Sky Show December 1: Jupiter, Venus, Moon Make "Frown"

Andrew Fazekas
for National Geographic News
Updated December 1, 2008
Skywatchers across the world are in for a celestial treat Monday night as two of the brightest naked-eye planets, Venus and Jupiter, join a thin crescent moon to create a brief "unhappy face" in the sky.

The planets appeared closest together—an event known as a planetary conjunction—Sunday night.

"This is set to be the best planetary gathering of the year, simply because it involves three of the brightest objects in the sky after the sun," said Geza Gyuk, director of astronomy at the Adler Planetarium in Chicago.

"As long as you have clear skies in the early part of the evening, this is one astronomical event that's hard to miss."

In fact, some historians think that a similar conjunction between Jupiter and Venus in 2 B.C. may be the source of the "star of Bethlehem" story related in the Bible.

The stellar pair would have appeared so close together, scholars think, that they might have seemed to meld into one brilliant beacon of light.

Blocked by a Thumb

Of the eight official planets in the solar system, five are visible at night without the aid of a telescope or binoculars.

Jupiter and Venus are particularly bright, partly because both have highly reflective clouds that completely envelop them, but also because Venus is Earth's closest neighbor while Jupiter is the largest planet in the solar system.

Both planets are currently easy to locate just after sunset in the southwestern sky.

During a conjunction, two or more of the naked-eye planets seem to be huddled close together—but this proximity is misleading.

For example, on the flat plane of the sky, Jupiter and Venus look like they have been drawing closer together over the past month. But in three dimensions the planets are actually separated by nearly 500 million miles (805 million kilometers).

On November 30 the two planets closed the gap to just over two degrees, or about the same space that four full moons would take up in the sky.

To the unaided eye, the planets appeared to be the same distance from each other on both viewing nights.

"They'll appear so close together on these two evenings that you could stretch out your arm and cover the pair up with just your thumb," Gyuk said.

Venus, the brighter of the two, will be slightly lower left of Jupiter, and when the crescent moon joins the show, it will sit to the upper left of Venus.

(Read more about the conjunction and see an animation of the event on the NatGeo News space blog.)

As an added bonus for observers in Western Europe and northwestern Africa, Venus will actually be eclipsed by the moon on Monday at 4:15 p.m. UTC and will reemerge later during twilight.

The next time a so-called planetary occultation with Venus will be visible from North America is on the morning of April 22, 2009.

Rare Events

Planetary conjunctions are relatively rare events, especially ones involving such a close encounter between Venus and Jupiter.

"They reach conjunction on average about once a year, although the time interval can range from 34 days to 412 days, and all but 23 percent of these are too close to the sun to observe," said Anthony Cook, astronomical observer at the Griffith Observatory in Los Angeles.

The next conjunction between Jupiter and Venus is on February 16, 2010, when the pair is just over half a degree apart, or the size of one full moon.

Unfortunately that event will be too close to the blinding glare of the sun for us to see.

The next visible conjunction will be on the evening of March 14, 2012, but the two planets will appear farther apart in the sky, separated by more than three degrees.

Heavenly Light

Historically, striking planetary groupings have held special meaning to ancient astronomers and astrologers.

A similarly close conjunction between Venus and Jupiter occurred in June of 2 B.C., and some scholars have connected the event with the Christian nativity story.

(Related: "'Noah's Flood' May Have Triggered European Farming" [November 20, 2007].)

According to the Bible, three magi in the East were alerted to the birth of Jesus and led toward Bethlehem by a superbright star—a celestial phenomenon that could be explained by two planets tightly grouped in the sky.

Astronomers say this year's planetary convergence is special because it's occurring at a particularly opportune time of day: in the early evening when nearly everyone worldwide might have a chance to witness it.

"The striking appearance of the conjunction will catch the attention of even those unfamiliar with the sky," Cook said.

"We are naturally attracted to close pairings of bright objects, and the presence of the slender crescent moon will only add to the beauty and awe of the event."

© 1996-2008 National Geographic Society. All rights reserved.