Ch 01-02 Angles and Distances

• Angles (What role do angles play in astronomy?)
  
  
  
  Angular size
  
  For α <<1  , α ~ D/d  (in units of radian)
  
  1 radian = 180^◦ / π ~ 57.3^◦                          (degree)
  
                         ~ 3437.75 '     ( ~ 10^3.5 ' )    (arcmin)
  
                         ~ 206265 "     ( ~ 10^5.3 " )    (arcsec)
  
  e.g.     α ~ 30' for the moon
  
             d = 384,000 km
  
             D ~ αd = 30 x 10^-3.5 x 384,000 km
  
                       ≈ 3800 km      (actually, 3476 km)
  
  Positions in the sky (See Chapter 2)
   
• Units of distance
  
  1 AU = 1.496 x10⁸km ~ 1.5 x10¹³cm
  
  1 ly = 9.46 x10¹²km ~ 63240 AU ~ 9.5 x10¹⁷cm
  
  1 pc = 3.09 x10¹³km ~ 3.26 ly ~ 206265 AU ~ 3 x10¹⁸cm
  
  kpc, Mpc, Gpc