Earth Motions

The Earth has two principal motions- rotation and revolution the movement of the Earth around the sun it takes 365.25 days how do we account for the .25 days? Effects: Change in seasons Change in constellations

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)

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)

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 1670 km/h 40,074 km/24 hr) at the poles ~ 0 km/hr

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

Revolution the earth is tilted from perpendicular (90 degrees) 23.5 degrees

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

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

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

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.

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

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

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)