This NASA Scientist Is So Excited About the Mercury Transit of 2019. Here's Why

Mercury's march across the sun today (Nov. 11) is due to a weird and wild set of planetary alignments, according to NASA. Usually the planet sails above or below the sun, from Earth's perspective, but today something stranger is happening — a transit event, which only happens 13 or 14 times a century. 

"The planets go around the sun in roughly a plane, like a pancake," NASA heliophysicist Alex Young told But it's not a perfect pancake, because the planets are slightly tilted with respect to one another.

But every once in a while, the planets do line up "just right," he said, and when that happens, "you see these little black planets go across the disk of the sun." In fact, Mercury and Venus are the only planets that can pass across the sun from our point of view, because they are closer to the sun than we are.

Mercury transited the sun on Nov. 11, 2019, as seen from Washington, D.C. (Image credit: NASA/Bill Ingalls)

Mercury's transit schedule is far more irregular than Venus', scattering transits across a century roughly 13 or 14 times. Due to planetary-alignment patterns, Venus transits happen in pairs, each pair separated by a century. This means if you missed the last Venus transit in 2012, you likely won't see it happen in your lifetime, as the next one is set for the year 2117. But the next Mercury transit may be more feasible for astronomy fans: It's set to happen in 2032.

Just don't try to find Mercury on the sun with your naked eyes — it's dangerous, and besides, Mercury is too small to see anyway. Use proper filters on your telescope or binoculars to check out the planet, which only ranges from about 1/160 to 1/190 the diameter of the sun. That's "very, very small," pointed out Young.

If you are a fan of watching celestial objects align, however, you'll have more frequent luck with eclipses. Solar eclipses happen when the moon passes in front of the sun from Earth's perspective; lunar eclipses happen when the moon passes into the shadow of our planet. These can be observed from anywhere on Earth and tend to occur about eight months apart, Young said. While lunar eclipses are totally safe to view with your unprotected eyes, you will need proper solar equipment for viewing a solar eclipse.

Historically, planetary transits such as today's Mercury phenomenon were great for science because they helped astronomers figure out the distance to our sun. While we use radar measurements today for distance measuring, we still use planetary transits in another way — to find other planets outside of our solar system as they swing across their own stars. 

NASA's Kepler telescope used this method to find thousands of exoplanets, Young said, and the agency's newer Transiting Exoplanet Survey Satellite (TESS) is expected to find tens of thousands. TESS will also act as a wayfinder for NASA's James Webb Space Telescope, which can look at some of the closer and larger ones for signs of their planetary atmosphere, he added.

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Elizabeth Howell
Staff Writer, Spaceflight

Elizabeth Howell (she/her), Ph.D., is a staff writer in the spaceflight channel since 2022 covering diversity, education and gaming as well. She was contributing writer for for 10 years before joining full-time. Elizabeth's reporting includes multiple exclusives with the White House and Office of the Vice-President of the United States, an exclusive conversation with aspiring space tourist (and NSYNC bassist) Lance Bass, speaking several times with the International Space Station, witnessing five human spaceflight launches on two continents, flying parabolic, working inside a spacesuit, and participating in a simulated Mars mission. Her latest book, "Why Am I Taller?", is co-written with astronaut Dave Williams. Elizabeth holds a Ph.D. and M.Sc. in Space Studies from the University of North Dakota, a Bachelor of Journalism from Canada's Carleton University and a Bachelor of History from Canada's Athabasca University. Elizabeth is also a post-secondary instructor in communications and science at several institutions since 2015; her experience includes developing and teaching an astronomy course at Canada's Algonquin College (with Indigenous content as well) to more than 1,000 students since 2020. Elizabeth first got interested in space after watching the movie Apollo 13 in 1996, and still wants to be an astronaut someday. Mastodon:

  • rod
    Fun 7 minute video here. While I viewed the Mercury transit yesterday using my 90-mm refractor telescope, at mid-point near 1019 or so EST, the immense difference in size of Mercury and the Sun was very obvious in the eyepiece field of view! Mercury close to center of the Sun at that time. Something to behold and ponder the varieties of the cosmos and size of the solar system :) My field of view was 1-degree across at 71x. The Sun's angular size near 1938 arcsecond, Mercury disk silhouette near 10 arcsecond size. The Sun was nearly 194x larger in size in the field of view! A spectacular sight if any did observe using solar filters and good telescopes and eyepieces.