Mercury is making a rare 'transit' across the sun. Here's how to watch. - NBCNews.com

Skywatchers around the world have the opportunity to witness a rare astronomical event Monday that occurs just 13 times each century.

Mercury, the smallest planet in the solar system, is set to inch across the face of the sun in what’s known as a “transit,” and several organizations are planning to broadcast the celestial event live online.

During the Mercury transit, the planet will pass between Earth and the sun, and while this chance alignment occurs, skywatchers here will be able to see Mercury appear as an inky black dot crossing the sun’s bright disk.

“The planet Mercury is a very small, terrestrial planet, and it’s quite a bit closer to the sun than we are, so it’ll just be a tiny little black spot,” said Patti Boyd, an astrophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.

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The transit will begin Monday at 7:35 a.m. ET, and the entire event will last a little more than five hours. People on the East Coast of the United States, Central America and South America will be able to witness the entire transit because Mercury will start marching across the solar disk after the sun has already risen.

For the rest of North America and parts of Europe and Africa, sunrise will occur while the transit is already in progress, but skywatchers should still be able to catch part of the event, weather permitting. The transit of Mercury will not be visible in Australia and much of Asia, but enthusiasts can still catch all the action, thanks to almost real-time images from NASA’s Solar Dynamics Observatory.

Slooh, an online observatory, is planning to livestream the event on YouTube, beginning at 7:30 a.m. ET. The Virtual Telescope Project, which collects images from remotely controlled telescopes around the world, will also broadcast the transit of Mercury online.

To watch the transit in person, do not look at the sun directly with the naked eye, including through binoculars or telescopes. Observing the sun without proper protection can lead to serious and permanent vision damage.

Rather, Boyd recommends using eclipse glasses, which are designed with certified solar filters to make viewing safe. But even with eclipse glasses, it will likely be difficult to spot Mercury.

“The dot will be very small,” she said. “Even for people with perfect vision, it’ll be a stretch to make out the faint, circular dot crossing the face of the sun.”

From Earth, it’s only possible to see transits of Mercury and Venus. Though a Mercury transit will occur again in 2032, the next one that will be visible from the continental United States is in 2049.

Transits of Venus are even more rare; the last one occurred in 2012, and the next one won’t take place until 2117.