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Transcript
The Year Two Indicators • Due to the Earth’s tilt the Length of the Day and Sun’s path through the sky vary. One year = returns to the same spot More dramatic further north (Stonehenge) • Which stars are overhead changes with seasons. Gives passage of year Passage of time at night also given by stars’ apparent motion Stars = Calendar and Clock Can be used to navigate (used up to about 1950) PHYS 162 Class 1 1 Yearly Motion: Earth orbits Sun PHYS 162 Class 1 2 Stonehenge PHYS 162 Class 1 3 Star and Planet Locator (aka Planisphere or Star Wheel) Date Time Pole star Northern hemisphere Horizon PHYS 162 Class 2 4 Star Wheel • Stars “move” East to West over the course of one Night (in circle about the North Star) • Stars “move” East to West by 2 hours per month and “return” to the same position after one Year • It’s just caused by Earth’s daily spin and yearly orbit about the Sun • Star wheel depends on latitude: northern vs southern hemisphere completely different PHYS 162 Class 2 5 Constellations • Stars which are “close” to each other (in angle) form patterns called constellations • Unchanging in 10,000 year timescales but position in sky varies with season • Geological timescales stars move 10 million AD Today PHYS 162 Class 2 6 Constellations - Orion PHYS 162 Class 2 7 Orion (Greek) or Osiris (Egypt) pyramids: in same pattern? PHYS 162 Class 2 8 Orion – Star Distances Stars are at different distances Rigel 773 Light Years(LY) Betelguese 427 LY Nebula Saiph 1300 LY 720 LY Ballatrix (rt shoulder) 243 LY PHYS 162 Class 2 9 Different seasons, different constellations PHYS 162 Class 2 10 Different seasons, different constellations PHYS 162 Class 2 11 Different hemispheres, different constellations Sky seen at North pole (up) and South pole (right) PHYS 162 Class 2 12 Polaris - The North Star • Polaris is almost directly overhead at the North Pole. • Polaris is about halfway up from the north horizon in DeKalb. • Polaris is at the north horizon at the Equator. • Polaris is not visible south of the Equator. • Polaris doesn’t “move” due to the Earth’s rotation. • No equivalent “South Star” PHYS 162 Class 2 13 Rising and Setting Stars • Other stars “move” in circles about Polaris. • It takes 1 day to complete the circle. • The rising and setting time of a star changes with the seasons. • Stars are directly overhead (zenith) 2 hours earlier each month. PHYS 162 Class 2 14 Star trails in the northern sky • As the earth spins on its axis, the sky seems to rotate around us. This motion produces the concentric trails traced by the stars in this time exposure of the night sky. The north celestial pole (NCP) is at the center. PHYS 162 Class 2 15 Star trails in the northern sky • The very short bright trail near the NCP was made by Polaris, commonly known as the North Star. • So this is proof that Polaris is not exactly due north. PHYS 162 Class 2 16 Star trails in the southern sky • While the bright star Polaris lies conveniently close to the North Celestial Pole, no bright star similarly marks the pole in the South. • Still, the South Celestial Pole is easily identified in the picture as the point in the sky at the center of all the star trail arcs. PHYS 162 Class 2 17 Star trails at mid-latitudes • Star trails from a lower latitude (trails set beneath the horizon) PHYS 162 Class 2 18 Compare rising and setting at high and low latitudes PHYS 162 Class 2 19 January and April – near Sunset PHYS 162 Class 2 20 July and October – near Sunset PHYS 162 Class 2 21 Winter triangle PHYS 162 Class 2 22 Summer triangle Vega Altair Deneb PHYS 162 Class 2 23 What to Remember - NS • Polaris (the North Star) doesn’t “move” due to the Earth’s rotation. • The angle of Polaris to the north horizon gives the observer’s latitude Polaris Location Latitude directly overhead 900 N (NP) about halfway up 420 N (DK) on horizon 00 (Equator) below horizon Southern hemisphere • Other stars “move” in circles about Polaris. 1 day to complete circle. Angle between star and Polaris gives declination (N-S location) PHYS 162 Class 2 24 What to Remember - EW • What time during the day a star rises, is overhead, and sets changes with the seasons • look up on Star Chart (right ascension is the EastWest location) • Changes 2 hours/month • Only on the Equator can all stars be viewed from a single location Hawaii or northern Chile a good place for telescopes (all of northern or southern hemisphere plus a little bit of the other) PHYS 162 Class 2 25 Zodiac Constellations (not on tests) • a line between the Earth and the Sun traces out a “circle” in the sky • called the ecliptic • the 12-13 constellations along the circle are the “signs” of the Zodiac • changes with the seasons PHYS 162 Class 2 26 Zodiac Constellations (not on tests) PHYS 162 Class 2 27 Zodiac Constellations “modern” may add Ophiuchus between Sagittarius and Scorpius PHYS 162 Class 2 28