2What about seasons? Why do we have them? Earth’s distance from the sun varies throughout theyear – doesn’t that cause the seasons?Earth is closest to our Sun (91 million miles) in winter—January 3Earth is farthest from our Sun (94 million miles) in summer –July 4
3Tilt!Because of the tilt of Earth’s axis, the amount of solar radiation received by Northern and Southern Hemispheres changes seasonallyNorthern Hemisphere has summer when it tilts toward the sun, winter when it tilts awaySouthern Hemisphere has summer when it tilts toward the sun, winter when it tilts away
5Earth’s axis is tilted – it always points in the same direction (Polaris, the North Star) as we orbit our Sun once a yearThis tilt causes the hemispheres to receive different amounts of sunlight throughout the yearMore information is atThis image shows the reason Earth experiences seasons. Points we discuss using this image are:1) Earth’s orbit around the Sun is only slightly elliptical2) 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.
6Radiation is more intense near the equator compared to the poles. For this reason, it’s warmer near the equator than at the poles.North PoleEquatorEarthSouth Pole
7Earth North Pole Equator South Pole When the north pole tilts toward the sun, it gets more radiation – more warmthduring the summerSUMMER (Northern Hemisphere)North PoleEquatorEarthWhen the north pole tilts toward the sun, the south pole tilts awaySo when it’s summer in the north,it’s winter in the southSouth PoleWINTER (Southern Hemisphere)
10Planets that are tilted on an axis can experience seasons.
11Equinoxes/SolsticesVernal Equinox – March 21/22 (1st day of spring) = amount of daylight and darknessSummer Solstice – June 21/22 (1st day of summer) longest daylight, shortest nightAutumnal Equinox – Sept 22/23 (1st day of fall) = daylight and darknessWinter Solstice – Dec 21/22 (1st day of winter) shortest period of daylight, longest night
12Seasons—a summary caused by the tilt of the earths axis As earth revolves around the sun its axis always points in the same directionEach hemisphere leans toward the sun during different parts of the orbitThe seasons N and S of the equator are always opposite of each otherThe Earths Rotation Axis