Motions of the Earth

Our goals for learning:• What are the main motions of the Earth in space?

• How do we see these motions on the ground? How does it affect our lives?

• How does the orientation of Earth's axis change with time?

What are the Main Motions of the Earth in Space?

There are 3 main movements:

• Rotation (daily)

• Orbit or revolution (annual)

• Precession (millennia)

1. Daily Motion

• Earth moves counterclockwise, so Sun appears to move clockwise

• Sun appears to rise in the East and set in the west.

• Speed average 15°/hr for 360° in 24 hrs

View of the Sun With LatitudeThanks to Earth’s rotation, the Sun appears to move in the sky. But, its location depends on the viewer's latitude.

Here is the equinox, when sunlight falls evenly everywhere on the Earth.

View of Daily motion from Ground

• Sun path & latitude Hand out

Sun’s Path in the Local Sky

View of Daily motion from Ground

Earth rotates west to east (counterclockwise), so, like the Sun, stars appear to circle from east to west. Stars will rise and set, depending on your location

View of Daily motion from GroundYou will always see ½ of the celestial sphere at one time.However which part you see depends on your location on the Earth.

•This affects which stars you see and the angle at which you see them.

•Some stars will rotate in and out of view.

View of Daily motion from GroundIf you are in the northern hemisphere then:

Stars near the north celestial pole never set. We cannot see stars near the south celestial pole. All other stars (and Sun, Moon, planets) rise in east and set

in west.

Your Location Your View

2. Orbital Motion

Thought Question

TRUE OR FALSE? Earth is closer to the Sun in summer and farther from the Sun in winter.

A. True

B. False

How do we mark the seasons? • We define four special points:

summer solstice (longest day)winter solsticespring (vernal) equinoxfall (autumnal) equinox

Summary: The Real Reason for Seasons

• Earth’s axis points in the same direction (to Polaris) all year round, so its orientation relative to the Sun changes as Earth orbits the Sun.

• Seasons are caused by.

• 1) Constant Axial Tilt• 2) Orbit around the Sun

Why doesn’t distance matter?• Variation of Earth-Sun distance is small —

about 3%; this small variation is overwhelmed by the effects of axis tilt.

View of Orbital Motion From Ground

Summer solstice: Highest path, rise and set at most extreme north of due east.

Equinoxes: Sun rises precisely due east and sets precisely due west.

Winter solstice: Lowest path, rise and set at most extreme south of due east.

When the sun is high in the sky, its light is more concentrated &

more effective in heating the Earth.

Why does Temperature Change Seasonally?

• Summer – High sun angle+ longer time in the sky

=> increased surface heating=> WARM

• Winter – Low sun angle+ shorter time in the sky

=> decreased surface heating => COLD!

Seasonal changes are more extreme at high latitudes

summer solstice at the Arctic Circle-no sunset

Thought Question

It is December 31 and you see 24 hours of daylight. Where are you standing?

A. North PoleB. EquatorC. South PoleD. San Mateo

The sky varies as Earth orbits the Sun

• As the Earth orbits the Sun, the Sun appears to move eastward along the ecliptic.

• The result is that the sun blots out different stars in different sesaons, while constellations opposite the Sun are visible

The sky varies as Earth orbits the Sun

• Different constellations swing into view at different seasons.

How does the orientation of Earth’s axis change

with time?

Although the axis seems fixed on human time scales, it actually precesses over about 26,000 years.

Earth’s axis precesses like the axis of a spinning top

The View of Precession from the Ground

Polaris won’t always be the North Star.

Positions of equinoxes shift around orbit; e.g., spring equinox, once in Aries, is now in Pisces!