On Monday, Nov. 11, Mercury and Earth will partner with the sun in a celestial swing dance known as a transit of Mercury, when this planet will pass directly in front of the sun as seen from Earth. From northern Arizona, the event will be visible (most easily with the aid of filtered telescopes) for about four hours.
Nearly every year, the moon crosses in front of the sun, partially or completely blocking it from view in what is commonly called a solar eclipse. The moon isn’t the only celestial object that crosses in front of the sun, however. The positioning of the orbits of Mercury and Venus between Earth and the sun means these bodies also occasionally move in front of, or transit, the sun’s face, as seen from Earth.
Compared to solar eclipses, these planetary transits are rare. Transits of Venus, for instance, generally happen a little less than twice per century. The last one occurred in 2012 but the next one won’t happen until 2117.
Partly because Mercury is closer to the sun than Venus and thus orbits the sun more rapidly and frequently, Mercury transits are more common than those of Venus, occurring 13 or 14 times per century — most recently in 2016. After the Nov. 11 event, the next Mercury transit will happen in 2031.
German mathematician and astronomer Johannes Kepler was the first person to predict Mercury and Venus transits but never observed one, dying a year before a Mercury event in 1631. But thanks to Kepler’s calculations, Frenchman Pierre Gassendi will forever serve as a footnote in the study of transits. A priest and scientist, Gassendi also taught philosophy courses and purportedly one of his students was Cyrano de Bergerac, the real-life author on which Edmond Rostand’s 1897 play was based. Gassendi used Kepler’s predictions to observe the 1631 Mercury transit, becoming the first person to document a planetary transit of the Sun.
In later years, scientists made some fundamental astronomical measurements while observing transits. Most significantly, in the 1700s English astronomer Edmund Halley proposed using transits to measure the sun’s distance from Earth. His method was first applied during a Venus transit in 1761 but the resulting estimate, which gave a range of 78,000,000 to 96,000,000 miles, was not very accurate, largely due to inclement weather on Earth that clouded out many observations. The estimate nevertheless served as a reasonable starting point and scientists observing subsequent transits narrowed down this number (the actual distance to the Sun averages a little less than 93 million miles).
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Modern-day astronomers have applied the study of planetary transits to search for planets around other stars. Using sensitive light detectors, they study the brightness of select stars. In some instances, the astronomers observe a temporary but repeated drop in the brightness of the star. This suggests the presence of a planet that is orbiting that star and blocking a small portion of the starlight.
Mercury transits are not visible to the naked eye for two reasons. First, direct observation of the sun without appropriate filters or other protection can cause serious eye damage. Second, Mercury is generally too small to distinguish without magnification (its diameter as seen from Earth is 1/194th that of the sun). The best way to observe a Mercury transit — short of watching a live stream of the event on the Internet — is by either looking through the eyepiece of a telescope that is fitted with a special solar filter or by projecting an image of the sun through a telescope and onto a piece of paper.
Astronomers define four critical stages of transits. First contact, or exterior ingress, marks the beginning of the event, when Mercury first appears to make contact with the edge of the sun. Second contact, or interior ingress, comes a couple minutes later, when the trailing edge of Mercury has moved completely inside of the sun. Just after this stage, observers may detect Mercury momentarily taking on a teardrop shape in what astronomers call the black drop effect.
The Nov. 11 transit begins at 5:35 a.m. Flagstaff time, but won’t yet be visible because the sun doesn’t rise until 6:57 a.m. While the first two stages of the transit thus can’t be seen from northern Arizona, the last two will. Third contact, or interior egress, happens at 11:02 a.m., when Mercury touches the opposite edge of the sun. The black drop effect occurs right before this stage. Fourth contact, or exterior egress, happens at 11:04 and marks the end of the transit, when Mercury is no longer in front of the sun.
Lowell Observatory is hosting “Mercury Transit Breakfast." This limited-seating event will take place from 6:30 to 10 a.m., before normal operating hours. Tickets cost $50 per person, including a catered breakfast and unlimited viewing of the Mercury Transit, and may be purchased from Lowell’s website at lowell.edu.