On Monday, Nov. 11, the planet Mercury will glide across the sun.
Mercury is the fastest planet, and if it orbited on the same plane as Earth we would see it pass in front of the sun every 166 days.
But Mercury’s orbit is tipped, so we only see it cross the sun in the rare November or May when Mercury rises or falls directly between the Earth and sun.
How to watch the transit
Do not look directly at the sun.
NASA’s Solar Dynamics Observatory will post images and video in near real time, or search for a local astronomy club. The transit will be most visible in the Americas, especially along the East Coast, and Mercury will already be crossing when the sun rises on the West Coast.
The transit of 2016
The last transit of Mercury was on May 9, 2016. A NASA timelapse shows the tiny dot of Mercury sliding across the sun.
The transit of 1631
The first documented transit of Mercury was on Nov. 7, 1631. The transit was predicted by Johannes Kepler, who died in 1630, and observed by the French astronomer Pierre Gassendi.
An observation of the 1631 Mercury transit by Pierre Gassendi. Karl Galle/Linda Hall Library
A transit on Mars
In June 2014, NASA’s Curiosity rover looked up to observe a transit of Mercury from the surface of Mars.
It was the first transit seen from a planet other than Earth. Mercury appeared as a faint dot moving faster than two larger sunspots.
An observer on Mars might also see occasional transits of Earth across the sun, though the next one won’t be until November 2084.
Transits in The Times
The first transit of the 20th century made the front page of The Times on Nov. 15, 1907. An observer noted a “diffused ring” around Mercury that was thought to be evidence of an atmosphere. (The planet does have an extremely thin atmosphere, called an exosphere, of atoms blown into space by the solar wind.)
An article after the 1940 transit noted that transits were less important since astronomers had stopped looking for the theorized planet Vulcan between Mercury and the sun, but that transits “will be watched as long as there are observatories and astronomers.”
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Sources: NASA; Fred Espenak (NASA Eclipse and EclipseWise). Images by NASA, except where noted.