Sky-watchers across much of the Western Hemisphere are gearing up for a rare celestial show on November 11, as the planet Mercury sails across the face of the sun. Known as a transit of Mercury, this is the last time humans will see this daytime sky show until 2032.
During a transit, Mercury passes between Earth and the sun, becoming a small, round silhouette against the yellow glow of the solar disk. Safe viewing is paramount at all times—never look directly at the sun without proper protection or you risk damaging your eyes.
Not that you’d be able to see much with your naked eyes during a transit; from Earth, the black dot of Mercury will be just 1/160th the width of the solar disk, so you will need relatively high-powered visual aids fitted with solar filters to watch the transit. If you don’t have the right gear, look for public viewing parties held by astronomy clubs, museums, planetariums, and colleges in many countries.
This year, Mercury will take about five and a half hours to complete its trek, making first contact with the disk at 7:35 a.m. ET (12:35 UT). The planet reaches the midpoint in its journey at 10:20 a.m. ET (15:20 UT), and the transit ends at 1:04 p.m. ET (18:04 UT).
Weather permitting, the best seats will be in places where the entire transit will happen during daylight hours. That includes the East Coast of North America, as well as South America, western Europe, and far-western Africa. For those on the western coast of the Americas, the transit will already be underway at sunrise, while viewers in most of Africa, Eastern Europe, and the bulk of Asia will still see the transit in progress as the sun sets.
Because they orbit closer to the sun than Earth, Mercury and Venus are the only planets that can make solar transits from our perspective. With its swift 88-day orbit, Mercury passes between Earth and the sun every four months or so. But the planet’s orbit is tilted compared to the plane of Earth’s orbit, so most of the time, the tiny world passes above or below the sun's disk from our line of sight.
That orbital configuration means Mercury transits happen just 13 to 14 times a century, with the most recent prior event taking place in 2016. Venusian transits are even rarer, happening on average only once a century. The last transit of Venus occurred in 2012, and we won’t see another one until 2117.
No one on Earth will see Mercury cross the sun again until November 2032. North Americans will have an even longer dry spell to contend with, as they will have to wait until 2049 for the next Mercury transit visible from their part of the globe.
One interesting sight to watch for during a transit is the so-called black drop effect, an optical illusion that happens when the planet either just enters or starts to leave the sun’s disk. When Mercury’s leading edge first touches the sun, the planet will appear to grow a narrow neck connecting it to the edge of the sun, making the silhouette look like a teardrop. This strange apparition happens again just as Mercury becomes engulfed by the sun’s disk. (Here’s why science fiction icon Kim Stanley Robinson is inspired by Mercury.)
Seeing a planet sail across our sun also offers a chance to witness a crucial method astronomers use to find planets beyond our solar system. NASA’s now retired Kepler mission was able to successfully identify and confirm 2,662 of these exoplanets across the galaxy using transit events like the one we will see up close on November 11.
In many instances, our line of sight is aligned so that telescopes on Earth can detect the tiny dips in starlight as an exoplanet transits its host star. From this data, astronomers can then calculate the size, orbit, and even some physical properties of these alien worlds.
Even if you are clouded out, in the wrong place, or don’t have the right gear, this transit can be enjoyed worldwide via live webcasts that will showcase the entire event. Virtual Telescope promises to have coverage from Earth-based telescopes, while NASA’s sun-watching satellite SDO will offer a dramatic perspective on the transit via its own livestream from space.