ay45c2-page2 -  1 mrad = 3:50  1 rad = 0:2100 2.1.2...

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Unformatted text preview:  1 mrad = 3:50  1 rad = 0:2100 2.1.2 Coordinate systems in the sky Positions of objects in the sky can be given in various coordinate systems. Equatorial Celestial Coordinates ( , ) or (RA, DEC) 23.5 spi 23.5 nv ect or The Earth is a pretty good gyroscope, so its axis points a constant direction in inertial space: the North Celestial Pole (NCP). Declination ( or DEC) is measured from +90 at that pole to 90 at the South Celestial Pole. The longitude-like coordinate is called \right ascension" ( or RA) and is measured (confusingly) in hours (24h= 360 ). The zero point is at the \vernal equinox" (where the Sun is in the sky at the beginning of Spring, also called the \ rst point of Ares"). RA increases in number in the direction the sky moves (as the Earth turns). That is, a xed telescope sees increasing RA positions with time (1 hour RA per hour of sidereal time). One peculiarity is that objects at rest on sky have RA/Dec which vary very gradually with time because of the Earth's precession. This occurs because the spin axis of the earth is not aligned with the Earth-Sun orbital plane. "6 mos. later" "now" 11 ...
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