A PLANETARY TRIAD
Dr. Terry L. Goforth
Associate Professor of Physics, SWOSU
June 2005

Go outside shortly after sunset and look toward the waning light of the sun near the northwestern horizon. Once the sun is down, the first heavenly body you will see is Venus. Venus at its brightest outshines all stars and other planets. Only the sun and the moon are brighter. In fact, Venus is often so bright that it is mistaken for a plane, a UFO, or some other “very bright and very close” object.

Positions of Mercury, Venus, and Saturn relative to Casto and Pollux on June 25, 2005 Over the next couple of weeks, Venus is going to have company in the northwestern sky. A close conjunction of the planets Venus, Mercury, and Saturn will be a must-see for sky enthusiasts. A conjunction occurs when two or more heavenly bodies (planets, stars, the moon, or the sun) appear to be very close to each other in the sky. Of course, this doesn’t mean the bodies are actually close–they all just lie in the same direction from Earth and so appear to be lined up. In reality, the objects in conjunction are at vastly different distances from Earth, so there’s no concern about a “collision.”

During the last weekend of June, these three heavenly bodies will be so “close” to each other that you can cover them all by holding your thumb at arm’s length and looking with one eye. By Monday, Venus and Mercury will be so close together that you can cover them with your pinky finger held at arm’s length!

To see this marvelous sight, just check out the sky near the northwestern horizon about 9:45 to 10:00 p.m. Venus, the brightest object, will be the obvious one. Mercury will be the closest object to Venus, but significantly dimmer. Three other objects will also be visible in this part of the sky. The southernmost of these three is Saturn, the northernmost is the star Castor, and the center one is the star Pollux. (Castor and Pollux are the “twins” of Gemini.)

Over the course of the next two weeks, Saturn, Castor, and Pollux will drop lower and lower in the sky, eventually disappearing in the glare of the sun by the end of the first week of July. Meanwhile, Venus and Mercury will rise higher in the western sky. They will be joined by the crescent moon July 7 and 8. After July 8, Mercury will begin dropping back down toward the sun and fade from view before the end of the month. Venus will continue to sparkle brightly, sharing the western sky with the planet Jupiter.

For sky maps and more information, check out Science@NASA (science.nasa.gov, Spectacular Conjunction) or Sky & Telescope (June 2005, p 56, or online at skyandtelescope.com, Three Planets Bunch Up in Twilight). Now go enjoy the view!

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Last update:  June 7, 2005