by 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). This alignment is known as an inferior conjunction—“conjunction” means both the alignment itself and the result that an Earth-bound observer sees Mercury and the Sun very near each other in our sky. The conjunction is “inferior” because Mercury is between Earth and the Sun, rather than on the Sun’s far side (in that case, you’d have “superior conjunction” instead).
Since Mercury orbits the Sun in about 88 days—much faster than our 365-day year—it can make several loops around the Sun in the time Earth takes to go around once, so several of these inferior conjunctions (and superior conjunctions) occur in an Earth-year. That’s hardly a “rare” occurrence—transits are rare because we need both an inferior conjunction and a second condition, at the same time.
Look again at Chart 1, and notice how it’s drawn in just two dimensions, like a flat pie plate—Mercury’s orbit and Earth’s both seem to lie on the same surface. In reality, Mercury’s orbit is tilted 7° from ours, so most of the time, Mercury lies either above or below the plane of Earth’s orbit—that is, too high or too low to line up perfectly with the Sun and show a transit from Earth’s point of view—see Chart 2, below. As Chart 2 shows with red dashes, we get a transit only when Mercury’s orbit crosses Earth’s during an inferior conjunction, a relatively rare combination.
**To see the transit, you’ll need a telescope specially equipped for solar viewing—DO NOT look at the Sun without this equipment, because permanent eye damage could result.
Here in Denver, several venues will have the right gear for the public to safely see the event (they’ll be open, good weather or not):
Note that the transit begins before sunrise in Denver, and thus won’t be visible at its start—events here begin when the Sun clears the local terrain; they’re listed below by start time. To see the whole event (or if there’s bad weather), go to the NASA Live Stream, linked at bottom.
Mile High Astronomy, Lakewood, CO, 7-11 AM
Denver Astronomical Society, Denver, CO, 7:30-11 AM **CANCELLED, DUE TO WEATHER
Fiske Planetarium and Sommers-Bausch Observatory, Boulder, CO, 8-11 AM
Live Stream from NASA Solar Dynamics Observatory, starting at 5 AM; the transit itself begins at 5:41 AM MST