1. Earth-Sun Relationships The Reasons for the Seasons

2. Solar Radiation The earth intercepts less than one two-billionth of the energy given off by the sun. However, the radiation is sufficient to provide 99.9% of the energy that heats the earth / drives our climate.

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

4. 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 diffused in the polar regions.

5. The Earth is curved and this cause differing angles of incidence.

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

7. 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)

8. Seasons Seasons on Earth are created by The tilt of Earth Axis Relative to The Plane of the Ecliptic Earth’s tilt allows the Subsolar Point to Migrate during the year

9. Earth’s Tilt Relative to Orbit Earth’s Axis is tilted 66.5 degrees from its Plane of Ecliptic (Plane of Earth’s Orbit) Sometimes this Is expressed as 23.5 degrees from a line Perpendicular To The Plane of the Ecliptic

10. 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

11. 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

12. 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

13. 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?

14. 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?

15. EQUINOXES VERNAL, MARCH 21 AUTUMNAL, SEPTEMBER 21

16. 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.