Stargazers, ready your (solar-filtered) telescopes: Mercury is passing directly across the sun for the first time in nearly a decade.
The innermost planet of our solar system will look like a small, dark circle cutting across the sun's disc. In the U.S., the transit begins shortly after 7 a.m. ET on Monday and will continue for more than seven hours.
In this animation, NASA shows Mercury's expected path, hour by hour:
At least part of the transit, which only happens about 13 times every century, will be visible across the Americas, Europe, Africa and large portions of Asia.
If you're hoping to watch it, eye protection is key. NASA stresses that "viewing this event safely requires a telescope or high-powered binoculars fitted with solar filters made of specially-coated glass or Mylar."
You won't be able to see the tiny dot of Mercury on its celestial crawl without magnification, NASA says.
Another option: Check out one of the multiple live-streaming events going on Monday. NASA says it will stream the transit here, here and here. Here's one of those feeds:
The European Space Agency also has a live stream:
Vox explains why transits like this are so rare:
"For a transit to occur, the sun, Mercury, and Earth all have to line up directly. But Mercury's orbit is inclined by about 7 degrees compared with Earth's. So there are only two spots where the two planets could conceivably line up with the sun — the places where Mercury crosses the Earth's orbital plane.
"Earth lines up with these intersection spots around May 8 and November 10 each year, give or take a few days. If Mercury, which takes 88 days to orbit the sun, is also wandering through at the same time, a transit occurs. This happens once every seven or eight years."
It's not all about the show — transits like this one have historically been, and continue to be, important research opportunities for scientists. First observed in 1631, the transits were later used to "measure the distance between the Earth and the Sun," NASA said.
Now, they provide scientists an opportunity to study the planets' exospheres — the thin layer of gases that make up their atmosphere.
"When Mercury is in front of the sun, we can study the exosphere close to the planet," NASA scientist Rosemary Killen said in a release from the Jet Propulsion Laboratory. "Sodium in the exosphere absorbs and re-emits a yellow-orange color from sunlight, and by measuring that absorption, we can learn about the density of gas there."
Additionally, scientists have found that a transiting planet causes a drop in the sun's brightness. The subtle change on the light curve can be seen in this example animation
This phenomenon is "the main way we find planets outside the solar system," .
The Kepler mission, which is searching for habitable planets, has found 1,041 planets to date using the transit method. The mission says it is able to determine the size of a planet by observing its transit.
Wondering what that black dot looks like up close? NASA recently released these mesmerizing images taken from the MESSENGER spacecraft, the first to orbit Mercury.
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