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Bolded words are in Glossary
By: Justin Li

Earths Rotation: ====We are always rotating. Everything in the universe is moving. The Earth moves by rotating on its **axis**, (an imaginary line that goes through the North and South poles). **Centrifugal** **force** happens because of the Earth's **rotation**; **gravity** is the reason centrifugal force can not make objects to fly into outer space. This works because the force of the Earth's gravity is 289 times stronger than its centrifugal force; gravity prevents objects from leaving the surface. Centrifugal force also causes the Earth to bulge at the equator, making it slightly ovoid in shape. ====


====The Earth rotates counter-clockwise. This is the contrary way the Sun and stars move. They spin East to West, while the Earth spins West to East. ====  ====The rotation of Earth can be verified in numerous ways. One is the Foucault experiment. This was initially done in 1851, by Frenchman Léon Foucault. What he did is he hung a heavy iron ball from a 200 feet wire, from the dome of the Pantheon in Paris, thus creating a **pendulum**. He put sand underneath the pendulum, and stuck a pin on the bottom of the ball, so it would leave a mark on its swing from side to side. On each swing, recorded for 24 hours, the mark in the sand would move to the right. The direction in the path showed the movement of the Earth against the swing of the pendulum. ====

==== Another way is through the prevailing winds. In the Northern Hemisphere, they blow counter clockwise, but in the Southern Hemisphere, they blow clockwise. ====

In conclusion, when we talk about the Earth's rotation, we are talking about the rotation
==== at Earth's surface. This rotation is used to define time. It takes the Earth approximately 24 hours to complete one full rotation. The Earth's axis is not perpendicular to the equator, it leans at a 23.45° angle, and the amount of daylight varies over the course of a year. ====

Earths Tilt and Rotations Effect on the Seasons: ==== We all know that the Earth completes a **revolution** around the sun once every 365.25 days. The change in season depends on the amount of sunlight that reaches the Earth. ====

==== The seasons occur because the Earth is tilted 23.45° on its axis. Summer transpires to the **hemisphere** tilted towards the Sun, and winter happens to the hemisphere tilted away from the Sun. Sunlight causes the hemisphere towards the Sun to be hotter because it is closer to the sun. That means that when it is summer in the Northern Hemisphere, it is winter in the Southern Hemisphere. The hemisphere experiencing summer, tilted towards the Sun, has longer days and shorter nights compared the hemisphere tilted away from the Sun which is the complete opposite. ==== ====Finally, summer and winter temperatures drop the further you travel from the equator. There are no seasons at the **equator** because each day the Sun strikes at about the same angle. Everyday the equator receives 12 hours of sunlight. The north and south poles always remain cold because they are never tilted toward the sun. ====

A little video on the Earth's Seasons: media type="youtube" key="DuiQvPLWziQ" height="344" width="425" Glossary: ** Axis: ** The line about which a rotating body, such as the earth, turns.


 * Centrifugal Force: ** An outward force on a body rotating about an axis, assumed equal and opposite to the centripetal force and postulated to account for the phenomena seen by an observer in the rotating body.


 * Equator: ** The great circle on a sphere or heavenly body whose plane is perpendicular to the axis, equidistant everywhere from the two poles of the sphere or heavenly body.


 * Gravity: ** The force of attraction by which terrestrial bodies tend to fall toward the center of the earth.


 * Hemisphere: ** Half of the terrestrial globe or celestial sphere, esp. one of the halves into which the earth is divided.


 * Pendulum: ** A body so suspended from a fixed point as to move to and fro by the action of gravity and acquired momentum.


 * Revolution: ** The orbiting of one heavenly body around another.


 * Rotation: ** The movement or path of the earth or a heavenly body turning on its axis.

Bibliography: ==== · Ritter, Michael E. "Seasons and Earth-Sun relations." //The Physical Environment//. 2006. 31 May 2009 . ====

 · Unknown, Unknown. "The Seasons." //The Seasons//. 31 May 2009 .
==== · Pogge, Richard. "Lecture 7: The Four Seasons." //Lecture 7: The Four Seasons//. 17 September 2007. 31 May 2009 <http://www.astronomy.ohio-state.edu/~pogge/Ast161/Unit2/seasons.html>. ====

====<span style="font-family: Tahoma,Geneva,sans-serif;"> · Unknown, Unknown. "EARTH"S Seasons." //Zoom Astronomy//. 1999-2000. 31 May 2009 <http://www.enchantedlearning.com/subjects/astronomy/planets/earth/Seasons.shtml>. ====

====<span style="font-family: Tahoma,Geneva,sans-serif;"> · Gardiner, Lisa. "Earth's Tilt Is the Reason for the Seasons!." //Windows to the Universe//. 31 October 2006. 31 May 2009 <http://www.windows.ucar.edu/tour/link=/earth/climate/cli_seasons.html&edu=high>. ====