Today (May 9), you can spot Mercury’s tiny shadow over the sun as it passes between Earth and our star between 7:12 am and 2:42 pm ET—if you’ve got the right equipment.
Mercury, the planet closest to the sun, is so tiny that in order to view its transit, you need either a telescope or binoculars. (It’s extremely important to make sure you have the proper kind of glass or plastic Mylar filter to look at the sun, to avoid damaging your eyes.)
If you don’t have the proper equipment handy, you can watch a live stream of the video from the Sloosh Astronomy Network. NASA will also be broadcasting the event from 10:30 am to 11:30 am in the eastern United States. For a visual approximation of the event, check out this NASA illustration below:
Mercury transits only happen about 13 times per century. The last one was back in 2006; the next ones will be in 2019 and 2032. This is because in order for the transit to happen, the Earth, the sun, and Mercury all have to line up—not a common occurrence, between Earth’s 365-day trip around the sun, and Mercury’s 88-day orbit. Mercury’s orbit is also at a 7° angle compared to the Earth’s, meaning there are only two months in which their orbits line up in each other’s planes: May and November.
Because they happen so rarely, astronomers have to make sure that they get all they can out of each transit observation. Mercury transits provide unique opportunities to study Mercury’s exosphere, a collection of gases that thinly surround the outer layer of the planet, similar to our own atmosphere but much less robust.
Additionally, the transit provides a chance to study exoplanets. Even a planet as tiny as Mercury can cause a dip in the sun’s brightness, which allows astronomers to look for other dips in the brightness of stars far away from our own that could indicate planets. And because astronomers know exactly where to expect Mercury and the sun’s alignment, they can use the event to fine-tune the directionality of current satellites to help them improve future observations.
Correction: This story originally said that Mercury transits happen once every 13 years. They actually happen about 13 times a century.