Photograph by Laurent Laveder, TWAN

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The Venus and Jupiter conjunction is seen from France this week.

Photograph by Laurent Laveder, TWAN

Venus-Jupiter Conjunction Peaks Thursday—Easy-to-See Sky Show

Spectacle will appear in Northern Hemisphere evening skies to the west.

This Thursday evening, look to the western skies as Jupiter and Venus—the two brightest planets to the unaided eye—stage a close encounter over the Northern Hemisphere.

Though the two planets will appear to converge all this week, they'll be at their closest March 15—separated by only 3 degrees in the sky, or the width of two fingers at arms' length.

When two worlds seem to line up in the sky, it's called a conjunction. But the apparent proximity is an optical illusion—in reality, Venus is nearly 75.9 million miles (122 million kilometers) distant from Earth, and Jupiter sits about seven times farther away at 524 million miles (844 million kilometers) from Earth.

Visible throughout the Northern Hemisphere, this week's sky show lasts for more than four hours after sunset, before the planets themselves sink below the horizon.

"While such conjunctions are without any particular scientific value, and we don't believe the planets control our lives any more, they are nevertheless beautiful and easy to see," said Geza Gyuk, astronomer at the Adler Planetarium in Chicago, Illinois.

In addition, "what is a bit special about this one is that it is so high up in the sky, away from the setting sun. Mars will also be in the sky on the other side, in the east."

Close Encounters of the Planet Kind

Venus-Jupiter conjunctions are fairly common, according to Gyuk, with the next one occurring on May 28, 2013. During that conjunction, the planets will appear three times closer together than they will this week.

"Another in 2014 will have the two planets come within [a fourth of a] degree of each other, equal to only half the width of the apparent disk of the full moon in the sky," Gyuk said.

But unlike this week's eye-catching conjunction, the 2013 and 2014 conjunctions will not be ideally placed for Northern Hemisphere observers. (Test your solar system knowledge.)

"We get a reasonably close encounter every two years or so, however the ultimate in conjunctions, when Venus transits in front of Jupiter, happens more rarely," he said. The last time such a conjunction occurred was 1818, and it won't happen again until 2065, Gyuk said.

One of the leading theories for the Star of Bethlehem legend involves a close conjunction of Jupiter and Venus in the sunset skies in June of 2 B.C. (Related: "Christmas Star Mystery Continues.")

"Both Jupiter and Venus are very bright objects—the second and third brightest, after the moon—in the night sky, so it's not surprising a conjunction would historically always be watched with interest, simply because both are so bright that they sort of command our attention," he said.

"One alone is ignorable, but both Venus and Jupiter together draw the eye."