The Earth’s Rotation, Revolution Around the Sun, and the Seasons Earth's Motion The Earth’s Rotation, Revolution Around the Sun, and the Seasons

I. Earth’s Rotation Earth spins around on an imaginary axis– This is called rotation. The axis of rotation is an imaginary straight line through Earth Between the North and South Poles. The axis is currently inclined at 23.5 from the plane of Earth’s orbit.

II. Earth’s Cycle of Precession Like a spinning top that rotates rapidly on its own axis while that axis slowly revolves about the vertical, Earth's axis changes its direction over the course of time; This is called precession. During a complete cycle of precession, which takes about 26,000 years, Earth's axis traces out a cone. Earth's axis currently points nearly toward the star Polaris. Some 12,000 years from now—nearly half-way through one cycle of precession—Earth's axis will point toward a star called Vega, which will then be the "North Star." Five thousand years ago, the North Star was a star named Thuban in the constellation Draco.

III. Parallelism of Earth’s Axis Throughout the year, Earth’s axis has always the same tilt and points in the same direction. This parallelism of the axis is important in the change of seasons and in the length of daylight and night.

IV. Evidence for Earth’s Rotation It is quite obvious from the motion of the sun, moon, and stars across the sky that the Earth rotates, but how do we know it is not them that rotate around us? The French physicist Jean Foucault provided proof of Earth’s rotation in Paris in 1851. He built a pendulum 60 meters high out of steel wire and a cannon ball and set it in motion in a north-south direction. What do you think happened?

V. The Effects of Earth’s Rotation Some of the first airplane pilots found themselves veering mysterious off course and long range cannons in WWI missed their targets. The Coriolis Effect deflects all free moving objects (winds, water, airplanes) to the right in the northern hemisphere and to the left in the southern hemisphere. This is due to the rotation of the Earth.

VI. The Rate of Earth’s Rotation The Earth completes 1 rotation in 24 hours. Given 360 in a sphere, Turns at a rate of 360/ 24 hours = 15/hr While every location on Earth moves at 15/hr, the speed of rotation in km/hr is not the same everywhere! The Earth actually rotates faster at the equator than the poles!

VII. Earth’s Revolution Around the Sun The movement of Earth in its orbit around the sun is called revolution.

VI_I. Evidence for Earth’s Revolution As Earth moves around in its orbit and/or it rotates on it axis, nearby stars appear to shift positions when compared to distant stars. This apparent shift in position is called parallax.

IX. Path and Rate of Earth’s Revolution Earth’s revolves counterclockwise (when viewed from above the North Pole) in an elliptical orbit around the Sun. At aphelion, on or about July 4th, Earth is furthest from the sun (about 152,400,000 km) At perihelion, on or about January 2nd, Earth is closest to the sun (about 147,600,000 km) Earth makes one revolution every 365.24 days. (.24*4  1extra day every 4 years) July 4th January 2nd The Earth is actually closer to the sun, by nearly 5 million km, in winter (for the northern hemisphere)!

X. The “Dome” of the Sky On the imaginary dome of the sky, the horizon is the rim of the dome and the zenith is the point straight overhead. The altitude of the sun is the vertical angle between the horizon and the sun.

XI. The 3 Basic Causes of Seasons (1) The tilt of Earth’s axis. Because of this tilt, parts of the Earth receive more direct radiation from the sun (angle of incoming solar radiation (insolation)) than others during different times of the year. (2) The revolution of Earth around the sun. (3) The parallelism of Earth’s axis.

Summer Solstice On June 21st, the Sun is directly overhead at the Tropic of Cancer (23.5° N). The number of daylight hours for the northern hemisphere is at its maximum and the sun does not set in the region within the arctic circle. What season is it in the southern hemisphere?

Land of the Midnight Sun Bylot Island in Baffin Bay, Canada, is far enough north of the Arctic Circle to have many weeks of continuous daylight in summer. This picture shows the island’s midnight sun photographed every 15 minutes from 11:15 P.M. to 1:00 A.M. on July 25-26.

Winter Solstice On December 21st, the sun is directly overhead at the Tropic of Capricorn (23.5° S). The sun has reached its lowest altitude in the northern hemisphere sky and the number of daylight hours is at its minimum. On this date, the sun never rises within the Arctic Circle and never sets within the Antarctic Circle.

Equinoxes When neither of the Earth’s hemispheres is tilted towards the sun both receive equal amounts of sunlight and the sun is directly overhead at the equator. Equinox means “equal nights”, and there are 12 hours of daylight and 12 hours of nighttime everywhere on Earth! What dates do the equinoxes correspond with?

XII. The Earth’s Seasons

The Sun’s Path The figure at the right illustrates how a person in the northern hemisphere would observe the sun during the solstices and equinoxes. What happens to the arc the sun travels through the sky as the seasons change?