If the skies stay clear, Maryland stargazers in the coming days will be spectators at a rare and beautiful dance of the planets.
Jupiter, Mars and Venus, -- already closely grouped in the western sky from our perspective on Earth at sunset -- will appear over the next several evenings to "rush" toward each other in the western sky.
They will pass within a whisker of each other Monday night and, by June 30, they will have reversed their relative positions in the sky.
"I've been out watching it, and almost night-to-night you can see them moving quite quickly. Boy, it's been like an express train in the sky," said Jim O'Leary, director of the Maryland Science Center's Davis Planetarium.
Unlike many other celestial "spectaculars," this one involves bright objects that are easy to find, and which appear at a convenient time of night.
"It's one of those things that, just by its nature is going to attract attention," he said. "And on the night of the 15th, the crescent moon will be there. It's a spectacular grouping of all these objects, and this is probably the time people will look up and say, 'What is all this?'"
It's only the fifth time in this century these three planets have grouped together in this sort of "grand conjunction," and they haven't been quite this close together since Dec. 23, 1769.
They'll join up again for the last time this century on Nov. 19, 1995, but not this closely.
All three planets can be seen now with the naked eye, appearing for several hours after sunset as three bright "stars" above the western horizon.
They are, in order from upper left to lower right: Jupiter, Mars and Venus.
Venus, on the right, is the brightest object in the sky, except for the moon. It is the second planet from the sun, and is actually in the foreground of the scene, 71 million miles from Earth.
The second-brightest of the three is Jupiter, on the left. Jupiter is the fifth planet from the sun, and lies in the distant background, a half billion miles beyond the sun. Only its huge size makes it appear nearly as big and bright as the much smaller, and closer, Venus.
Mars is the much fainter, reddish "star" that appears between the other two planets. It now appears quite close to Jupiter, although it is actually about 340 million miles in front of the giant planet.
As the celestial minuet begins, Jupiter will appear to overtake Mars tomorrow night, appearing a scant two-thirds of a degree apart. To the naked eye, they will seem to be practically on top of each other.
The scene will be made even more impressive by the arrival Friday evening of a slender, waxing crescent moon. It will appear just below the planetary grouping on Friday, then ascend to their left by Saturday evening.
By Monday night, Jupiter and Mars will have drifted still lower in the sky, joining Venus in a grouping barely 1.8 degrees across. That's less than the width of three full moons.
"That may sound like a big distance, but when you look at it, it is surprisingly close," O'Leary said.
For the rest of June and July, Jupiter will appear to fall rapidly toward the spot where the sun set each night, with Mars and Venus following. They will by then have reversed their order in the evening sky from the positions they held at the beginning of June.
On July 13, there will be an even grander display.
Venus, Mars, Jupiter, another crescent moon and the bright star Regulus will all be visible in close company near the western horizon at dusk. Even the planet Mercury -- the closest planet to the sun and most difficult naked-eye planet to see -- will have joined the assemblage as a tiny point of light beside Jupiter.
The apparent movement toward the sunset by Jupiter and Mars this month is an optical illusion, and contrary to their actual orbital movement. The illusion is the result of Earth's own relative orbital motion.
You can visualize it if you imagine yourself as Earth. In front of you, stationary in the foreground is a person representing the sun. Beyond the sun, and slightly to the left stands Mars, with Jupiter standing off behind Mars in the distant background.
If you walk to your right, Jupiter and Mars will appear to move behind or "into" the sun, disappearing from your view.