The Great Planet Race

July 20, 2010

Authored By James Wooten

James is the Planetarium Astronomer at the Houston Museum of Natural Science. He teaches students every school morning in the planetarium, and also answers astronomy questions from the public.

During July and August 2010, you can watch a great planet race as Venus and Mars close in on and then pass Saturn. Observing this will also give you a sense of how the ancients, even thousands of years ago, could distinguish the planets from the stars and from each other.

Thousands of years ago, the earliest astronomers noticed that they could form patterns with the stars.  They also noticed that these patterns remained the same throughout their lives and even across generations.

In contrast to these “fixed stars,” there were seven “wandering stars.”  Consistent observation revealed that five points of light in the night sky shifted position noticeably from night to night.  After a year, even the slowest of these was clearly “out of place.”   The other two “wanderers” were the sun and the moon.  From the Greek word for “wanderer,” today we call these moving objects planets.  (The sun and moon were thus “planets” until we understood the solar system better.)

Cellarius ptolemaic system

With more careful observation, we can clearly distinguish the planets from one another using only the naked eye.  One of the planets far outshines all the others, and in fact outshines everything in the sky except the sun and moon.  Ancients named this one after the goddess of love and beauty–Aphrodite for the Greeks and Venus for the Romans.  Another planet has a distinctly reddish tint compared to all of the others, whose light is closer to pure white.  This one has therefore borne the names of gods of war, such as the Greek Ares and Roman Mars.

“Jupiter of Smyrna” currently residing
in the Louvre in Paris, France.

An important way to distinguish among the planets was to observe them when several were close together and to note which ones moved faster compared to others.  The fastest moving planet, the one that always passed up the others (unless it appears to stop and head the other way), received the name of the swiftest god.  For the Greeks, this was the messenger god Hermes, for the Romans it was Mercury.  On the other hand, there were two planets so slow that the motion was barely noticeable from night to night, but could be detected over months or years.  These two were considered rulers of heaven since they were the farthest away.  After all, ancients noticed that faraway objects, such as ships sailing at the limit of their vision, seemed to be going slower than similar objects close by.    The very slowest planet is also the dimmest; any other planet at its brightest outshines it.  The second slowest, on the other hand, outshines all stars at night and all planets except Venus.  Thus, the Greeks identified the slowest planet with the former, deposed ruler of heaven (Kronos/Saturn).  The planet which is brilliant despite its great slowness and distance was the true ruler of heaven (Zeus/Jupiter).

Now, you can go outside and make these same types of observations. In July 2010, face west at dusk to find three planets.  Venus is by far the brightest, outshining all the planets and stars.  Mars and Saturn are to the left and  slightly higher in the sky.  Although they aren’t nearly as bright as Venus, Mars and Saturn easily outshine all stars in their immediate vicinity and are therefore just as noticeable.  Mars is between Venus and Saturn and slightly below a line joining those two.

During the rest of July, Mars will close the gap on Saturn, until by July 31 it appears less then 2 degrees under the ringed planet.  Meanwhile, Venus will have closed to less than 8 degrees to the right of the pair.  Keep watching in August as Mars pulls ahead of Saturn while Venus begins to form a compact triangle with them both.  Venus is less than 3 degrees below Saturn on August 8 as it continues to gain on Mars.  Finally, Venus catches up to Mars and is less than three degrees below it on August 19.  By the end of August, the three planets have reversed their order; Venus has become the leftmost of the three, while Saturn will be on the right.

As you keep observing, you will also notice that Venus begins to slow down a little once it has “won” the race.  Also, it begins to move off of the imaginary line joining Mars and Saturn.  This is due to the geometry of our solar system.  Mars and Saturn are traveling slower than earth on their outer orbits.  Thus, Earth is leaving them behind and will pass on the far side of the Sun from them.  That’s why we see Saturn (in September) and Mars (in December) drop into the Sun glare.

Venus, on the other hand, is orbiting inside Earth’s orbit and is therefore going faster than Earth.  We’re seeing Venus come out from behind the Sun, and then around to our side of the Sun.  That’s why Venus seems to slow down–it’s starting to move towards the Earth.  Once Venus is on our side of the Sun, we see it move backwards (or retrograde) against the background stars.  In October, when Venus “laps” the Earth, we can’t see it at all.

Viewers with perfectly clear horizons who observe right at dusk may also glimpse Mercury.  The elusive Messenger is to the lower right of the other three planets and roughly in line with them.  Mercury won’t catch up to the other three however.  By August, Mercury will have come around to our side of the Sun, so we’ll see it head back towards the Sun’s glare before it aligns with Saturn, Mars, or Venus.

The King of Planets, Jupiter, sits this one out.  he makes a grand entry into the eastern sky at dusk this September, is up all night long on September 21, and dominates the evening sky throughout the fall.

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