The Earth-Moon-Sun System

Motions of the Earth 1.Rotation 2.Revolution 3.Precession

Rotation The turning or spinning of an object on its axis Axis of the Earth: An imaginary straight line running through the center of the earth from the North Pole to the South Pole. The earth is said to rotate (spin) on this axis, which is "tilted" in relation to the sun. This causes our seasons.

Revolution

The earth’s distance from the sun varies because it moves in an elliptical pattern –Perihelion is when the earth is closest to the sun, which occurs on January 3 of every year –Aphelion is when the earth is farthest from the sun and occurs about July 4

The Seasons

What would happen if the earth did not tilt on it’s axis? Turn to your neighbor and discuss. Would a. All the days be longer b. There be no seasons c. All the days be shorter d. Seasons would be extreme (i.e. colder winters)

Precession A very slow movement of the Earth in which the direction of the Earth’s axis changes. The angle remains the same but the axis point traces a circle in the sky.

Cause of Precession The cause of the precession is the equatorial bulge of the Earth, caused by the centrifugal force of the Earth's rotation. That rotation changes the Earth from a perfect sphere to a slightly flattened one, thicker across the equator. The attraction of the Moon and Sun on the bulge is what makes the Earth precess.

Precession Currently, this annual motion is about 50.3 seconds of arc per year or 1 degree every 71.6 years. The process is slow, but cumulative. A complete precession cycle covers a period of approximately 25,765 years.

More Motion 1.Entire solar system moves in the direction of Vega at 20 km/second 2.Sun revolves around the galaxy at a speed of about 250km/sec 3.Milky way galaxy is moving toward the Great Galaxy