1. Earth Motions

2. The Earth has two principal motions- rotation and revolution
the movement of the Earth around the sun it takes days
how do we account for the .25 days?
Effects:
Change in seasons
Change in constellations

3. Rotation as the earth moves around the sun it is spinning on its axis
it takes 24 hours to complete a rotation
Effects of Rotation
day and night
movement of the sun across the sky (the sun rotates west to east, counterclockwise)

4. Evidence for rotation Coriolis effect
the apparent deflection of the winds
if the earth did not rotate, winds would blow from the poles ( high pressure) to the equator (low pressure)

5. Rate of rotation
The earth completes one complete circle (360 degrees every 24 hours) so every location on earth moves at a rate of 15 degrees per hour
360o/24 hr = 15 degrees/hr.
the speed of rotation differs, it is based on latitude (remember that latitude runs north to south)
at the equator - earth spins at km/h 40,074 km/24 hr)
at the poles ~ 0 km/hr

6. Revolution slightly elliptical we are not always
the same distance from the sun
Perihelion- closest to the sun occurs ~ Jan. 3rd 147,000,000 km
Aphelion- farthest from the sun ~ July 4th 152,000,000 km

7. Revolution
the earth is tilted from perpendicular (90 degrees) degrees

8. December North Pole points away from the sun occurs on Dec. 21
Sun strikes the Tropic of Capricorn at 90 degrees angle which has a latitude of 23.5oS
Winter solstice

9. Dec. 21st marks the beginning of winter in Northern Hemisphere
sun follows lowest path across the sky
Places North of Arctic Circle have 24 hours of darkness
From this day forward the hours of daylight begins to increase
shortest hours of daylight

10. Spring Equinox
Sun’s rays are at 90 degree angle at the equator (0 degrees)
Occurs on or near March 20th
Called the Spring or Vernal equinox
marks the beginning of Spring
hours of daylight and darkness are the same

11. Summer Solstice On June 21- the North Pole tilts toward the sun
The sun is striking the earth at a 90 degree angle along the Tropic of Cancer Which has a latitude of 23.5oN.
This is called the summer solstice, it marks the beginning of SUMMER in the Northern Hemisphere.

12. Summer con’t most hours of daylight, longest day of the year
the further North you are, the more hours of daylight
Arctic Circle has 24 hours of daylight
From this day the hours of daylight begins to decrease

13. Fall equinox Occurs on ~ Sept 22nd
sun’s rays at 90o at the equator( 0 o )
called the Autumnal Equinox
marks beginning of fall in Northern Hemisphere
Equal amounts of daylight and darkness

14. Three of the four main causes of seasons.
Tilt of the earth’s axis
Revolution of the earth around the sun
Parallelism of axis ( the axis of the earth always points to Polaris)