Sky watchers over large parts of the western hemisphere are on their way to a rare celestial show on November 11, when the planet Mercury sails across the sun's face. Known as a transit of Mercury, this is the last time people will see this day sky until 2032.
During a transit, Mercury passes between Earth and the sun, becoming a small, round silhouette against the sun's yellow glowing disk. Safe viewing is always paramount – never look directly at the sun without proper protection or you risk damaging your eyes.
Not that you could see much with your naked eyes during a transit; from Earth, the black dot of Mercury will be the 1
This year, Mercury takes about five and a half hours to complete its hike and makes the first contact with the counter at. 7:35 pm ET (12:35 pm ET). The planet reaches the midpoint of its journey at. 10:20 ET (15:20 UT), and the transit ends at 01:04. ET (6:04 pm).
Weather permitting, the best seats will be in places where the entire transit will take place under daylight. It includes North America's east coast, as well as South America, Western Europe and West-West Africa. For those on the west coast of America, transit will already take place at sunrise, while viewers in much of Africa, Eastern Europe, and most of Asia will still see transit going on as the sun goes down.
As they orbit the sun more than Earth, Mercury and Venus are the only planets that can make solar transmissions from our perspective. With its fast 88-day orbit, Mercury passes between the Earth and the sun every four months or so. But the planet's orbit is tilted compared to the planet's orbit, so for the most part, the little one passes the world over or below the solar disk from our line of sight.
The orbital configuration means that mercury transitions happen only 13 to 14 times a century, with the most recent previous event occurring in 2016. Venusian transits are still rare, and on average only occur once per century. The last transit of Venus occurred in 2012, and we will not see another 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, because they will have to wait until 2049 for the next Mercury transit seen from their continent.
An interesting sight to look for during a transit is the so-called black drop effect, an optical illusion that happens when the planet either just enters or starts leaving the solar disk. When Mercury's leading edge first touches the sun, the planet appears to grow a narrow neck that connects it to the sun's edge, making the silhouette look like a teardrop. This strange revelation happens again just as Mercury is engulfed by the solar disk. (That's why science fiction icon Kim Stanley Robinson is inspired by Mercury.)
Seeing a planet sail across our sun also provides 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 modes of transport that we will see near November 11.
In many cases, our line of sight is adapted so that telescopes on Earth can detect the small dips in starlight when an exoplanet transmits its host star. From this information, astronomers can then calculate the size, orbit and even certain physical properties of these alien worlds.
Even if you are obscured, in the wrong place or do not have the right gear, this transit can be enjoyed worldwide through live broadcasts that show the entire event. Virtual telescope promises coverage from terrestrial telescopes, while NASA's Sunshine Satellite SOHO will offer a dramatic perspective on transit through its own live stream from space.