Seasons

Seasons happen because of Earth’s tilt on its axis AND revolution around the Sun. Axis- imaginary line drawn from the north to the south pole. Rotation- the spinning of the earth on its axis. 1 rotation on its axis =24 hours

Revolution- the movement of the earth around the sun Revolution- the movement of the earth around the sun. Takes Earth 1 year to make one revolution. Orbit- the path the earth takes around the sun.

What Causes Earth’s Seasons? Earth’s axis is tilted 23.5 degrees – it always points in the same direction (Polaris, the North Star) as we orbit our Sun once a year This tilt causes the hemispheres to alternate in the amount of our Sun’s light and heat they receive through the year. When our part of Earth (Northern Hemisphere) is tilted TOWARD the sun, we are having summer. The NH is getting the Sun’s direct rays. In summer, days are long, nights are short. When our part of Earth (Northern Hemisphere) is tilted AWAY from the sun, we are having winter. The NH is getting LESS direct rays from the sun (weak). Days are short, nights are long. More information is at http://www.lpi.usra.edu/education/skytellers/seasons/about.shtml This image shows the reason Earth experiences seasons.  Points we discuss using this image are: 1) Earth’s orbit around the Sun is only slightly elliptical 2) Earth’s path around the  Sun brings us closer to the Sun in January.  Many students think we have seasons because Earth is sometimes closer and sometimes farther from the Sun.  This is correct, however, we actually are closer to the Sun in January in the Northern Hemisphere! 3) Earth’s seasons are caused by Earth’s tilt on its axis (~23 degrees).  Earth’s axis essentially is fixed  - it always points to the same place in the sky (on the celestial sphere) – towards Polaris. As we orbit the Sun each year, first one polar region is tilted toward the Sun, and then the other is tilted toward the Sun. When the north polar region is tilted toward the Sun (summer) the south polar region is tilted away (winter). Notes: Earth’s tilt does change over very long time periods, but for the most part, it moves between 22 and 23 degrees. Earth’s axis also wobble a bit, but over time periods of thousands of years, pointing toward different stars. http://www.lpi.usra.edu/education/skytellers/seasons/about.shtml

Winter Solstice Winter Solstice- (December 21 or 22) The sun shines on the Tropic of Capricorn. The shortest day of the year will occur on Dec. 21 or 22. After Dec. 21 or 22 the days will begin to get longer

Vernal Equinox Vernal Equinox- (March 20-21) Sun shines directly on the equator. Equal hours of daylight and darkness.

Summer Solstice Summer Solstice- (June 20-21) Sun Shines directly on the Tropic of Cancer. Longest day of the year will be June 20 or 21. After June 20/21 the days will begin getting shorter.

Autumnal Equinox Autumnal Equinox- (September 22 or 23) Sun shines directly on the Equator. Equal hours of daylight and darkness.

Effect of different angles of sun: Summer solstice=rays are more concentrated Winter solstice = rays are less concentrated, spread out Summer solstice = hot weather on the way. Winter solstice = cooler weather.

True color images December March June September An understanding of seasons can begin with observations. What is it like here in December? In June? What is it like in other cities? In other countries? Teachers can use satellite photos like these, or look at newspapers with temperatures for various cities around the world throughout the year. Photos from http://www.nasa.gov/vision/earth/features/blue_marble.html Using data from the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard NASA’s Terra satellite, scientists and data visualizers stitched together a full year’s worth of monthly observations of the land surface, coastal oceans, sea ice, and clouds into a seamless, photo-like mosaic of every square kilometer (.386 square mile) of our planet. Changes in ice are most obvious for the northern hemisphere; changes in vegetation can be seen in Africa and South America. A separate animation can be downloaded and played here: http://library01.gsfc.nasa.gov/svs/html/SVS000435.html http://www.nasa.gov/vision/earth/features/blue_marble.html