Holiday Sky Show: Five Planets to Shine in Predawn

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Dimmer and harder to identify: Mars and Mercury, which will appear near Venus.

Jupiter will shine brightly overhead, and Saturn, though very bright, will share the western sky with the also-bright twin stars of Gemini—Castor and Pollux. The brightest of that three-star grouping will be the ringed planet.

(See Earth and Sky's five planets chart here.)

The five planets will share the same nighttime sky until early to mid-January. From December 10 to 28 and again in mid-January, they will also appear in their "correct" order: Mercury, Venus, Mars, Jupiter, and Saturn.

"There's nothing particularly 'correct' about the order," Beatty noted. "But it is the most familiar lineup, as it represents the order that we know them outward from the sun."

Mercury, said to be the most elusive of these visible planets, is the key to the rare joint appearance.

"Mercury is the innermost planet to the sun, and it travels around the sun in only 88 days," Byrd said. "What that means in our sky is that Mercury moves from the morning to the evening sky six times a year. Most other planets don't do that."

The planet also never rises far above the horizon and does so only near dawn and dusk. Most of the time the sun's glare obscures Mercury to the naked eyes of sky-watchers.

"It comes and goes quickly," Byrd added. "The longest you'll ever see it in one place in the sky is a month. But usually it's only [in one location] for a week or ten days. That's why it has the name Mercury, after the fleet-footed messenger god."

During the current convergence, Mercury "is just getting to the place in its orbit where there's the greatest distance between Mercury and the sun on the sky's dome," she said. "That's when we can see it."

The orbiting planets are visible from Earth in part because all are currently on the same side of the sun. They are not, however, in any type of alignment.

"They are not in some kind of straight line in space," Byrd explained, noting that NASA scientists have used computers to project the location of the planets millions of years into the future. "In all that time, there's never a time when all the planets make a straight line in space."

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