Observer's Notebook for June 1999

Planets of Summer

by John Rummel

The motions of seven celestial bodies perplexed ancient astronomers for thousands of years. The motion of the fixed stars was relatively well understood - the were part of the "sphere of the stars" and as such, moved in harmony with one another. The moon and the sun also had their own spheres, and thus moved independent of the stars, but still in a way that could be reliably predicted. What stumped them was the motions of the other starry wanderers - the five planets. Mercury and Venus were a natural grouping, never straying far from the sun, and displaying predictable patterns of alternately being visible before sunrise or after sunset.

What defeated early astronomers were the remaining three of these seven important bodies: Mars, Jupiter, and Saturn. Their movements seemed to defy all models of the universe which worked so well for the other heavenly bodies.

These three objects drifted against the backdrop of the fixed stars for awhile, following a predictable west to east motion (as measured against the backdrop of the constellations), but would seeming stop, reverse their course for awhile, stop again, and then resume easterly motion. Early astronomers invented some fairly elaborate theories to account for this apparent backward motion, but it was not fully understood until Copernicus and Galileo firmly placed the sun at the center of the solar system, and Kepler and Newton worked out the motions of the planets.

The backward, or retrograde, motion of the planets is simply caused by our uniquely Earthbound vantage point. As the Earth swings around the sun, it overtakes the more distant planets, which move about their orbits more slowly. As the Earth overtakes and passes the outer planets, they appear to move backwards for a time against the backdrop of the stars.

Since mid-March, Mars has been pursuing its retrograde (backward) path creeping closer and closer to the bright star Spica. In early June, it will reach its closest approach, reverse its path, and begin its easterly journey again. As June opens, Mars is just about 2 degrees to the upper left (northeast) of Spica. After seeming to hover in place for a few nights, it will begin to slowly move off to the left. By the end of June, it will be nearly 5 degrees to the left. By mid-July, that distance will have doubled to 10 degrees. Because of its proximity to such a bright star, its motion will be easy to observe as the summer progresses.

The other summer planet, Venus, is holding court as the brilliant evening star, fully dominating the western sky after sunset for the entire spring and summer. As June opens, Venus is entrenched in Gemini, flying in formation with bright Castor and Pollux on June 1st. As the month progresses, Venus departs Gemini and moves into the dim constellation Cancer, who's stars are tough to make until the sun is well down and the sky is very dark. Cancer is home to a wonderful binocular star cluster - M44, more popularly known as the Beehive cluster. By June 10th, Venus is just 2 degrees to the right of the Beehive. With binoculars, you should be able to make out a dense cluster of dim stars just to the left of the brilliant arc of the planet. By June 12, the planet is less than a degree away, nearly superimposing itself on top of the distant formation of stars. By the time it gets dark enough to really appreciate the view, the whole group is sinking quickly toward the western horizon, so start your viewing while the sky is still a bit dusky. With Venus as your guide, seeing the Beehive will be a lock.

If you have a telescope, watch Venus throughout June as its phase slowly changes from a half-moon to a clear crescent, and as its size continues to expand as Earth slowly overtakes it throughout June and July.

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