1. AST 111 Lecture 6 Seasons, Phases of the Moon and Planets

2. Earth’s Seasons Light is a form of energy that can heat things up Consider light coming from the sun hitting different sized areas – Which heats up faster?

3. Earth’s Seasons Now consider light impinging upon a large sphere. – Which parts get hottest? Coolest? – Consider how “sections” of light spread out

4. Earth’s Seasons Equal amounts of sunlight hit unequal amounts of area.

5. Earth’s Seasons What if Earth’s axis were perpendicular to the plane of the solar system? Temperature would vary with latitude, but not time of the year.

6. Earth’s Seasons In this picture, say you’re in Arizona. – Hotter in July or January?

7. So what causes the temperature to change, over the year, at a FIXED latitude?

8. Earth’s Seasons Earth’s axis is tilted. – This causes the seasons. – Tilted 23.5 o to Solar System plane

9. Earth’s Seasons Sunlight hits different parts of the Earth more directly or less directly during different parts of the year.

10. Earth’s Seasons Northern Hemisphere: Winter Northern Hemisphere: Summer

11. Earth’s Seasons Southern Hemisphere: Summer Southern Hemisphere: Winter

12. Tropics Summer in Northern Hemisphere: – Sun at zenith in Tropic of Cancer Summer in Southern Hemisphere: – Sun at zenith in Tropic of Capricorn What are the latitudes of each tropic?

13. Earth’s Seasons The vertical motion in the picture goes along with the seasons. The Sun is: – Closest to zenith in summer – Closest to horizon in winter

14. Terminology (applies to Northern hemisphere) Summer Solstice (June 21): – Most direct sunlight – Sun highest in the sky Winter Solstice (December 21): – Least direct sunlight – Sun lowest in the sky Spring (Vernal) Equinox (March 21): – Axis: tipped away to tipped toward Fall Equinox (September 21): – Axis: tipped toward to tipped away

15. Earth’s Seasons Common misconception: ellipticity of Earth’s orbit causes the seasons – Distances do not even correspond to the seasons. – North and South hemispheres would not have opposite seasons. – Earth’s orbit only slightly eccentric.

16. Elliptical Orbit Eccentricity greatly exaggerated. Summer Winter

17. Seasons Jupiter has an axial tilt of 5 degrees. Saturn has an axial tilt of 27 degrees. – Does Jupiter have mild seasons? – Saturn?

18. Phases of the Moon Waxing CrescentFirst QuarterNew MoonWaxing Gibbous Full MoonWaning GibbousThird QuarterWaning Crescent

19. Phases of the Moon How much of the Moon is illuminated by the Sun at any given time?

20. Phases of the Moon Moon orbits Earth every 27 1/3 days How does its orbit around Earth affect how much of the Moon we see?

21. Phases of the Moon The Moon shows phases because the half that is lit faces Earth differently.

22. Synchronous Rotation The Moon spins on its axis in the same time it completes an orbit around Earth. What does this imply about how we see the Moon?

23. Near Side of the Moon

24. Far Side of the Moon

25. Inner Planets Have Phases

26. Full Venus or Mercury? Do we ever see a “full Venus”? Or a “full Mercury”?

27. Inner Planets Have Phases

28. And the outer planets? From Earth: – The inner planets have phases – The outer planets do not Why?

29. Outer Planets Do NOT Have (significant) Phases Consider the geometry of the solar system Earth is always looking at the lit-up face of the outer planets

30. Outer Planets do NOT Have (significant) Phases Earth is much closer to the source of light than the outer planets, and so we see them more “from the Sun’s point of view”