This weekend, skywatchers around the world will get to see a striking triple planetary meetup in the evening skies the likes of which won't be repeated until 2026.
"When we see a single planet out by itself on a given night, we might not pay much attention to its status as a planet, as it appears mostly like a bright star," explained Ben Burress, staff astronomer at the Chabot Space and Science Center in Oakland, California.
"But when they are discovered together like this, in a close-knit configuration we know we haven't seen lately—if ever—our minds acknowledge the fact that they move, and in this case, have moved together.
"It reminds us that the sky isn't static and unchanging, but [is] alive with motion and has a dynamic personality," Burress added.
What is a triple planet conjunction?
When multiple worlds appear to align in the sky, it's called a conjunction. But their apparent proximity to each other is an optical illusion—in reality, Mercury is 105 million miles from Earth, Venus is more than 150 million miles away, and Jupiter is a whopping 565 million miles away. (Read about a two-planet conjunction in 2012.)
What should we look for?
All three planets can be found pretty easily without any optical aids. But if you have binoculars or a small telescope, it's definitely worth a look—and can allow you to start finding the trio before they're easily viewable with the naked eye.
"It's visually exciting to see this clustering; Jupiter and Venus especially are brighter than the brightest stars, and seeing just those two so close to each other is remarkable," said Burress.
Venus will appear as the brightest of the trio, and will pop into view soon after sunset. Earth's twin will act as a convenient guidepost to finding Jupiter and Mercury. Venus is the most challenging to find since it never travels far from the glare of the sun, so binoculars will help.
When are the best viewing times?
The best time to see the cosmic trio is between a half hour to an hour after local sunset. Timing is critical because the planets are so close to the horizon and will follow the sun, quickly sinking below the horizon.
"Start looking as soon as the sun has set, but naked-eye discovery should become better at about 30 minutes after sunset," said Burress.
How will the planets change positions?
Starting on May 24, the three planets will appear within five degrees of each other—easily fitting behind a golf ball held at arm's length.
By May 26, the conjunction will be at its tightest, with the planets forming a striking equilateral triangle spanning only 2.5 degrees. The planets will be huddling so close together that they can be covered up by a thumb held at arm's length.
On May 27 the two brightest planets, Jupiter and Venus, will come together in their own conjunction—only 1.5 degrees apart.
In the remaining days of May, Jupiter will continue to sink toward the horizon, while Venus and Mercury will climb higher in the west and dominate twilight skies by early June.
Where is the best spot to watch this event?
While timing is important to catch the event, most of the world's population will be able to see this sky show except for those at extremely high latitudes.
"It will actually be impossible if you're too far north—above the Arctic Circle—where the sun is currently above the horizon 24 hours a day," said Burress.
"But in latitudes where most of us live, there's a chance, as long as you have a clear view of the western horizon."
No matter your geographical location, it will be important to find an observing spot with a clear line of sight to the low western horizon.
What if I miss this one?
Close encounters of three bright planets are considered fairly rare—occurring every few years. The next triple conjunction is scheduled for October 2015. But the wait will be longer for one this tight—the next one will occur in 2026. (Read about the moon and stars forming their own celestial cluster.)