It will be six decades before an alignment this striking will be visible again.
From today through early next week, Mercury, Venus, Mars, Jupiter and Saturn are all visible in the west-northwestern sky just after sunset. On May 6, Venus, Mars and Saturn will form a planetary trio, defined as three planets fitting into a circle with a minimum diameter smaller than 5 degrees. (A clenched fist held at arm's length is equal to roughly 10 degrees of sky.)
The trio will crowd into a circle that's just 2.7 degrees across at 9 p.m. EDT on May 6. And Mercury won't be far away.
"The overall grouping is so tight, you'll be able to blot out four planets with your fist on an outstretched arm," said Robert Roy Britt, SPACE.com's Senior Science Writer. "Visually, it will be striking. Perhaps more important, it's a great opportunity to use the sky as a textbook and teach yourself or your kids something about how the solar system works."
Britt explained that the planets are close together because each orbits the Sun at a different pace, and right now Earth and five others are more or less lined up on the left side of the Sun, as seen from above the solar system.
"From our vantage point, we see the planets grouped in the sky," Britt said. "In reality, they are very far apart, all nearly lined up, one behind the other."
No equipment is required to enjoy the show, because the planets are bright enough to be easily spotted. Astronomers use a magnitude scale to measure the apparent brightness of objects in the sky. Bright stars are typically categorized as zero or first magnitude. Negative magnitudes are reserved for the most brilliant objects. The brightest star is Sirius, at magnitude -1.4.
Venus, the brightest point of light in the sky right now, is at magnitude -3.9, a beacon impossible to miss. Mars, the dimmest of the planets visible to the unaided eye, is at magnitude +1.6.
"The planets are so bright that even city dwellers can enjoy this event," Britt said. "All you need is an unobstructed view of the western horizon."
There will be several other interesting close groupings over the next few nights: on May 4, Mars and Saturn will be 2.2 degrees apart; on May 5, Venus, Saturn and Mars form a nearly perfect equilateral triangle; on May 7, Venus and Saturn will be 2.4 degrees apart; on May 10, Venus will move to within one-third of a degree of Mars, nearly touching it.
During all this, Mercury sits just below and to the right of the planetary trio. It will gradually fade to invisibility during the second week of May. Jupiter, very bright right now at magnitude -1.9, hovers high overhead and slightly to the left of the main event.
Similar compact gatherings will occur in the years 2040, 2060 and 2100. But the 2040 and 2100 events will take place when some of the planets are barely above the horizon and only visible through binoculars.
Britt suggests the following tips for turning the planet alignment into an educational experience:
"Each night, notice how the planets draw closer together. Here's why: The inner planets move around the Sun much more quickly, effectively lapping the lumbering outer planets. All of them occupy roughly the same imaginary plane in space. From time to time, as now, a few of the planets are in roughly the same line of sight from our vantage point.
"Stretch your arm out, make a fist, and see how many planets you can block out at once. Your fist in this configuration covers about 10 degrees of sky. On May 5 and 6, your fist should blot out Mercury, Venus, Saturn and Mars!
"Go out early and watch the planets appear one by one as darkness falls. Then watch them set. Depending on your view of the horizon, Mercury can be gone just an hour or two after sunset. If one of the planets gets near a reference point -- a tree, power line or even the rearview mirror of your parked car, note its location and close your eyes for 10 full seconds. Open your eyes and see how the planet has moved. It's direction of movement will be slanted along the line that the planets form in the sky, a line that points to where the Sun had earlier gone down. This happens because the Sun and the planets move across our sky from east to west, a fact controlled by Earth's rotation."
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