Transit of Mercury: Everything You Need to Know About Rare Celestial Phenomenon

Next week, skywatchers will be able to observe the rare event of the planet Mercury passing directly in front of the sun in a phenomenon known as "transit."

As Mercury transits across the disk of the sun, it will block some of the star's rays, appearing as a tiny black dot.

From our perspective, transits only happen with Mercury and Venus, given that they are the only two planets closer to the sun than the Earth.

Mercury—the smallest planet in the solar system and the closest to the sun—will begin its transit at 7:35 a.m. EDT on November 11, with the whole event lasting around five hours, according to

Transits of Mercury do not occur very often. In the period between 2000 and 2199, for example, they will happen only 27 times. The last one occurred in 2016 and the next won't be visible for another 13 years.

Before the year 1585, these transits occurred in the months of April and October. However, they have gradually been shifting over time, to the point where they now take place later in the year—in May and November.

Transits only take place in these two windows because the orbits of Mercury and Earth around the sun are slightly tilted relative to one another, only overlapping at two points, known as nodes, reported.

These two nodes are located at points in Earth's orbit where transits could be possible in May or November. However, because the Earth and Mercury take different amounts of time to complete a full circle around the sun, we don't see transits every year.

transit of mercury
The planet Mercury is seen in silhouette, lower third of image, as it transits across the face of the sun Monday, May 9, 2016, as viewed from Boyertown, Pennsylvania. NASA/Bill Ingalls

For a transit to be possible, the Earth and Mercury both need to arrive at the nodes at precisely the same time. In May, Mercury is further away from the sun and closer to Earth, meaning it appears to be a fraction larger from our perspective during these transits compared to those which take place in November.

The entire transit process can be split into four "contacts." The first is when the leading edge of the black dot representing Mercury comes into contact with the outside edge of the sun's disk. The second contact is when the trailing edge of the planet is in line with the edge of the sun's disk.

Meanwhile, third contact is when the the silhouette first touches the inside edge of the sun's disk on the opposite side of the star. And finally, fourth contact marks the end of the transit when the trailing edge of Mercury is touching the outside edge of the sun's disk.

To be able to see Mercury during transit you will need telescopes or binoculars because the silhouette is too tiny to see with the naked eye. However, take note that you will need to use a special protective sun filter so as not to damage your eyesight.

The transit on November 11 will be visible to people in most of North America, however, those in central and western parts of the continent will miss some parts of it because the event will already have started by the time the sun rises. People in the east, on the other, hand will be able to see the whole transit.