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

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

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 ...
View Full Document

Ask a homework question - tutors are online