By
Editor's Note: Visit Space.com on Nov. 11 to see live webcast views of the rare Mercury transit as shown from telescopes on Earth and in space, along with complete coverage of the celestial event. If you SAFELY capture a photo of the transit of Mercury and would like to share it with Space.com and our news partners for a story or gallery, you can send images and comments into spacephotos@space.com.
On Nov. 11, people across most of the world can catch the planet Mercury passing across the sun. This rare event won't be seen from Earth again until 2032.
The smallest planet in the solar system is also the closest to our star, and occasionally it crosses in front of the sun's bright disk from our perspective here on Earth. The last time this happened was in 2016, but after this upcoming transit, we'll have to wait another 13 years to see the next one.
Mercury will begin its journey across the sun on Nov. 11 at 7:35 a.m. EST (1235 GMT), and the entire transit will take roughly 5 and a half hours, ending at 1:04 p.m. EST (1804 GMT), according to NASA.
The planet will look like a tiny, traveling blemish on the sun's face as it passes in front of the sun. The transiting world will be so small that skywatchers will need special gear — telescopes or binoculars equipped with protective solar filters — to see it.
Mercury's transit explained
Mercury and Venus are the only planets that can pass in front of, or transit, the sun as seen from Earth because their orbits are between the sun and Earth's orbit. Mercury's average distance from the sun is 35,983,095 miles (57,909,175 kilometers), or about 30% of the average distance between the Earth and the sun.
Transits are rare events. From Earth, Mercury can be seen moving across the sun's face about 13 times every hundred years, on average. For transit to occur, Mercury's got to be in the right place at the right time.
Let's use breakfast to illustrate. Picture the solar system as a cosmic fried egg. The sun is the yolk, and the planets orbit it within the flat plane of the egg white. Mercury and Earth move in orbits slightly tilted to one another, so if these orbits were drawn out in 3D within the egg white, and if you were to look toward the middle of this egg from the side, there would be two points at which Mercury's orbit and Earth's orbit would appear to overlap.
Currently, these points occur in early May and early November; that's the window of time when Mercury transits could be visible from Earth. People on Earth don't see Mercury transit every year, however, because each planet takes different lengths of time to orbit the sun, so Mercury and Earth don't always coincide at those two places where the orbits overlap, called nodes, at the same time.
There are four key parts to the entire event, beginning with the first contact, or the moment Mercury's silhouette first touches the edge of the sun, Dean Pesnell, a project scientist from NASA's Solar Dynamics Observatory (SDO), said in an Oct. 23 blog post. Another way to describe it is to say, “when the planet's disk is externally tangent to the sun,” according to astrophysicist Fred Espenak, aka “Mr. Eclipse,” who runs the skywatching website eclipsewise.com.
The second contact occurs at the instant that Mercury appears to have moved completely in front of the sun, Pesnell wrote. Third contact is when Mercury begins to cross over the edge of the sun's disk near the end of the transit, and the fourth contact is the last moment Mercury's shadow touches the edge of the solar disk, marking the end of the transit.
Where to see it
Mercury's transit will be visible from most of North America; all of South America; all of Africa; and parts of Europe, Asia, and Antarctica. According to a map developed by Time and Date, skygazers in cities like New York, Montreal, and Sao Paulo will be able to see the entire transit, and people in Honolulu, Rome, and Cairo can catch part of Mercury's trip.
Viewers on the U.S. East Coast can watch the entire transit because the sun will already have risen by the time the event begins. Certain Midwesterners, like those viewing from Columbus, Ohio, can also catch the tiny silhouette of Mercury entering and leaving the sun's disk. But skywatchers west of the Mississippi River will miss part of the event because the transit will already be underway by the time the sun rises there. There will be plenty of time to enjoy it, however, because the event will last for 5 hours, 28 minutes, 47 seconds, according to Time and Date. For example, folks in San Francisco, California, will have over 3 hours of viewing time after sunrise.
Visit this interactive map on Time and Date for local transit and sunrise (or sunset) times.
What to expect
Viewers who pay close attention to Mercury's transit can catch some interesting visuals.
Time-lapse photos or videos, for example, will reveal that Mercury moves in a curved line over the sun, and the planet's path will vary according to your location on Earth. From New York, it will appear to begin at the bottom-left section of the sun, curving upwards towards the right and then tapering off. But from Santiago, Chile, Mercury will first appear on the bottom right, then approach the center of the solar disk before curving downwards.
Then there's also the “blackdrop effect,” in which the tiny shadow looks like a teardrop connected to the blackness of space when it's near the edge of the solar disk. Astronomers believe this effect is due to diffraction, as light waves bend around the sides of Mercury.
How to see it (safely)
Even if you don't have access to safe sun-gazing equipment, you still have many ways to view the event. One of the most exciting will come directly to your internet browser, put together by NASA's solar scientists, engineers, and web programmers.
Beginning at about 7 a.m. EST (1200 GMT) on Nov. 11, just as Mercury moves over the sun's corona, or its outermost layer, SDO will begin recording videos and images of the transit and adding self-updating movies to NASA's Mercury Transit website.
