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June 2005 Feature: A Planetary Trio Adorns the Evening Twilight



This June you'll be able to witness a rather rare astronomical attraction -- a trio, a grouping of three planets in a small section of the sky. A similarly tight planetary grouping won't happen again till December 2006, and that trio will be too close to the Sun's glare for decent viewing. This month will provide the best opportunity to see a planetary trio until September 2008.

On the evening of June 25 & 26, look low in the west some thirty to forty-five minutes after sunset to catch Mercury, Venus and Saturn all close enough together to fit inside a 1.5-degree circle. (The width of your little finger held at an arm length away covers about 1.5 degrees of sky.) Remember to hunt for these planets before twilight turns into total darkness.

At a latitude of 45 degrees North latitude (Potsdam, NY) this planetary threesome sets about one hour and twenty minutes after the Sun. At more southerly latitudes, the window of opportunity lasts a little longer.

With the exception of the Moon, Venus is easily the brightest heavenly body to grace the night, beaming at dusk shortly after sunset. Though all three planets are technically visible to the naked eye, you can extend your viewing pleasure by using binoculars. Once you spot Venus, aim binoculars at this brilliant point of light to find two other worlds nearby. Mercury is the closer planet to Venus while Saturn is the farther.

Incidentally, you don't have to wait till June 25 & 26 to enjoy this planetary spectacle. Starting around June 22, there's a good chance of seeing Mercury, Venus and Saturn in the same binocular field till the end of June.

Even before June 22, you can find all three planets if you have a little persistence. First find Venus, looking for Mercury to the lower right of this beacon planet. Then draw a line upward from Mercury through Venus to locate Saturn to Venus' upper left. Remember that these planets move around from night to night, and that Saturn will drop below Mercury and Venus toward the end of the month. (Click here for some graphics.)

By definition, a trio refers to three planets residing within a circle whose diameter spans less than five degrees of sky. I don't know how this "less-than-five-degree" figure came about, but I applaud the choice. A planetary trio readily fits within the approximate six-degree field of view of ordinary binoculars.

Mercury and Venus can be seen in a single binocular field from about mid June to mid July. What's more, Mercury and Venus stage the closest two-planet conjunction of the year this month, standing a scant 1/10 degree from one another on June 27. (For reference, the diameter of the full Moon spans about 1/2 degree of sky.) Also, on July 8, look for the thin crescent moon to join Mercury and Venus for a picturesque display in the evening twilight.

The Five Visible Planets

As a bonus, this June presents a good view of all five naked-eye planets. If you catch the trio, you've already bagged three of them: Mercury, Venus and Saturn. And Jupiter just can't be missed in June. Jupiter, the sky's second brightest planet, shines boldly in your southwest sky first thing at dusk. Jupiter, the king of the planets, glorifies the sky all evening long and sets well after midnight. Be sure to see the slightly gibbous Moon near Jupiter on the evening of June 15.

As for the fifth visible planet -- Mars -- look for this ruddy gem to rise in the east after Jupiter sets in the west. Mars looks like a respectably bright star in the southeast sky before and during dawn. A fat waning crescent Moon shines near Mars on the morning of June 29.

copyright 2005 by Bruce McClure

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May 2005 Feature * July 2005 Feature