Earth’s Motions

Unit 4

I. Rotation

1. The earth spins on anaxis

2. The axis of rotation is animaginary line through the earth from north to south pole

•The earth rotates around this axis

•The axis is tilted23½º from vertical

3. The axis always points towardsPolarisThis is calledParallelism of the axis.

A. Results of Rotation

1. Sunrise and set2. Moonrise and set3. Planet rise and set4. Coriolis effect5. Motion of the foucault

pendulum6. Circumpolar motion of stars

7. Location of sunrise/setis due to the direction of rotation…the earth rotates fromwest to east orcounterclockwise

B. Rate of Rotation

1. Angular rate:360º 24 hours15º/hr

2. Velocity is dependent upon latitudeAs latitude increasesvelocity decreases

Equator 40,080 km

1670 km/hr

42º 31,200 km

1300 km/hr

60º 20,140 km

839 km/hr

90º 0 km 0 km/hr

Latitude CircumferenceRate ofRotation

C. Time, the Sun and Earth’s Rotation

•We use the apparent motion of the sun to tell time.

•Time is based upon the actual movement of the earth

•The earth’s rotation causes the apparent rise and set of the sun

•The apparent path of the sun is an arc

•Local solar noon is always the time at which the sun reaches its highest point in its arc–Solar noon in NYS is

never directly overhead!!

Z

II. Revolution

1. The earth travels around the sun

2. The orbit of the earth isan ellipse withthe sun at one of the foci

3. The direction of revolution is counterclockwise

A. Results of Revolution

1. Seasons2. Seasonal changes in

visible constellations3. Parallax4. Retrograde motion

B. Rate of Revolution

1. Angular rate:360º365.25 days~1º/day

2. Velocityradius of revolution is 1.5 x 108

therefore v=2(pi)r Tv = ~ 2,580,822 km/day

(1548493 mi/day)

•Comparative velocity:•The Earth travels fastest when we are closest to the sun andslowest when we are farthest from the sun

III. Seasons in the Northern Hemisphere

Seasons and seasonal changes are due to the revolution of the earth combined withthe tilt of the earth on its axisand parallelism of the axis

A. Summer1. Summer solstice2. June 21st

3. Northern hemisphere is tipped towards the sun

4. Earth is at its greatest distance from the sun (aphelion)

5. Altitude of solar noon is highest

6. Days are longest7. Shadows are shortest8. The sun appears smallest9. Sunrise occurs north of

east10.Sunset occurs north of

west

11.The zenith sun is found at 23½ºN…Tropic of Cancer

12.The north pole experiences 24 hours of daylight

13.The south pole experiences 24 hours of darkness

B. Fall

1. Autumnal equinox2. September 23rd

3. Neither hemisphere is tilted towards the sun

4. The distance to the sun is decreasing

5. The altitude of solar noon is decreasing

6. The length of day is decreasing

7. Shadow length is increasing

8. The sun is looking bigger

9. Sunrise occurs due east10.Sunset occurs due west11. Zenith sun is found at

the equator…0º12.We experience 12

hours of daylight and 12 hours of darkness

C. Winter1. Winter solstice2. December 21st

3. The northern hemisphere is tilted away from the sun

4. The earth is closest to the sun

5. The altitude of solar noon is lowest

6. The length of day is shortest

7. Shadow length is longest8. The sun appears biggest9. Sunrise occurs south of

east10.Sunset occurs south of

west

11. The zenith sun is found at the Tropic of Capricorn…23½ºS

12.The north pole experiences 24 hours of darkness

13.The south pole experiences 24 hours of daylight

D. Spring

1. Vernal equinox2. March 21st

3. Neither hemisphere is tilted towards the sun

4. The distance to the sun is increasing

5. The altitude of solar noon is increasing

6. The length of day is increasing

7. Shadow length is decreasing

8. The sun is looking smaller

9. Sunrise occurs due east10.Sunset occurs due west11.The zenith sun is found

at the equator…0º12.We experience 12

hours of daylight and 12 hours of darkness

Shadows in the northern hemisphere

A shadow castby an object at solar noon in NYS will always pointnorth

June

Sept

DecJanFeb

MarAp

MayJulyAug

OctNov

J

A

S

O

N

D

J

F

M

A

M

J

JS/MD

IV. The Moon

A. Movements1. Revolution

The moon travels a path thatis an ellipse withthe earth at one of the foci(the other foci is called the barycenter)

Its period of revolution is27 1/3 days and is called asidereal month

2. The moon’s period of rotation is 27 1/3 days …its rotation is synchronous with its revolution

•Because of this synchronicity, the same side of the moon always faces the earth.

•Until we began exploring space, the far side of the moon remained unobserved.

B. Lunar Phenomena

1. Phases of the moon:the changing shape of the visible moon due to the revolution of the moon around the earth

•Because the earth is revolving around the sun as the moon revolves around the earth, one complete cycle of phases takes longer than one complete revolution

•One cycle of phases takes29½ days and is called asynodic month

waxing

waning

crescent

New moon

crescent

quarter

¾ or gibbous

Full moon

¾ or gibbousquarter

http://www.ac.wwu.edu/~stephan/phases2.html

Waxing…when the visible portion of the moon is becoming larger

Waning…when the visible portion of the moon is becoming smaller

2. Eclipsesa. Terms:

i. Umbra: The darkest part of the shadow cast by the earth or the moon. The surface is completely blocked from view.

ii. Penumbra: The area of partial shadow surrounding the darkest part of the shadow cast by the earth or the moon. The surface is only partially blocked from view.

b. Lunar eclipse:when the moon is in the shadow of the earthThe moon must be in the full moon phase:S – E – M

c. Solar eclipse:when the earth is in the shadow of the moonThe moon must be in the new moon phase:S – M – E

                                             

d. We don’t experience eclipses with every new and full moon because the plane of the moon’s orbit is tilted 5º to the plane of the earth’s orbit

e. When the moon is at perigee and the sun is at aphelion, the surface of the sun is completely “blocked”When the moon is at apogee and the sun is at perihelion, we see an annular eclipse in which the sun is seen as a ring around the moon’s shadow

3. Tides•The cyclic rise and fall of ocean water on the earth.

•They are caused by the gravitational attraction between the moon and the earth’s waters…and, to a lesser degree, the sun and the earth

•Tidal events occur every6 hours

•The position of the moon affects the degree of the tides

a. S – E – M = Full MoonorS – M – E = New Moonthe sun and the moon work “together” to create extreme tides…Spring tidesvery high, high tides and very low, low tides

The sun and the moon work against each other to create minimal tides

b.

S E

M

Quarter Moon

or

M

ES

•These tides are calledneap tidesvery low, high tides andvery high, low tides

•Without the moon, the earth would still experience tides, but certainly not to the extent that we do with the moon’s influence.

                                                                         

                 

4. Moonrise and Moonset

•Caused by therotation of the earth

•Occurs 50 minutes later each day because of the revolution of the moon