Sun, Moon and Planetary Observational Facts

by John P. Pratt
©1998 by John P. Pratt. All rights Reserved.

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The following are useful facts to remember when you are actually going to look at the planets. First, the moon and visible planets are all orbiting the sun within 7° of the plane of the earth's orbit (the ecliptic), and so they all appear to go through the same background stars. Those stars are divided into 12 constellations, called the zodiac constellations. Even though we know the earth is orbiting the sun, it still appears that the sun is orbiting the earth, just like it did before Copernicus came along. Thus, the sun appears to move through the zodiac constellations once a year, but of course, we can't see them with the sun there. The effects is that the stars opposite the sun which are up at night slowly change during the seasons. Because all of the planets are orbiting the sun in the same direction, the sun, moon and planets all appear to move from west to east (most of the time) across the zodiac constellations. The outer planets (from Mars on out) all appear to have retrograde motion (east to west) for a few months (when the rise about sunset), as the earth passes them in orbit. The inner two planets (Mercury and Venus) both appear as both evening and morning stars because they can never be opposite from the sun in the sky, and they are also often seen in retrograde motion.

ObjectOrbital PeriodObservational Fact to Remember
Moon30 days
(period of phases)
Moves east its own width (1/2°) hourly,
or about 12° per day
Rises about 50 minutes later every night.
Mercury1/4 year
(So it has 4 - 1 = 3 evening/morning star cycles each year)
Like a bright star, but near the sun
Evening 40 days
Morning 40 days
Invisible 40 days
Venus8/13 year
(So it has 13 - 8 = 5 evening/morning Star cycles in 8 years)
Blazing Bright.
Evening 9 months
Morning 9 months
Invisible 2 months
Sun1 year (as seen from earth)
Moves 1š daily east through stars, or one zodiac constellation monthly
Sun's apparent motion thru stars causes stars to rise 4 minutes earlier each night, or two hours earlier every month
Mars2 years
(Both sidereal and synodic)
Like a red star.
Retrograde for 2-3 months.
Invisible for 4 months of 2 years.
Jupiter12 years
(one zodiac constellation per year)
Brighter than any star.
Retrograde 4 months yearly.
Invisible 1 month.
Saturn30 years
(counts years like moon counts days)
Looks like a bright star.
Retrograde 5 months yearly.
Invisible 1 month.
Uranus84 = 12 x 7 years
(One zodiac constellation every 7 years)
Visible in binoculars.
Retrograde 5 months yearly.
Invisible 1 month.