FOUR PLANETS AND A MOON
Dr. Terry L. Goforth
Professor of Physics, SWOSU
July 2010

Have you heard about Mars? If you’ve received the emails or internet links telling about Mars being closer to Earth than it ever has been, be warned–it just ain’t so. These are recycled and exaggerated stories based on the actual close pass (called a perihelic opposition) of Mars and Earth in August 2003. That event has come and gone. But don’t despair! Nature is giving us a great midsummer show right now in the western sky just after sunset.

Step outside and look to the west just after sunset. In late July and August (of 2010), four planets are visible to the naked eye in the early evening hours. As of late July, the line-up is Mercury, Venus, Mars, and Saturn, with the latter two being very close together, separated by less than 2°. (For reference, at arms length, your index finger, viewed with one eye closed, spans about 1°.) This conjunction of Mars and Saturn–their appearance very close to each other in the sky–does not represent an actual close passing of the planets. Each remains safely in its own orbit, and the distance between the two is almost 300 million miles. Venus is unmistakable as the brightest object in the western sky after sunset. Mercury lies near the horizon in the west-northwest, not far from where the sun has just set (around 8:45 p.m. DST in Weatherford). The tiny planet Mercury is usually difficult to find. It is never far from the sun and is generally lost in the glare, even after the sun has set. However, because the solar system is “flat”–that is, the planets all orbit in a more or less flat plane–you’ll see that they line up neatly in the sky. You can trace the line formed by the other three planets downward toward the horizon. Mercury will lie very near that line and within a fist’s width (at arm’s length) of the horizon.

Over the next few weeks, you’ll notice that the sun sets about one minute earlier each evening. Stars rise and set about four minutes earlier each consecutive night (due to Earth’s motion around the sun), so if you go out at the same time each night, the sky will be just a bit darker each night, and the stars will be slightly lower in the sky. Over the next few weeks, Saturn will remain in the same position relative to the stars around it, but the other planets will shift to positions higher and farther south relative to the stars. By August 4, Mars, Saturn, and Venus will form a tight triangle in the west southwest. Venus is the brightest of the three and in the lowest position in the sky. Mars is at the upper left of the triangle and will have a slightly orange or reddish color to it. The center planet in the triangle is Saturn, brighter and whiter than Mars but dimmer than Venus. Mercury has shifted up and left relative to the stars, but lower in the sky, and may be hard to spot.

On August 7, a conjunction of Saturn and Venus will bring them within 3° of each other. On August 11, a thin crescent moon will join the party, positioned below and slightly left of Mercury almost due west just after sunset. Each night the crescent moon will wax (grow, or increase) and shift noticeably higher and farther south, moving past the Mars-Venus-Saturn triangle.

The final act in this planetary dance will be a conjunction of Venus and Mars on August 19 when our two nearest planetary neighbors will appear within 2° of each other. After this the planets will separate. At the end of August, Venus and Mars will sandwich Spica, a blue-white star in the constellation Virgo, for an encore.

For nightly star maps showing the positions of the planets, the moon, and the major stars, visit spaceweather.com.

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Last update:  July 27, 2010