1. Earth in Space EARTH, THE SUN, AND THE SEASONS

2. Earth, the Sun, and the Seasons  Why is Earth colder in winter than in the summer?  Not because Earth is closer to the sun in summer.  We are at our farthest approach (aphelion) on July 4 (152 million km)  We are at closest approach (perihelion) on January 3 (147 million km)  The Northern Hemisphere does receive less solar radiation in the winter  Due to the tilt of Earth’s axis  23.5° from vertical  Insolation – the amount of solar radiation received by Earth (or any planet)  Is greatest when the sun is directly overhead and decreases as the sun’s rays make a lower angle with Earth’s surface

3. Earth, the Sun, and the Seasons  Equator – imaginary line that circles Earth at 0° latitude  Tropic of Cancer – 23.5° North of the equator  Tropic of Capricorn – 23.5° South of the equator  Approximately ½ of the solar radiation that illuminates Earth makes it to Earth’s surface  The rest is reflected back into space by clouds and atmospheric particles  The poles receive less solar radiation than the equator, because solar rays must travel farther to strike land.  The farther the rays travel, the more likely they are to be reflected

4. Earth, the Sun, and the Seasons  The curvature of Earth’s surface results in heat energy more distributed at the poles  The equator receives 2.5 more times solar radiation than the poles  Circulation patterns in the atmosphere and oceans distribute solar heat to moderate Earth’s climate

5. Why Day Length Changes  The sun is directly over the equator on the vernal and autumnal equinoxes (March 20 and September 22) – day and night are both 12 hours long  The sun is directly over the Tropic of Cancer on June 21  Summer Solstice in the Northern Hemisphere – longest day of the year  The sun is directly over the Tropic of Capricorn on December 21  Winter Solstice in the Northern Hemisphere – longest night of the year  Seasons are reversed for the Southern Hemisphere