Celestial Sphere Explained: Understanding Earth's Patterns in the Sky

1. Lecture Slides CHAPTER 2: Patterns in the Sky—Motions of Earth and the Moon Understanding Our Universe SECOND EDITION Stacy Palen, Laura Kay, Brad Smith, and George Blumenthal Prepared by Lisa M. Will, San Diego City College

2. Patterns in the Sky What do we observe on the sky due to the: • Rotation of Earth? • Revolution of Earth? • Axial tilt of Earth? • Orbit of the Moon?

3. Rotation of the Earth • One day is 24 hours. • One rotation with respect to stars = 23 hr 56 min = sidereal day • One rotation with respect to the Sun = 24 hours = solar day • Earth rotates counterclockwise and also orbits the Sun in the counterclockwise direction.

4. Class Question The Sun rises in the east and sets in the west. Which way is Earth rotating? • Toward the west • Toward the north • Toward the east • Toward the south

5. Celestial Sphere • The celestialsphereis a projection of Earth’s axes and equator into space. • The celestial sphere rotates around the north and south celestial poles each day.

6. Celestial Sphere: Celestial Equator and Ecliptic • Celestial equator: midway between the two poles. • Ecliptic: apparent path of the Sun, inclined 23.5 degrees to equator.

7. Celestial Sphere: Earth’s North Pole • At Earth’s North Pole, we would see half of the celestial sphere. • The north celestial pole would be at the zenith, the point directly overhead.

8. Celestial Sphere: Circumpolar • No star rises or sets =>circumpolar. • Observers never see the south celestial pole.

9. Celestial Sphere: Circumpolar (Cont.)

10. Celestial Sphere: Circumpolar (Cont.)

11. Celestial Sphere: Circumpolar (Cont.)

12. From what location on Earth would all of the stars on the celestial sphere be visible? Equator North Pole Prime Meridian South Pole Class Question

13. Celestial Sphere: Hemispheres • At Earth’s equator, all stars rise and set. • The celestial poles are on the northern and southern horizons.

14. What would the sky look like from a latitude of 30° N? All stars would be circumpolar. All stars would be visible. No stars would be visible. Some stars would be circumpolar; some would never be visible. Class Question

15. Celestial Sphere: Stars Visibility • Some stars are circumpolar. • Some stars are never visible.

16. Revolution of the Earth • The average distance to the Sun is called the astronomical unit, or AU. • 1 AU = 150 million km = 8.3 light-min

17. Revolution of the Earth: Zodiac As Earth revolves, the Sun is seen against different constellations—the zodiac. The constellations are along the ecliptic.

18. Axial Tilt of the Earth • Earth’s axis is tilted at an angle of 23.5°with respect to the ecliptic plane.

19. Axial Tilt of the Earth: In Summer The tilt causes the seasons. In summer: • The angle of sunlight is more direct. • Energy is more concentrated and days are longer.

20. Axial Tilt of the Earth: In Summer (Cont.)

21. Axial Tilt of the Earth: In Summer (Cont.)

22. Axial Tilt of the Earth:Summer and Autumnal Seasons • Summer solstice: • Sun is at its highest point above the ecliptic (June 21). • Longest day. • Autumnal equinox: • Sun on the celestial equator (Sept. 22). • Equal hours of day and night.

23. Axial Tilt of the Earth: Winter and Vernal Seasons • Winter solstice: • Sun at its lowest point below the ecliptic (December 21). • Shortest day. • Vernal equinox: • Sun on the celestial equator (March 20). • Equal hours of day and night.

24. Axial Tilt of the Earth: Winter and Vernal Seasons (Cont.)

25. Axial Tilt of the Earth: Winter and Vernal Seasons (Cont.)

26. Class Question If the ecliptic were aligned with the celestial equator, what would happen to the seasons? • Each season would last longer. • Nothing. They would be the same as now. • We would not have seasons at all. • Cannot tell from the information given.

27. Axial Tilt of the Earth:Changes Over Years • Earth’s axial tilt changes orientation over a period of 26,000 years due to precession. • The location of the poles slowly shifts. • Currently the north celestial pole is near the bright star Polaris.

28. Orbit of the Moon Synchronous rotation. The Moon completes one full rotation in one full revolution around Earth.

29. Orbit of the Moon:Moon From the Earth The Moon shines because of reflected sunlight. Half of the Moon is always illuminated by the Sun.

30. Phases of the Moon The phase of the Moon is determined by how much of the bright side we see.

31. Phases of the Moon: New and Crescent Moon • New Moon: Moon between Earth and the Sun. • Crescent: Less than half of the Moon is lit up from the Earth’s point of view.

32. Phases of the Moon: Quarter, Gibbous, and Full Moon • Quarter: Moon at right angles with Earth and the Sun. • Gibbous: More than half of the Moon is lit up from Earth’s point of view. • Full Moon: Moon on opposite side of Earth from the Sun.

33. At what phase of the Moon is a solar eclipse possible? First Quarter Full New Third Quarter Class Question

34. Eclipses: Solar Eclipse Solar eclipses happen at new Moon. Only a small portion of Earth can witness each one.

35. Eclipses: Solar Eclipse (Cont.)

36. Eclipses: Solar Eclipse (Cont.)

37. Eclipses: Types of Solar Eclipse Three types of solar eclipses: Total: the Moon completely blocks the Sun. Partial: only part of Sun is blocked. Annular: the Sun appears as a bright ring surrounding the Moon.

38. At what phase of the Moon is a lunar eclipse possible? First Quarter Full New Third Quarter Class Question

39. Eclipses: Lunar Eclipse Lunar eclipses occur at full Moon. Visible over a wider area of Earth. Longer duration than solar eclipses.

40. Eclipses: Lunar Eclipse (Cont.)

41. Eclipses: Lunar Eclipse (Cont.)

42. Eclipses: Lunar Eclipse (Cont.)

43. Eclipses: Lunar Eclipse (Cont.)

44. Do we observe eclipses every month? Yes No Class Question

45. Orbit of the Moon • Eclipses do not occur every month because the Moon’s orbit is tilted about 5 degrees with respect to Earth’s orbit around the Sun (the ecliptic).

46. If Earth rotated in 12 hours but its orbit were the same, which of the following would NOT be true? Each season would happen twice per year. We would still see only one hemisphere of the Moon. The cycle of lunar phases would last as long as it does now. Observers at the North Pole would only see one half of the celestial sphere. Class Question

47. Chapter Summary • The daily patterns of rising and setting => Earth’s rotation. • The annual patterns of the stars and constellations in the sky => Earth’s revolution. • The changes in the length of day and intensity of sunlight that create the seasons => Axial tilt of the Earth. • The phases of the Moon => Moon’s revolution around the Earth.