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Yearly Motion: Earth orbits Sun PHYS 162 Class 2 1 Stars and Planets • Stars are “fixed” relative to each other. They produce their own light which is independent of Sun’s location (thus indicating they are very far away - the Greeks understood this) • Planets have complicated (but predictable) orbits when viewed from the Earth. Wanderers. Brightness does depend on Sun. Small numbers of such objects (5 planets visible to unaided eye) PHYS 162 Class 2 2 Stars and Planets • What stars you see in a given night changes through the year. And depends on latitude (NorthSouth location) • Planets are located along the ecliptic and which you can see at any time changes each month/year Today, mini-Planetarium program followed by a simple “star wheel” and star locations. PHYS 162 Class 2 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 • short bright trail near the NCP was made by Polaris • Show Polaris is not exactly due north. • If use for navigation have correction factor. See Celestial Air Navigation, War Department Technical Manual TM 1-206, 1941 (carried in kit bag by all USAAF navigators) 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). Viewing particular constellation at certain time/day also gives observer’s latitude 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 East-West 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) • Stars (plus accurate clock) give EW location (longitude). Used for navigation pre-1950 (in WW II) 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 Solar System • Sun plus 8 (or 9 with Pluto) planets many of which have moons • plus “debris”: comets, asteroids, meteors, etc • We’ll go over historical understanding of motion (which is “complicated” when viewed from the Earth) and later look at Solar System formation, planetary atmospheres, and planets discovered in other star systems PHYS 162 Class 2 29 Solar System distances to Sun clearly not to scale) PHYS 162 Class 2 30 Sun vs Earth. 100 times larger radius 1,000,000 times larger volume and 300,000 times larger mass PHYS 162 Class 2 31 Solar System - Orbits PHYS 162 Class 2 32 Mean Distance Sidereal Mass from Sun Orbital Period AU Mercury 0.387 Venus 0.723 Earth 1.000 Mars 1.524 Jupiter 5.203 Saturn 9.537 Uranus 19.191 Neptune 30.069 Pe 0.241 0.615 1.000 1.881 11.857 29.424 83.749 163.727 PHYS 162 Class 2 Me 0.055 0.815 1.000 0.107 317.828 95.161 14.536 17.148 33