"Conjunction" is a technical term for when several planetary objects line up, as seen from our Earthly perspective.
It's been a while since I've paid any attention to a conjunction, largely because I've been quick to dismiss them as no big deal: Is it rare for two planets to be in the same general vicinity in the sky? Not really. In our sky, the planets are confined to wander through the band of stars that make up the twelve Zodiac constellations (Leo, Sagittarius, Cancer, Libra, etc.) Like racecars, they're all on the same racetrack, so to speak, so it's not surprising that occasionally they pass one another up.
Nonetheless, when Venus, Mercury, and Jupiter formed a triangle in the evening skies in early summer of 1991, I was mesmerized. Maybe it was because I was ten years old and easily impressed. More likely, however, is the fact that here was an easy, sure-fire way for me to distinguish planets from stars in the night sky. (I see that Wikipedia doesn't even list it as a "notable conjuction" of recent times, which suggests it probably occurs fairly often.)
I hadn't actually thought about that conjunction of 1991 until tonight, when I looked at the conjunction happening now. Right now, Venus and Jupiter are paired nicely together in the western sky after sunset. Something about their brilliant light makes them especially eye-catching. And best of all, on the evening of Monday, December 1st, for that night only, the Moon will sit beside them. This will be the best evening of all to witness the conjunction.
Is it rare? Not exceedingly so. (The Moon more or less follows the band of the Zodiac as well.) But it really is a pretty sight, and a grand opportunity to easily pick out the planets Venus and Jupiter. Venus is the brighter of the two.
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Addenda:
(1) You don't need binoculars or a telescope to appreciate this. But if you have either, bring them along! A good pair of binoculars will allow you to resolve some small "stars" around Jupiter, which are in fact some of Jupiter's moons. A small telescope will reveal Venus to have a phase, just as the Moon does... only tomorrow night, Venus will be a little more than half full; whereas the Moon will be a crescent.
(2) For all this talk of mine that conjunctions aren't rare, there are some rare conjunctions that have been especially worth viewing. In 2002, Saturn, Jupiter, Mars, Venus, and Mercury (the five planets visible to the naked eye) lined up in April, visible shortly after sundown. This won't happen again until 2060--and will only be visible before dawn (which poses no problem for those of us who will be old fogies by then, already awake for an hour).
(3) In case you're thinking about it, but too afraid to ask: Do planetary alignments have any adverse effects on the Earth? Earthquakes? Tidal waves? I direct you to Phil Plait:
http://www.badastronomy.com/bad/misc/planets.html
