National Geographic News
A triple planetary conjunction.
Seen from Central Asia last week, Venus, Saturn, and Mars form a celestial triangle.

Photograph by Babak Tafreshi, TWAN

Andrew Fazekas

for National Geographic News

Published August 11, 2010

The famed Perseid meteor shower isn't the only sky show peaking Thursday. That same night, four planets, an asteroid, and the moon will huddle together in the western sky in a celestial traffic jam, astronomers say.

After more than a month of slowly converging, the planets Mars, Saturn, and Venus, will create their most impressive formation yet: a tight upside-down triangle in the evening sky. Adding to the spectacle, Mercury and the asteroid Vesta should be visible near the planet triangle.

(See related picture: "Venus, Jupiter, Moon Smile on Earth" [2008.])

Called a conjunction—when celestial objects get close to each other in the sky—this is one event that observers worldwide won't need binoculars or telescopes to enjoy.

"Four out of the five planets visible with the naked eye can be seen all at once if you have an unobstructed view of the western horizon from the tenth to the fourteenth" of August, said Raminder Singh Samra, resident astronomer at the H.R. MacMillan Space Centre in Vancouver, Canada.

Jupiter, which will rise in the eastern sky, will not be part of the conjunction.

Wide Cast of Cosmic Bodies in Sky Show

The brightest of the trio, Venus, will be the first planet to pop into view.

"Venus is so bright it will be visible very shortly after sunset as the brightest thing in the sky," said Geza Gyuk, staff astronomer at the Adler Planetarium in Chicago.

"A few degrees away to its upper right will be Saturn ... while Mars will be to its upper left." A degree is a measure of a span of sky that's comparable to two full moon disks side by side.

All the planets will easily fit inside the view of a pair of binoculars, Gyuk said—but to see Saturn and Mars, you'll have to wait until the sky is darker.

Keen-eyed viewers can also catch sight of Mercury, the innermost planet in the solar system and Vesta, the second largest known asteroid. (Related: "NASA to Visit Asteroid Predicted to Hit Earth?")

"Observers with telescopes might want to spot the asteroid Vesta, as it will be just three degrees [six full moon disks] above and to the right of Saturn," the space centre's Samra said.

As an added treat on Thursday, the waxing crescent moon will hang below the planetary trio. By Friday night, the moon will climb up to the left of the conjunction.

Earth a Gem Among Many

Such an alignment of several worlds occurs only every few years or so, Gyuk said. To many cultures, these events have always held special meaning—and still do today.

"Looking up at the sky and seeing four planets at once reminds me that we are ourselves on a planet, as small and precious as the gems in the night sky," he said.

"It puts things in perspective and lets me start to appreciate the solar system as a whole."

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