The%20equatorial%20%20%20%20%20%20%20co-ordinate%20system

1
The equatorial co-ordinate system

• Right Ascension
• defined within equatorial plane
• measured from 0-24 hrs, increasing eastwards
• 0h is direction of sun at vernal equinox
• Declination
• measured perpendicular to RA
• due north is 90 deg, due south -90 deg.
• Precession
• axes precess due to earths 26000yr wobble, so
  co-ordinates need to be defined at a given epoch
  (e.g. J2000).

2
East and West..
N
Declination values increase as you go up (north)
in an image
E
Right Ascension increases to the East (left)
3
RA, Dec worked example

• Two quasars are located at
• RA 15h46m37.45s, Dec 170625.6"
• and
• RA 15h46m58.13s, Dec 170347.1"
• What is the angular separation of these two
  quasars on the sky (in arcsec)?

4
Question on Right Ascension and
Declination(from Sept 2000 past exam paper)