A lot has changed since November 8, 2006, the last time time Mercury passed directly between the Earth and the sun: Pluto had recently been "demoted" to dwarf planet, George W. Bush was still president of the United States, and Daniel Craig had yet to debut as the new James Bond in Casino Royale. Nearly 10 years later, on May 9, Mercury's orbit will once again draw the planet in between the Earth and the sun, making the small planet visible against the star's fiery glow.
While it won’t be as dramatic as a total solar eclipse, the transit of Mercury is one of this year’s most awaited astronomical events. According to NASA, this event only occurs around 13 times per century. The next transit won’t happen until 2019.
The reason this event is so rare is because Mercury orbits the sun at a different angle than Earth. While the small planet's short tilted orbit means it crosses between Earth and the sun every 116 days, it usually appears to pass either below or above, the Royal Astronomical Society reports. A transit can only take place when Mercury, Earth and the sun are perfectly aligned in three-dimensional space.
The transit will officially begin at 7:12 A.M. on May 9 and will take 7.5 hours for Mercury to complete the venture across the sun’s face. Over the course of the event, it will be visible from nearly everywhere on Earth at some point during the day. People in parts of western Europe and Africa, eastern North America, and most of South America will be able to watch the entire transit, given the proper equipment, Geoff Gaherty writes for Space.com.
Mercury isn’t the only planet to transit across the sun’s surface occasionally—Venus does as well. However, because Venus has a larger orbit than Mercury and also orbits at a different angle from the Earth, the planet transits in front of the sun fewer than two times per century, Gaherty reports. The last time Venus passed across the sun’s face was in 2012; its next transit won’t occur until 2117.
A word to the wise: because Mercury is only a tiny fraction of the size of the sun, it won’t be visible to the naked eye or even with binoculars. And directly observing the sun is dangerous. Only people with the proper equipment should attempt to watch the transit directly, warns the Washington Post’s Blaine Friedlander, Jr. If you try to look right at the sun with a bare telescope or binoculars, the intensity of the light will blind you.
Luckily, NASA will post live images of the transit as it occurs, and Harvard-Smithsonian Center for Astrophysics partnered with Sky and Telescope to broadcast a livestream of the event, including live images from NASA's Solar Dynamics Observatory spacecraft. However you choose to watch it (and provided it’s not a cloudy day), the transit of Mercury should be a stunning sight to see.