AST 1002: "Study Guide" Table 2

[Sections Taught by Prof. H.L. Cohen]

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Last updated January 1, 2003
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Table 2. Primary and Secondary Motions of the Heavens
Motion Object Direction Greatest   
Elongation (1)		 Approximate Average
Rates of Motion on Sky
Primary
Daily or
Diurnal		 All
Celestial
Objects		 East to West . . . . . 360°/day = 360°/24 hours
              = 15°/hour
Secondary
Annual or
Yearly		 Sun West to East . . . . . 360°/year = 360°/365 days
               = 1°/day
               = 2 diameters/day
Moon
("Monthly")		 Moon West to East 180°
(opposition)		 360°/27.3 days = 13°/day
                       = 13°/24 hours
                       = ½°/hour
                       = 1 diameter/hour
Inferior Planets (2)
  Mercury West to East prograde) (3)
East to West (retrograde)		 18° to 28° 360°/88 days   = 4°/day
  Venus West to East (prograde)
East to West (retrograde)		 45.9° to 46.7 360°/225 days = 1½°/day
Superior Planets (4), (5)
All
Others
(except
    Earth) (6)		 Mars
Jupiter
Saturn
Uranus
Neptune
Pluto 		West to East (prograde) (7)
East to West (retrograde)
From just before
to just after
opposition		 180°
(opposition)		 360°/1.8  years = 0.52°/day   (Mars)
360°/12   years = 0.08°/day   (Jupiter)
360°/29   years = 0.03°/day   (Saturn)
360°/84   years = 0.01°/day   (Uranus)
360°/165 years = 0.006°/day (Neptune)
360°/248 years = 0.004°/day (Pluto)
    Table Notes
  1. Elongation is the angular separation of an object from the Sun on the sky. Both Mercury and Venus reach
    greatest elongations     much less than 180°. (Notice the greatest elongation of Mercury is highly variable.)
  2. Inferior planets have average rates faster than the Sun (see last column of table).
  3. The usual direction of an inferior planet is West to East (prograde).
  4. Superior planets have average rates slower than the Sun (see last column of table).
  5. Superior planets always appear brightest when at opposition to the Sun (an elongation of about 180°).
  6. The Earth is neither an inferior nor a superior planet. (The Earth is not a celestial object!)
  7. The usual direction of a superior planets is West to East (prograde).
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