The University Record, July 9, 1996

13th constellation visible in July evening skies, astronomer says

July's evening sky brings Michigan skywatchers a view of the 13th constellation in the Zodiac, the band of constellations through which the sun passes on its yearly journey among the stars, according to U-M astronomer Richard G. Teske.

"In mid-sky after dark is Ophiuchus, the Snake-Holder, who stands upon Scorpius holding aloft the constellation of Serpens, the Snake. The sun enters Ophiuchus in December each year. The constellation is visible in our evening sky now because the sun will not get there for another five months," Teske says.

"The sun's daily and yearly movement on the sky reflects our Earth's rotation on its axis and its orbital revolution around the sun. Less familiar is the sun's yearly circuit among the stars, because we cannot see the stars in daytime. Yet, like Ol' Man River, it keeps moving slowly along, covering the width of two full moons on the sky every 24 hours. Ancient astronomers knew about the sun's yearly trip and identified the pathway of constellations through which it travels. Today we call this strip the `Zodiac.'

"Even before the year 2000 B.C., Babylonian skywatchers in the Middle East had detected the sun's movement through the star patterns of the constellations and mapped the ones through which it passed. They divided its path into 12 sections, because the 12 full moons of a year occur in them successively. Each section was identified with a different star pattern. These became the 12 `houses' of our Zodiac."

In July, Michigan skywatchers who find the constellations Scorpius, Sagittarius and Ophiuchus after sunset will be looking at the places occupied by the sun at the end of each year.

"The brightest visible `star' low in the Southeast in July is the planet Jupiter. It guides observers to the location of Sagittarius, the Archer, where Jupiter can be seen just above the `Milk Dipper' of Sagittarius. During the last week of December each year, the sun is poised almost exactly where Jupiter is right now. The sun moves from right to left on the sky, heading eastward. All during December it crosses the feet of the Snake Holder Ophiuchus," Teske explains.

Over many centuries and through many human cultures, he notes, the shapes and dimensions of the constellations were constantly transfigured. "Even those in the Zodiac didn't escape alteration. At one time there were only 11 recognized constellations along the sun's yearly course. The Romans, to complete the 12 Zodiacal constellations once again, cut off the claws of Scorpius, the Scorpion, and assigned their stars to Libra, the Scales.

"Like the Romans, modern astronomers moved the constellation boundaries around to suit their own needs. The boundaries were defined exactly and adopted officially in the year 1930 by the International Astronomical Union. As a consequence, each December the sun now travels for a few weeks through celestial territory assigned to Ophiuchus, and there are really 13 official Zodiacal constellations along its route."