Evening stargazers and dog-walkers are in for a treat next week as Venus and Jupiter, the two brightest planets in the heavens, cross paths in the western sky.
"This should be a head-turner," said Alan M. MacRobert, a senior editor at Sky & Telescope magazine. "People will notice this and say, 'What's that?' "
The two planets have been converging for weeks now. Venus is currently the brightest starlike object in the western sky, appearing as the dusk gathers in the hour after sunset.
Jupiter (named for the chief god of the Romans) is creeping closer to Venus (the Roman god of love and beauty) each night. The dimmer of the pair, Jupiter is closing in from above and to the left of Venus. Although the giant gas planet is far larger - Jupiter is nearly 12 times Venus' diameter - it appears dimmer because it is more than five times farther away.
By Thursday, Jupiter will move to within 1.2 degrees of Venus as seen from Earth - that's about the width of your index finger held at arm's length.
Such close conjunctions of these two planets are fairly common, occurring every 13 months or so, MacRobert said. And often, they appear to pass even closer together.
In 2 B.C., for example, the two planets appeared so close together that they seemed to merge into one very bright star. The event has been cited as a good candidate for the Christmas star - the one that the Gospel of Matthew describes as leading visitors to the baby Jesus in Bethlehem.
Next week won't be one of those closer conjunctions. "On May 17, 2000, they were only about a 60th of a degree apart. But they were buried very deep in the glow of sunrise," MacRobert said.
That's a problem with many Jupiter-Venus conjunctions. Because Venus orbits so much closer to the sun than Earth does, these pairings often occur with the planets aligned too far within the sun's glare to be seen from Earth.
But not this time. Anyone with clear skies and a good view of the western horizon 40 to 60 minutes after sunset should be able to see the two planets slip by each other over several nights. (The sun sets at 7:37 p.m. in Baltimore on Sept. 1.)
There's no need for any optical aids, although binoculars will enhance the view. The glasses might also reveal the star Spica, just to the left of the two planets but only about one-fifteenth as bright as Jupiter, MacRobert said.
The appearance of Jupiter and Venus together, seemingly in nearly the same place at the same time, is an illusion of both time and space.
On Sept. 1, Venus, the second planet from the sun, will actually be about 106 million miles from Earth. Jupiter, the fifth planet, is far in the background, about 575 million miles away.
And Spica is 1.5 quadrillion miles beyond - 260 light-years away. It's visible only because Spica is a star that emits its own light, whereas Jupiter and Venus only reflect the sun's light.
We're also looking back in time. Because light travels at a finite speed, it takes longer to reach Earth from more distant places.
So, the light we'll see reflected from Venus on Sept. 1 will have traveled from that planet for only about 10 minutes by the time it reaches our eyes. That will reveal the planet as it was 10 minutes before.
The reflected sunlight reaching us from Jupiter will be 52 minutes old. And the light that falls on our eyes from Spica at the same instant will have been traveling our way since Thomas Jefferson was a toddler - 1745, according to Sky & Telescope.
After Thursday's close conjunction, Jupiter will continue moving downward into the sunset. Venus will appear to rise higher above the sunset each night this fall, before turning in its orbit around the sun and dropping by year's end into the sun's glare.