Planets to waltz across night-sky ballroom

From our seat here on Earth, the rhythms of the universe rarely seem like must-see viewing. Oh, there's the occasional comet or meteor shower. But, for the most part, the heavens appear to slip by much as they have since the beginning of time.

If you happen to miss one week of stargazing, not to worry. They'll be there for the rest of eternity.

For the next few weeks, however, the night sky is conspiring to offer a scene that only comes around once a lifetime or so: All five planets visible to the naked eye will be packed into a tiny swatch of western sky shortly after sunset.

Given the planets' different paths and speeds, a similar convergence hasn't been seen since 1940 Â- and won't be seen again until 2040.

Yet until early June, this ever-restless dance of planets will give earthbound viewers a unique look at the solar system's thin necklace of night lights Â- not to mention an education on their mysteries and movements.

"Just to see all these planets together will be spectacular," says David Aguilar of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

Tonight at dusk, and for most of the rest of the month, the planets will stretch in a long line.

Anchoring the string will be Mercury Â- which will be visible just above the horizon where the sun sets Â- then Venus, Mars, Saturn, and Jupiter will unfurl upward at a roughly 60-degree angle.

Even without binoculars, the character of the solar system begins to reveal itself in a view like this. At the bottom, Mercury seems little more than a space pebble, the smallest of the inner planets Â- and so close to the sun that it, too, sets not long after the sunset glow is gone.

Above it, white Venus Â- our closest neighbor Â- is the brightest object in the night sky (besides the moon). Farther upward, Mars sulks in deep red, and the great, outer gas giants of Saturn and Jupiter glow massive and pale higher in the sky.

Yet the nature of the planets themselves is but one part of the show. The other is their nightly waltz.

From early May until early June, the planets will be constantly shifting and reordering themselves in a half-dozen different combinations of both scientific and historic significance. During that time:

• Venus will get two different dance partners, passing so close to Mars that they will seem to conjoin on May 10, then swinging by Jupiter on June 3, well after the main show has finished, and all the other planets have dropped below the horizon. "It will look like a very bight binary star off in the twilight," says Alan MacRobert of Sky & Telescope magazine.

• The three planets Mars, Venus, and Saturn will form a colorful equilateral triangle on May 5. In the Middle East, the triangle will sit directly over Bethlehem in the West Bank, just as it did on April 1, 2 BC. That fact has led some to suggest that this star formation is the Biblical "Star of Bethlehem," which led the three Magi to the newborn Jesus.

• The planets will again line up in a necklace on May 14 and 15, but this time more tightly and in a different order. A crescent moon will also nestle between the top two planets, Venus and Jupiter.

The moon's passage and the tidy line of the planet's ascending arc are no romantic coincidence.

Because the solar system is flat like a frisbee, each planet is on roughly the same plane. From Earth, that means the sun, the moon, and every planet follow the same track through the sky, called the ecliptic.

Usually, though, the planets are spaced out enough that the line isn't apparent. Saturn, for instance, takes 80 Earth years to circle the sun, while Mercury takes only 88 days. So finding a time when all five of the easily visible planets line up near each other and on one side of the sun Â- from Earth's perspective Â- is unusual.

Some slightly more skilled astronomers even plan to stay up all night to wait for Pluto, Neptune, and Uranus to rise in the wee hours. Amateur astronomers (and even people who don't ordinarily give the sky a second thought after dark) will be able to sit outside with CD walkmans listening to Gustav Holst's "The Planets" as they gaze at all nine planets with just the aid of a modest telescope.

Says Mr. Aguilar: "It's the only time in my life, I'll be able to see the whole solar system in one night."