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Chapter 2

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John Lester

Chapter 2 - Discovering the Sky (Sept. 12) Lecture Notes Constellations - Stars in a constellation may not be close to each other in space - just appear grouped from Earth (star charts on p A-28 to 31 in Appendix 1) - Constellations serve as a background used to detect the motions of solar system objects Local Sky - Our location on Earth determines our horizon, zenith and meridian Angles - Degrees - 360 full circle - Minute of arc - 1’ = 1 degree / 60 - Second of arc - 1” = 1’ / 60 = 1 degree / 3600 Angles and Distance - We see the angle of an object - That angle depends on its distance from us Cosmic Calculation with Angles angular size / 360 degrees = diameter / circumference=2pi x distance - We measure angular size and distance, solve for diameter diameter = angular size x 2pi x distance / 360 degrees Apparent Rising and Setting - Earth’s counterclockwise rotation causes all astronomical objects to appear to rise/set every day Sept. 14 Sun’s Rising/Setting - Solar Time - The meridian tracks the sun (meridian being at the highest point of the sun) - Sun arrives on the meridian line at noon and midnight - Spinning and orbiting around the sun - Star (Sidereal) Time - same idea based on meridian - Spinning, NOT orbiting around the star(s) Seasons and Calendars - A common misconception - seasons are caused by Earth being closer to or farther from the Sun - WRONG - Earth is actually closest to the Sun in January because the Sun’s angular size is largest (looks larger because we are closer to it) - Earth is farthest from the Sun in July - smallest angular size - Summer in Toronto = winter in Australia - The tilt of Earth’s spin (23.5 degrees) has two effects: - Concentration of sunlight on the ground - When Earth is tilted toward the Sun the sunlight hits the ground more directly, away from the Sun hits at a slanting angle ▯ - Number of hours of daylight ▯ ▯ - When Earth is tilted toward the Sun it is up for more than 12 hours, away ▯ ▯ from the Sun less than 12 hours Moon - Does the Moon orbit Earth? Yes, how do we know? - We see the Moon move in front of the background star patterns - Orbit takes about 27.3 days - the Moon returns the the same constellation of stars - The orbit is not circular Why does the Moon have Phases? - Waxing crescent - increasing crescent from night to night - Also due to the Moon’s orbit around the Earth - Reflects sunlight, does not “glow” - Day side - facing toward the Sun - Night side - facing away from the Sun - As the Moon orbits Earth, we see different amounts of the day and night sides - New moon - when only the night side is visible (therefore not visible at all) - Full cycle of phases takes 29.5 days, no
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