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Cycle of the Sky • • To understand the apparent motions of celestial objects To introduce the seasons So MANY objects….How Do We Make Sense of it ALL?? The Sun is physically 400 times larger than the Moon. Why is their angular size is the same? Angle diameter (in radians) = diameter/distance Full circle = 2 (in radians)=360 degrees=360o 1o=60 arminutes = 60’ 1’ = 60 arcseconds = 60” Angular Size Some Examples: Horizon to zenith (point overhead) 90o Your fist at arm's length 10o Your fingernail at arm's length 1o = 60' Sun or Moon seen from Earth 0.5o = 30' Smallest detail visible to naked eye 1' = 60'' Smallest detail visible by a single telescope from Earth's surface: 1'' Angular size of Pluto: 0.15” Betelgeuse (largest star) seen from Earth 0.004'' Constellations 88 official constellations divide the sky into areas with clearly boundaries. The names of constellations are in Latin. But most bright star names derived from ancient Arabic. The original constellations were invented by farmers over 5000 years ago. Remember that the constellations are not real! Projection Big Dipper ---- an asterism How long to go all the way around? 1 day b. 1 week c. 1 month d. 1 year Are there stars in the sky during the daytime? There are two ways to think about the motion of the skey: - the C.S. rotates - or the Earth rotates while the sky sits still. Sometimes one works better than the other … Luminosity and Magnitude • mV = -2.5*Log(I) + C • m1 – m2 = -2.5*Log(I1/I2) • I1/I2 = 10-0.4*(m1 – m2) = 2.512(m1 – m2) Latitude and longitude make a convenient coordinate system for locating objects on the Earth. Altitude and Azimuth give us the direction to look on the celestial sphere. The altitude of Polaris above the horizon is approximately the same as the observer's latitude in the Northern Hemisphere. Time Zones and Universal Time Time The time for many astronomical events is given in Universal Time (UT), which is (approximately) the local time for Greenwich, England --- the Greenwich Mean Time or GMT. Cycles of the Sky Earth's rotation causes the sun and stars to rise in the east and set in the west daily. But not everywhere! Near the poles, the sun rises and sets every half year, and circumpolar constellations such as Ursa Major, never set. What other cycles do you know? The constellations along the ecliptic are called the zodiac What causes the seasons? What are the differences between days in summer and winter? Longer days stronger Sun in summer than in winter Are the seasons due to the changing distance from the sun? Distance doesn't matter here! (Not to scale) Seasons are due to the tilt of the earth. Assigned Reading • Chapter 2