Rise before dawn on Sunday, prepare a thermos of hot cocoa, bundle up, drive to some spot where mountains don't obstruct the eastern horizon — and watch an astronomical grouping not seen since the Civil War.

As the Denver Astronomical Society points out, before dawn the planets Mars, Mercury and Jupiter will be grouped within a 1 degree circle. Mercury, the smallest planet, will seem only 0.1 degree from Jupiter, the largest, the society adds on its Web site, www.denverastrosociety.org/events.html.

"Literally, they're going to appear so close together that even with a pair of binoculars, which shows you only a small corner of sky, you'll see three planets in the field of view," said Patrick Wiggins, NASA solar system ambassador to Utah and northern Nevada.

He said they should rise in the southeast around 7 a.m., and will be easiest to see as they rise. Once dawn is in full throttle, the brightening sky will wash out the view of the planets.

The best place to observe them is a location with a low, flat eastern horizon clear of trees, buildings and other obstructions, he said. Nobody alive has seen these three planets so close together, Wiggins said.

Wiggins said Mars may be difficult to see with the naked eye "because it's not that bright" and is a tiny planet.

The other two will be easier to spot, he sad. "Literally the last time this happened with these three planets was back during the American Civil War."

The date was Sept. 2, 1861, when the armies of the North and South were at each other's throats.

"It'll be a long time before it happens again," Wiggins said. In fact, he noted, the next time the three planets gather so closely will be in 2053, when he said he will be 104 years old. That doesn't faze him. "I plan on sticking around until 2061 so I can see (Comet) Halley again."

The apparent closeness of the planets is only an optical illusion, he said. They happen to be lining up in their orbits with our line of sight. In reality, Mars is four times as far away as Mercury, and Jupiter is four times as far away as Mars.

"They're not going to run into one another and have an 'astrological' nightmare," he joked.

From the west side of Salt Lake Valley, residents should see the planets rise together before dawn.

But from the valley's east side, people might not be able to see the trio. By the time the planets rise above the intervening mountains, "we'll be approaching sunrise," and the sky may be too bright, he said.

Another option is to drive to a place where the eastern horizon is flat or unobstructed. One way to do that would be to drive to Little Mountain east of Salt Lake City, where the view to the east is good.

While waiting for the three to rise, glance upward. Saturn will be close to the moon, high in the southwest, according to Wiggins.

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E-mail: bau@desnews.com