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AS101 Lecture 5

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Patrick Mc Graw

Lecture 5 1/21/2013 12:24:00 PM Cycles of the Sky, Part I Reading: Chapter 2. Quasar: a type of extremely bright galactic core containing a super-massive black hole. A teaser:  At what time of year is the Earth closest to the sun?  The earth is closest to the sun at the beginning of January.  Our winter is at the same time as the Southern Hemisphere’s summer, and vice versa.  So. Any explanation of the seasons must explain why they are opposite in the northern and southern hemispheres, and cant depend only on our distance from the sun. Quick Review:  Declination and right ascension are the equivalent of latitude and longitude for the celestial sphere: o Declination: angular distance north or south from the celestial equator (negative if south) o Right ascension: an angle measure east from the vernal equinox, usually measured in hours.  A few terms describing the local sky wherever you are standing: o Zenith: the point directly above your head:  Meridian: a north-south line passing through the zenith. Sometimes also called the celestial meridian.  Horizon: all points at 90˚ away from the zenith. (where the ground meets the sky) Other occasionally useful terms:  Great circle: o On the surface of any sphere, a circle whose centre is the same as the centre of the sphere (for example, the equator is a great circle, but other latitude lines are not.)  Hour circle: o Any great circle passing through both celestial poles—or, a line of right ascension, equivalent to longitude lines on Earth. Circumpolar Stars:  A star that never sets, but always stays above the horizon.  Which starts are circumpolar depends on your latitude o At the north pole, all visible starts are circumpolar. o At the equator, none are circumpolar. How do we figure out which stars re circumpolar?  Suppose your latitude is 40˚N.  Then the north celestial pole is 40˚ above the horizon.  Then any star less than 40˚ away from the NCP can never be below the horizon.  That means, circumpolar stars have declinations bigger than 90 minus whatever your latitude is. (90 – 40 = 50˚)  More generally, circumpolar stars have declinations bigger than 90 minus y
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