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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- Fall '09
- Coordinate systems, various coordinate systems, Equatorial Celestial Coordinates