How to Watch Mercury’s Rare Transit Across the Sun
Step one: Protect your eyes
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On the morning of November 11, skywatchers around the world will have the opportunity to witness a rare celestial treat. Mercury is due to pass across the sun, an event known as a transit. The smallest planet’s next transit won’t be visible from Earth until 2032, and those who live in the United States won’t be able to see a transit until 2049. So provided that you have the proper equipment—or access to a good webcast—now is your chance to take a look.
For planetary transits to be visible from our vantage point, several factors have to align. For one, we can only witness the transits of Mercury and Venus because their orbits are closer to the sun than that of Earth.
Mercury’s orbit is inclined seven degrees to the ecliptic, or plane of Earth’s orbit, as Bob King of Sky and Telescope explains. And Mercury’s orbit intersects with the ecliptic twice during each revolution around the sun, currently in early May and November. But we don’t see transits every year because planets take different amounts of time to make their way around the sun, and thus do not always meet at the two points of overlap, according to Doris Elin Urrutia of Space.com. If Earth and Mercury arrive at these points, called nodes, at the same time—voila, we have a transit.
On average, there are 13 transits of Mercury each century. Venus transits happen in pairs, with eight years separating the two events—and more than a century passes between each transit duo. We won’t be able to see Venus’ next jaunt across the sun until 2117, reports David Dickson of Science Alert.
Fortunately for those who are eager to see a transit in action, the time is right on November 11. In North America, people on the east coast are well poised to see the entire event. In more westerly locales, the transit will have started by the time the sun rises, so it won’t be possible to witness the transit in its entirety. But there is still plenty of time to get a glimpse. Mercury will start its transit at 7:35 a.m. EST, and the event will last for five hours and 28 minutes.
All of South America will be able to view the entire transit, as will parts of Central America and Africa. “[F]arther east in Europe, Africa, and the Middle East, the sun will set with the planet still in transit,” King notes. “China, Australia, Indonesia, much of Asia, and the western half of Alaska won't see the show.”
On the day of the transit, King adds, Mercury will appear to be just one-194th the size of the sun—a small, dark speck moving across a huge, blazing surface. The planet is, in fact, small enough that you won’t be able to see it without a telescope or pair of binoculars—and you must make sure that your equipment is kitted out with a safe solar filter. Do not try staring directly into the sun; you won’t see anything, and it’s dangerous. And don’t use solar eclipse glasses to look through binoculars and telescopes. The lenses will amplify the sunlight hitting your eyes, according to Urrutia.
If you don’t have the proper viewing equipment, you can watch short movies of the transit on a NASA platform, in “almost real time.” Webcasts are also available through platforms like Slooh, which livestreams telescope feeds, and the Virtual Telescope Project.
One nifty phenomenon to watch out for is the “black-drop effect,” which happens when the planet is about to enter or leave the solar disk. If you happen to see it—the effect is easier to spot with Venus—Mercury will temporarily look as though it’s anchored to the edge of the sun, forming a tear-drop shape. “[M]odern research has suggested that it is due to a combination of two key effects,” according to the European Space Agency. “One is the image blurring that takes place when a telescope is used (described technically as ‘the point spread function’). The other is the way that the brightness of the Sun diminishes close to its visible ‘edge’ (known to astronomers as ‘limb darkening’).”
Stay safe and happy skywatching.