1. Earth-Sun Relationships The Reasons for the Seasons

2. Why is it hot in the tropics and cold at the poles?

3. The Earth is curved and this causes differing angles of incidence.

4. Intensity of incoming solar radiation (insolation) is related to angle of incidence. Higher angles = higher intensity.

5. Why is it hot in the tropics and cold at the poles? The Earth’s surface is curved relative to the incoming solar radiation. Direct rays (most intense) are always in the tropical latitudes (23.5 º S to 23.5º N) Poles don’t get as much solar radiation Indirect rays (least intense) are received by latitudes north and south of the tropics decreasing towards the poles

6. What causes the seasons? rotation of the earth on its axis revolution of the earth around the sun 23.5 O tilt of the earth axis from perpendicular to the plane of the ecliptic polarity (axial parallelism)

7. Rotation of the Earth Rotate west to east or ‘counter-clockwise’ (when viewed from the North Pole) –sunset and sunrise Rotation period is 24 hours per solar day (23 hours 56 minutes with respect to other stars)

8. Revolution of the earth Period of revolution is 365.25 solar days Elliptical orbit (plane of the ecliptic) Perihelion - 91.5 million miles (Jan. 4) Aphelion - 94.5 million miles (July 5) 93 million miles average distance

9. Tilt of the earth on its axis. The earth is tilted 23.5º from perpendicular to the plane of the ecliptic. Tilt is currently constantly toward Polaris

10. SOLSTICES SUMMER, JUNE 21 WINTER, DECEMBER 21 Summer Solstice: -tropic of cancer gets most direct rays - Northern Hemisphere has longer days - Arctic Circle has 24 hours of day light Which parts of the earth are in darkness and light? For how long?

11. Tropic of Cancer at 23.5º N Tropic of Capricorn at 23.5º S Arctic Circle (66.5 º N) Antarctic Circle (66.5º S) Winter Solstice: - Tropic of Capricorn gets the most direct rays - Southern Hemisphere has the longest days - 24 hours of day light at the Antarctic Circle

12. EQUINOXES VERNAL, MARCH 21 AUTUMNAL, SEPTEMBER 21

13. Effects of the Seasons Changes in solar intensity Changes in solar altitude Changes in day length Changes in temperature All of these changes are most extreme at high latitudes and minimized at the equator.

14. Daylight Savings Time Day starts earlier April 1 - October 31 Started during war time to save energy (maximize morning sun and increase work day). Arizona and Hawaii don’t use it. Navajo and Hopi do - causes great confusion.