What Causes the Seasons?

What about seasons? Why do we have them? Earth’s distance from the sun varies throughout the year – doesn’t that cause the seasons? Earth is closest to our Sun (91 million miles) in winter—January 3 Earth is farthest from our Sun (94 million miles) in summer –July 4

Tilt! Because of the tilt of Earth’s axis, the amount of solar radiation received by Northern and Southern Hemispheres changes seasonally Northern Hemisphere has summer when it tilts toward the sun, winter when it tilts away Southern Hemisphere has summer when it tilts toward the sun, winter when it tilts away

The earth is tilted 23.50

Earth’s axis is tilted 23.50 – 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 receive different amounts of sunlight throughout the year 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.

Radiation is more intense near the equator compared to the poles. For this reason, it’s warmer near the equator than at the poles. North Pole Equator Earth South Pole

Earth North Pole Equator South Pole When the north pole tilts toward the sun, it gets more radiation – more warmth during the summer SUMMER (Northern Hemisphere) North Pole Equator Earth When the north pole tilts toward the sun, the south pole tilts away So when it’s summer in the north, it’s winter in the south South Pole WINTER (Southern Hemisphere)

Planets that are tilted on an axis can experience seasons. http://www.solarviews.com/eng/solarsys.htm

Equinoxes/Solstices Vernal Equinox – March 21/22 (1st day of spring) = amount of daylight and darkness Summer Solstice – June 21/22 (1st day of summer) longest daylight, shortest night Autumnal Equinox – Sept 22/23 (1st day of fall) = daylight and darkness Winter Solstice – Dec 21/22 (1st day of winter) shortest period of daylight, longest night

Seasons—a summary caused by the tilt of the earths axis As earth revolves around the sun its axis always points in the same direction Each hemisphere leans toward the sun during different parts of the orbit The seasons N and S of the equator are always opposite of each other The Earths Rotation Axis http://highered.mcgraw-hill.com/sites/0072482621/student_view0/animations.html#

http://www.physicalgeography.net/fundamentals/6hsummer.html http://www.physicalgeography.net/fundamentals/6hwinter.html http://www.physicalgeography.net/fundamentals/6hequinox.html