The Mercury Transit Explained

by Zachary Singer

Diagram of a transit of Mercury.
Diagram of a transit of Mercury.

(All illustrations © Zachary Singer.)

If you’ve heard about the November 11 Transit of Mercury (visible 6:39 AM – 11:04 AM, Mountain Standard Time, here in Denver), you may have a simple question: “What is it, and why is a transit a rare event?” The “what” is quick to answer; the “why” takes a bit longer, but isn’t hard to understand.

During a transit of Mercury, we see Mercury’s silhouette pass across the face of the Sun. It’s very much the same idea as a solar eclipse (when the Moon passes in front of the Sun), except that Mercury is so much farther away that it hardly covers the solar disk—all we see (through a telescope set up for safe solar viewing) is the planet’s “little black dot” move across the Sun over several hours.

If we could watch this event while looking down from a vantage high above our solar system, we’d see Mercury, the Earth, and the Sun arranged in a fairly straight line, with Mercury between the Earth and Sun (see Chart 1, below). Continue reading “The Mercury Transit Explained”