Night Watchman: Close Encounters

Witness the striking meeting ofthree planets and a sliver of moon.

By Bob Berman|Sunday, March 01, 1998
Astronomers often imagine that everyone enjoys sky gazing. Alas, the confusing tapestry of the stars and the obscure nature of most constellations mar its appeal. For most of us the nightly mosaic makes as much sense as income tax forms. We seldom glance up.

But when a truly striking celestial event comes along, even the casual stargazers—those who don’t know that the three brightest nocturnal objects are the moon, Venus, and Jupiter, for example—pay attention.

This month’s superbright conjunction is a perfect example. On April 23 the sky’s three luminaries form a tight little triangle that easily qualifies as the year’s most impressive exhibition. That Venus can meet Jupiter is not surprising, since the planets orbit in nearly the same plane. Much more amazing is that the moon joins them. Every other major satellite circles its planet’s equator. Our moon ignores the tilted geometry of Earth’s equatorial plane and travels planetlike in the solar system’s zodiacal disk. It’s a strange and unique thing for a moon to do, and it allows the moon to glide next to the striking Venus-Jupiter pair.

About 45 minutes before dawn, look to the right of where the sun will rise. Dazzling Venus will sit just half a degree from not-quite-as-brilliant Jupiter, with the nearby crescent moon providing a finishing touch. (If it’s cloudy, try again the next morning, when the three will still be noteworthy but much farther apart.) The visceral beauty of the event nicely illustrates why ancient cultures made such a fuss over this sort of tight, brilliant planetary configuration.

The trio sit together, separated from the soon-to-rise sun by an angle of 45 degrees. With any small telescope, you’ll see how they are illuminated differently: the moon is a slender crescent, Venus looks a bit fatter than a half-moon, and Jupiter displays a full phase.

At first it makes no sense: If you look at a few friends standing together in the sunlight, all show the same illumination. But what if one person’s head were half lit up from the right, another lit from the left, and a third fully illuminated? It would be like a bizarre nightmare. Yet that’s what we’ve got in the predawn sky. What gives?

The secret lies in the enormous disparity between the moon’s and the planets’ distances from us. Although the three appear to be lumped together, the moon is less than two seconds away from us at light speed, Venus ten minutes, and Jupiter an hour. Picture it: The moon hovers between us and the sun, so it’s lit up from behind. Voilà—a crescent. Venus sits near the westernmost edge of its orbit, almost at right angles to us and the sun. Thus it appears roughly half bright and half dark. Jupiter lies on the very far side of its orbit, beyond the sun. The gassy giant is so distant from both the sun and the Earth that its disk appears fully lit.

April’s predawn meeting won’t be the only planetary spectacle this season. Three weeks earlier another conjunction—not as bright or instantly obvious as the later rendezvous—graces the horizon. On March 28, about half an hour after sunset, look very low in the western sky almost directly above the point of sunset. Binoculars help cut through twilight’s glare to a trio of stars just to the right of the hair-thin crescent moon. In descending order of brightness, they’re Mercury, Saturn, and Mars.

Consider: All five naked-eye planets plus the moon now star in separate triple and quadruple conjunctions. It would have driven the ancient Maya into a frenzy. Today the choice between celestial grandeur and extra sleep separates sky-watching dilettantes from devotees.
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