The two brightest planets are gliding closer together in the early evening sky, and their celestial dance culminates with an ultra-close pairing on June 30th.
Anyone who pays even cursory attention to the evening sky has surely noticed that the two brightest planets, Venus and Jupiter, have been drawing closer together in the west in the evening twilight. At the beginning of June, the two planets were 20° apart in the sky, about twice the width of your fist held at arm's length. Week by week, Jupiter and the stars behind it have gradually slipped lower in the evening twilight. But Venus, due to its rapid orbital motion around the Sun, has stayed high up.
The resulting slow-motion convergence is setting the stage for a dramatic sky sight. The warm-up act came on June 19th and 20th, when the planetary duo was joined by a thin and lovely crescent Moon. Farther to their upper left, and fainter, was Regulus, the alpha star of Leo. (I was texting all my friends: "Go outside. Now. Look west!")
But now the spectacle is taking an even more dramatic turn — one you just can't miss. For eight nights beginning June 27th, these two bright planets will be within 2° of each other — close enough to cover both with the thumb of an outstretched hand. In the midst of that weeklong run, on June 30th, Venus and Jupiter will appear so close together — just 1/3° apart — that they'll look like a tight, brilliant double star in the evening sky. You'll be able to cover both with the tip of an outstretched pinky finger. Amazing stuff!
Surely, this spectacle must be some kind of omen. Well, yes, it's a sign from the heavens to get outside and look! As my S&T colleague Alan MacRobert points out, a spectacular conjunction like this often gets people started in our wonderful pastime. "These planetary groupings in the sky have no effect on Earth or human affairs — except for one," he says. "They can lift our attention away from our own little world into the enormous things beyond. That's what amateur astronomers do all the time."
In fact, such conjunctions, or close pairings, of these two planets are not particularly rare. The orbit of Venus is tipped just 3.4° with respect to Earth's, Jupiter even less at 1.3°. So these close conjunctions are destined to occur. For example, the two appeared slightly closer together (though not as high up) before dawn last August 18th, and they'll be separated by about 1° before dawn on the morning of October 26th.
Sky & Telescope Contributing Editor Fred Schaaf brings up a very interesting point: this current trio of Venus-Jupiter conjunctions closely resembles a similar series in 3-2 BC that has been suggested as the Star of Bethlehem. "As has been the case in 2014–15," he explains, "the first two conjunctions back then were extremely close, the last one separated by about 1°, all three occurred not far from Regulus, and all were similarly high up in the sky."
Have a telescope or binoculars handy on June 30th. Both planets will crowd into the same telescopic field of view, Venus appearing as a fat crescent and round Jupiter accompanied by its four largest moons. The two planets will appear nearly the same size — but Jupiter, though much larger in reality, is also much farther away. On the 30th Venus is 49 million miles (78 million km) from Earth, and Jupiter is more than 10 times farther out at 564 million miles (908 million km). Their globes will also contrast dramatically in brightness, with Venus's crescent appearing dazzlingly white compared to Jupiter's duller, striped cloud deck.
And be sure to take pictures! Share your best shots with other S&T.com visitors by posting them in our Online Photo Gallery.
For a comprehensive guide to upcoming sky events, one that S&T's editors use every day, get the indispensable RASC Observer's Handbook 2015.