angle of the sun affect the amount of light energy received by the Earth.

One common misconception is One common misconception is that the Moon is only visible at that the Moon is only visible at night. However, the time of night. However, the time of day in which the Moon is in day in which the Moon is in our sky varies depending on our sky varies depending on its phase. This picture clearly its phase. This picture clearly displays the Moon, visible displays the Moon, visible

during the day.during the day.