Solar Scientists are accustomed to seeing spots on the sun--irregular islands of magnetism that sometimes erupt, producing strong solar flares.

On May 9, 2016, they will see a spot of a very different kind--a dark circle moving across the solar disk.

This spot is no ordinary sunspot. It's the planet Mercury, making a rare transit of the sun.

Mercury passes directly between the sun and Earth about 13 times every century. The last time it happened was ten years ago in 2006, and the next time will be Nov. 11, 2019.

This year’s transit will be widely visible from most of Earth, including the Americas, the Atlantic and Pacific Oceans, Europe, Africa and much of Asia.

In the USA it begins on the morning of Monday, May 9, around 7:15 AM EDT. This means it begins before sunrise on the west coast, but that’s no problem. The transit lasts for more than seven hours, so Mercury will still be gliding across the solar disk when the sun comes up over places like California and Alaska. Everyone in the USA can experience the event.

Caution: Take care when viewing the transit. Mercury's tiny disk—jet black and perfectly round—covers only a tiny fraction of the sun’s blinding surface. Looking at the sun with unprotected eyes on May 9 is as dangerous as ever.

With a proper filter, however, viewing the transit of Mercury can be a marvelous experience. A telescope with a safe solar filter will be required to see the tiny disk of Mercury crawling across the face of the sun. Mercury is too small to be seen without magnification.

You may wish to call your local astronomy club and ask if they have a solar telescope. Amateur astronomers love to show off the heavens. The event will provide volunteers the opportunity to bring their 'scopes to classrooms for the transit.

If you can't find access to a good telescope, NASA's Solar Dynamics Observatory will also witness the entire transit and make it available in real time on its website.

NASA scientist Rosemary Killen and colleagues plan to use the transit to study Mercury’s ultra-thin atmosphere or exosphere. The atoms in Mercury’s exosphere come from the surface of Mercury itself. They are blasted into space by solar radiation, solar wind bombardment and meteoroids. This gives Mercury a comet-like tail stretched out as long as 1.2 million miles. You cannot see this tail during the transit, however.

Killen says, “When Mercury is in front of the sun, we can study the exosphere close to the planet. 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.”

She says, “We will be observing the transit from the National Solar Observatory, or NSO, in Sunspot, New Mexico.” Killen, Carl Schmidt of LATMOS at the French National Research Agency, and Kevin Reardon of the NSO will be on site making observations.

The Transit of Mercury offers something to professional astronomers and backyard sky watchers alike—from scientific discovery to simple wonder. Mark your calendar for May 9 and enjoy the show.

See more coverage of the 2016 transit of Mercury at http://mercurytransit.gsfc.nasa.gov

For more news about rare events in the heavens, stay tuned to science.nasa.gov.