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ay45c2-page3 - If the earth was a sphere this would not a...

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Unformatted text preview: If the earth was a sphere, this would not a ect the Earth's spin axis (there would be no coupling), but tides and rotation distort the earth, so it feels a net torque from the gravity of the sun/moon. This torque makes the polar axis (direction to NCP) precess with a period of 26,000 years. spin axis now precession of spin axis 23.5 Every 26,000 years, the RA goes through a big loop, and DEC changes by 23:5. Positions of objects in the sky change by tens of arcseconds per year. This is easily detectable, as we can measure the angle to 10 3 arcsec. Coordinates quoted for objects are therefore refered to a particular date or \standard epoch" to remove e ects of precession. Standards are  B1950 (going out of use)  J2000 (coming in) Here the \B" and \J" refer to technicalities of the model, while \1950" and \2000" are the reference dates (typically noon on January 1 of the date). Galactic Coordinates (l; b) The equator of the galactic coordinate system is the galactic plane (the Milky Way). l, the longitude coordinate, is zero in the direction of the Galactic Center. b, the galactic latitude goes from +90 at the North Galactic Pole (NGP) down to 90 at the SGP. 12 ...
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