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Class 3: End of Ch 2 and Ch. 3

Class 3: End of Ch 2 and Ch. 3. Last time: 2.1. Patterns in The Sky: Stars and constellations, TODAY: celestial coordinates 2.2 Seasons,…Precession 2.2 (cont.). 2.3. The Moon and Eclipses Lunar Phases and Eclipses 2.4 Ancient Mystery of the Planets:

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Class 3: End of Ch 2 and Ch. 3

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  1. Class 3: End of Ch 2 and Ch. 3 Last time: 2.1. Patterns in The Sky: Stars and constellations, TODAY: celestial coordinates 2.2 Seasons,…Precession 2.2 (cont.). 2.3. The Moon and Eclipses Lunar Phases and Eclipses 2.4 Ancient Mystery of the Planets: Apparent Retrograde motion of planets Parallax Ch 3 (Histrory of Astronomy)

  2. The Celestial Sphere North & South celestial poles the points in the sky directly above the Earth’s North and South poles celestial equator the extension of the Earth’s equator onto the celestial sphere ecliptic the annual path of the Sun through the celestial sphere, which is a projection of ecliptic plane

  3. The Celestial Sphere

  4. The Milky Way You’ve probably seen this band of light across the sky. What are we actually seeing?

  5. The Milky Way • Our Galaxy is shaped like a disk. • Our solar system is in that disk. • When we look at the Milky Way in the sky, we are looking along that disk.

  6. Measuring the Sky • Full circle = 360º • 1º = 60 arcmin • 1 arcmin = 60 arcsec We measure the sky in angles, not distances.

  7. Measuring Angles in the Sky

  8. The Local Sky zenith the point directly above you horizon all points 90° from the zenith altitude the angle above the horizon meridian due north horizon zenith due south horizon

  9. Review: Coordinates on the Earth • Latitude: position north or south of equator • Longitude: position east or west of prime meridian (runs through Greenwich, England)

  10. Review: Coordinates on the Earth • Latitude: position north or south of equator • Longitude: position east or west of prime meridian (runs through Greenwich, England) • In Celestial sphere the equivalent coordinate are • Declination: (more later) • Right Ascension: (more later)

  11. The Daily Motion • As the Earth rotates, the sky appears to us to rotate in the opposite direction. • The sky appears to rotate around the N (or S) celestial poles. • If you are standing at the poles, nothing rises or sets. • If you are standing at the equator, everything rises & sets 90 to the horizon.

  12. Time Exposure Photograph:Star Trails

  13. The Daily Motion daily circles --- CCW looking north, CW looking south

  14. Annual Motion • As the Earth orbits the Sun, the Sun appears to move eastward with respect to the stars. • The Sun circles the celestial sphere once every year.

  15. 2.3 Seasons • What is the cause of the seasons on Earth?

  16. Annual Motion • The Earth’s axis is tilted 23.5° from being perpendicular to the ecliptic plane. • Therefore, the celestial equator is tilted 23.5° to the ecliptic. • As seen from Earth, the Sun spends 6 months north of the celestial equator and 6 months south of the celestial equator. • Seasons are caused by the Earth’s axis tilt, not the distance from the Earth to the Sun!

  17. Annual Motion ecliptic the apparent path of the Sun through the sky (also the plane of Earth’s orbit) equinox where the ecliptic (the Sun) intersects the celestial equator solstice where the ecliptic (the Sun) is farthest from the celestial equator zodiac the constellations which lie along the ecliptic

  18. The Cause of the Seasons

  19. Coordinates on the Celestial Sphere (not in book) • Latitude: position north or south of equator • Longitude: position east or west of prime meridian (runs through Greenwich, England) • Declination: position north or south of celestial equator (in degrees) • Right Ascension: distance (in hours, 0 to 23h 59 min.) East of vernal equinox (where the sun crosses the celestial equator going North)

  20. Question 1 What point on the celestial sphere is defined by a Right Ascension of zero hours and a Declination of zero degrees?

  21. Question 2 What point on the celestial sphere is defined by a Right Ascension of 12 hours and a Declination of zero degrees?

  22. Question 3 What is an equinox?

  23. Question 4 What point on the celestial sphere is defined by being on the ecliptic at a Declination of +23.5 degrees?

  24. Question 5 What point on the celestial sphere is defined by being on the ecliptic at a Declination of minus 23.5 degrees (i.e. 23.5 degrees South)?

  25. Axis tilt causes uneven heating by sunlight throughout the year.

  26. When is summer?(in the Northern Hemisphere) • The solstice which occurs around June 21 is considered the first day of summer. • However, it takes time for the more direct sunlight to heat up the land and water. • Therefore, July & August are typically hotter than June.

  27. 2.4 Precession • What is the Earth’s cycle of precession?

  28. Thought QuestionWhat is the arrow pointing to?A. The zenithB. The north celestial poleC. The celestial equator

  29. Thought QuestionWhat is the arrow pointing to?A. The zenithB. The north celestial poleC. The celestial equator

  30. Precession • The Earth’s axis precesses (wobbles) like a top, once about every 26,000 years. • Precession changes the positions in the sky of the celestial poles and the equinoxes. • Polaris won't always be the north star. • However the tilt in the axis is the same (23.5 degrees) as the Earth’s axis precesses

  31. Question A. Will Polaris be the North star in another 13,000 years?

  32. Question B. Will Polaris be the North star in another 26,000 years?

  33. 2.3 The Moon, Our Constant Companion • Why do we see phases of the Moon? • What conditions are necessary for an eclipse?

  34. Lunar Motion Phases of the Moon’s 29.5 day cycle • new • crescent • first quarter • gibbous • full • gibbous • last quarter • crescent waxing waning

  35. Lunar phases and the month • A cycle of lunar phases is approximately 30 days and is the reason why we divide the year in 12 months • Why do lunar phases occur on different calendar dates from one year to the next?

  36. Lunar phases and the month • A cycle of lunar phases is approximately one month and is the reason why we divide the year in 12 months • Why do lunar phases occur on different calendar dates from one year to the next? • Because one year (365 days) is not exactly 12 times the period of the Moon’s phases (its actually 12.4 times).

  37. Why do we see phases? • Half the Moon illuminated by Sun and half dark • We see some combination of the bright and dark faces

  38. Moon’s position at sunset for waxing phases (numbers are days since new Moon)

  39. Phases of the Moon

  40. Why do we see the same side on our Moon? Rotation period = orbital period

  41. Eclipses • The Earth & Moon cast shadows. • When either passes through the other’s shadow, we have an eclipse. • Why don’t we have an eclipse every full & new Moon?

  42. Eclipses When the Moon’s orbit intersects the ecliptic (node): at new moon solar eclipse • you must be in Moon’s shadow to see it • within umbra: total solar eclipse • within penumbra: partial solar eclipse at full moon lunar eclipse • everyone on the nighttime side of Earth can see it

  43. Solar Eclipse

  44. Question What will be the Right Ascension and Declination of the Moon during a total solar eclipse on September 21?

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