The aqua-colored giants Uranus and Neptune aren’t often on people’s minds. Lumbering along at just a few miles per second, Uranus takes 84 years, a generous human lifetime, to circle the sun. Neptune’s a bit bluer and sports a few white spots and dark spots, but otherwise it seems like Uranus’s corpulent, more distant twin. Its 165-year orbital path keeps it in an oblivion twice as deep as Uranus’s. Indeed, since Neptune’s discovery in the middle of the last century, it hasn’t completed a single circuit.
So why pay attention to these distant relatives now? Because they’re about to align, to go into conjunction, to come together in our sky. It’s the rarest astronomical conjunction of our lifetime. These two frozen planets haven’t met since 1821, when Neptune was unknown. It’s an unusual spectacle, to say the least, and it’s observable even with binoculars.
Because our clockwork solar system lies on a flat plane, it’s guaranteed that the moon and planets will periodically line up. For sky watchers since early times, such conjunctions have been major nocturnal events, the Flintstone equivalent to the Super Bowl. The better ones probably inspired all-night vigils, cave painting, drumbeating, and sacrifices. Nowadays the primary conjunction-induced aberrant behavior involves bleary-eyed amateurs tiptoeing to the driveway with their telescopes, determined not to miss the show.
Happily, some conjunctions are both frequent and spectacular. At least once each month the moon passes every planet. Aesthetically, the moon’s most satisfying naked-eye configurations include Venus, the night’s brightest star. The moon always assumes a crescent phase for such a conjunction, which normally appears low in the sky around twilight. The moon also aligns with real stars, though only a select handful can participate, thanks to the moon’s repetitive path against the starry background. Currently, for example, the moon stands near Spica on the night of May 13, and it will be near Antares on June 13.
But for the greatest spectacles, look to the tight conjunctions of planets. This year Jupiter and Venus will appear dazzlingly close together in the western twilight on August 22; Venus will meet Saturn on December 21; and two days after Christmas the crescent moon, Venus, and Saturn will make a little triangle in the sky.
And if you want to see something really unusual, look to the snail’s-pace planets beyond Saturn. For years amateur astronomers have watched slow-moving Uranus gradually overtaking even slower-moving Neptune. Now, finally, they are meeting in the southern sky among the stars of Sagittarius. For the next four months both will hover their nearest to Earth and assume their brightest guise of the year. They’re now sufficiently close together to fall easily within the field of view of most binoculars. If you center Uranus, Neptune will appear as a very dim star about halfway to the edge of the field. If you own a star chart, look for them just to the southeast of Pi Sagittarii. Uranus appears moderately bright and somewhat green through binoculars and can be glimpsed in dark skies with the naked eye alone.
The planets’ very closest alignment--their official conjunction-- won’t occur till next year, on January 26. But with the two already elbowing each other in the same binocular field, this leisurely rendezvous of the distant giants deserves our notice now. Miss it, and you’ll have to wait until the twenty-third century for another chance.