1. Earth-Sun Relationships The Reasons for the Seasons

2. 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

3. 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) As a result, solar radiation is diffuse in the polar regions.

4. 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)

5. 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)

6. 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

7. SOLSTICES SUMMER, JUNE 21 WINTER, DECEMBER 21 Where are the overhead rays of the sun on these days? Which parts of the earth are in darkness and light? For how long?

8. 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) Notice these four important parallels. Where do they occur? Why?

9. EQUINOXES VERNAL, MARCH 21 AUTUMNAL, SEPTEMBER 21

10. Rays Direct Occurs when the energy from the sun hits the earth in a concentrated manner Indirect When energy from the sun contacts the earth at an indirect angle.