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Constellations

Constellations. Constellations -- groups of stars named by ancient cultures to honor gods, animals, legends, etc. They provide us with ways to recognize and identify individual stars and groups of stars (i.e., they are a crude “map” of the sky). How Many Constellations?.

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Constellations

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  1. Constellations Constellations --groups of stars named by ancient cultures to honor gods, animals, legends, etc. They provide us with ways to recognize and identify individual stars and groups of stars (i.e., they are a crude “map” of the sky).

  2. How Many Constellations? In all, there are 88 constellations; 12 in the zodiac (13 really). To learn more about constellations see: The Stars by H.A. Rey http://StarrySkies.com/ Sky & Telescope and Astronomy magazines

  3. Celestial Sphere The celestial sphere: Stars seem to be on the inner surface of a sphere surrounding the Earth They aren’t, but can use two-dimensional spherical coordinates (similar to latitude and longitude) to locate sky objects

  4. Eastern Sky in the Winter Orion is a primary Winter constellation

  5. Eastern Sky in the Summer The Summer Triangle

  6. Constellations, cont Stars that appear close in the sky may not actually be close in space:

  7. Big/Little Dippers From our latitude on the Earth, Polaris (the North Star) and the stars around it can be seen all night long. The stars appear to rotate around Polaris.

  8. Constellation Boundaries A constellation is not just the stars that make up a “picture”, it is an area of sky that borders the surrounding constellations.

  9. Celestial Sphere The celestial sphere: Stars seem to be on the inner surface of a sphere surrounding the Earth They aren’t, but can use two-dimensional spherical coordinates (similar to latitude and longitude) to locate sky objects

  10. Right Ascension and Declination • Declination: degrees north or south of celestial equator • Right ascension: measured in hours, minutes, and seconds eastward from position of Sun at vernal equinox

  11. The Celestial Sphere Equator => Celestial Equator North Pole => North Celestial Pole South Pole => South Celestial Pole Celestial Sphere is imaginary, not real. It is used for convenience.

  12. What happens when you need to know where an object is more accurately than “the fourth brightest star in Taurus”? Define a new coordinate system: the Celestial Sphere The celestial sphere is essentially a projection of Earth’s coordinate system of longitude and latitude out into space. longitude Right Ascension (RA) latitude Declination (Dec)

  13. So, instead of saying “The object is just to the left of Rigel in the constellation of Orion”, we have coordinates much like longitude and latitude on the Earth.

  14. SkyGazer Demo

  15. Rotation of the Earth Earth’s rotation around its axis: • Causes night and day (Solar day = average time between consecutive “noontimes”). • Causes (apparent) motion of the stars

  16. Years Time for Earth to orbit once around Sun, relative to fixed stars, is sidereal year Tropical year follows seasons; sidereal year follows constellations – in 13,000 years July and August will still be summer, but Orion will be a summer constellation

  17. Daily/Nightly Motion of the Stars The rotation of the Earth causes the stars in the night sky to rotate. Depending on where you are on the Earth, the pattern of rotation is different. Stars and constellations that never set (as seen from your location) are called circumpolar (they rotate around the pole).

  18. Circumpolar stars rotate around Polaris (in the Northern Hemisphere).

  19. At the North and South Pole, the stars move horizontally.

  20. On the Equator, the stars move vertically.

  21. At mid-latitudes, the stars move “diagonally” on an arc across the sky.

  22. Daily Motion Position tutorial page 1-2

  23. Yearly Motion of the Stars So far we have just talked about the motion of the stars over the course of a night (due to the Earth’s rotation). Over a day, the Sun will not appear to move with respect to the constellations. But over the course of months or a year, the Sun appears to move.

  24. Yearly Motion of the Stars 12 constellations Sun moves through during the year are called the zodiac; path is the ecliptic

  25. Yearly Motion Ecliptic tutorial page 13-17 (I-III, IV opt) Seasonal Stars tutorial page 7-9 (take home if don’t finish), maybe do this after Horoscopes depending on time. (Fall 2008 – completely ran out of time!)

  26. Today’s Horoscope click here

  27. Horoscopes Read your horoscopes. Which one sounds most like you? Your sign The previous sign The next sign Six signs later

  28. Precession – Why Your Sign is Wrong Precession: rotation of Earth’s axis itself; makes one complete circle in about 26,000 years

  29. Years: Two Types Time for Earth to orbit once around Sun, relative to fixed stars, is a sidereal year A tropical year is the time it takes for the Earth to be in the same configuration with respect to the Sun again (as opposed to distant stars). Tropical year follows seasons; sidereal year follows constellations – in 13,000 years July and August will still be summer, but the “Summer Triangle” will be a winter constellation.

  30. Next Time Seasons and Lunar Phases Reading: E2, E3 5.2 (12 pages) HW #1 Due

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