Ch 02-02 Celestial Coordinate Sysems and the Precession of Earth Rotation

[Celestial coordinate systems]

• Horizontal
  Azimuth ( A ), 0^◦ ~ 360^◦, from north to the direction of east
  Altitude ( h ), or, zenith distance ( z )
  
  
  
   
• Equatorial
  
  
  
  Right ascension ( R.A., α), 0^h ~ 24^h, starting from the vernal equinox eastwards
  Declination ( Dec., δ), 0^◦ ~ +/- 90^◦
  
  
  
   
• Ecliptic
  
  
  
  Ecliptic longitude (λ),  0^◦ ~ 360^◦, starting from the vernal equinox eastwards
  Ecliptic latitude (β), 0^◦ ~ +/- 90^◦
   
• Galactic
  Galactic longitude ( l ), 0^◦ ~ 360^◦ starting from the galactic center eastwards
  Galactic latitude ( b ), 0^◦ ~ +/- 90^◦
  
  The galactic coordinate origin (l = 0, b = 0), defined by IAU in 1958, is at α = 17^h45^m37.2^s   δ = -28^◦56'10"    (Epoch 2000)
  
  Sgr A*, discovered in 1974, which is considered to be the rotational center of the Galaxy, is at α = 17^h45^m40^s   δ = -29^◦00'28"    (Epoch 2000).

[The precession of earth rotation]