8 Fast Facts You Must Know About Mercury’s Last Transit Until 2032
On May 9, 2016, the prior transit of Mercury occurred, and was photographed many times on a practically continuous basis by NASA’s Solar Dynamics Observatory. Transits of Mercury are rare, occurring approximately during only 4% of all inferior conjunctions (when it passes between Earth and the Sun) of Mercury. (NASA / SOLAR DYNAMICS OBSERVATORY)
On May 9, 2016, the prior transit of Mercury occurred, and was photographed many times on a practically continuous basis by NASA’s Solar Dynamics Observatory. Transits of Mercury are rare, occurring approximately during only 4% of all inferior conjunctions (when it passes between Earth and the Sun) of Mercury. (NASA / SOLAR DYNAMICS OBSERVATORY)
If you make the effort to see it for yourself, you can reap a unique set of both visual and scientific rewards.
The last transit of Mercury, in 2016, saw the innermost planet cross the Sun’s disk over a timespan exceeding 5 hours. This was the prior transit of Mercury to the 2019 transit, and another one won’t arrive until 2032. Transits of Mercury, as seen from Earth, only come in either May or November. (NASA’S GODDARD SPACE FLIGHT CENTER/SDO/GENNA DUBERSTEIN)
Enjoy these 8 fantastic facts now, as the next transit won’t arrive for 13 years.
At any point between 12:35 and 18:04 UT, Mercury’s shadow can be seen somewhere on the Sun’s disk. This map shows where the entire transit will be visible (white area), where none of the transit will be visible (dark area), and where it will be partially visible (grey area) but either the beginning or end will be obscured because the Earth is in the way. (FRED ESPENAK / ECLIPSEWISE.COM)
During the partial phases of a lunar eclipse, the shadow of Earth can be seen on the surface of the Moon, indicating quite clearly that it casts a roughly circular shape. The longest-duration eclipses on Earth are lunar eclipses, which can have a duration of nearly four hours from the beginning of the first partial phase through totality to the end of the final partial phase; transits of Mercury last some 40% longer. (E. SIEGEL; ECLIPSE SEQUENCES BY WIKIMEDIA COMMONS USERS ZAERETH AND JAVIER SÁNCHEZ)
2.) Mercurian transits outlast any eclipse. Enduring for nearly 5.5 hours, even the longest lunar eclipses (~4 hours) fall short.
Although transits of Mercury were predicted even in pre-telescope times by Johannes Kepler, the first one was not observed until 1631, when Pierre Gassendi accomplished (and documented) the feat. (PIERRE GASSENDI (ORIGINAL), VIA KARL GALLE / LINDA HALL LIBRARY)
3.) Humanity didn’t witness one until 1631. Only with enhancements to human vision in magnification and/or resolution can Mercurian transits be seen.
By setting up a magnifying device (a telescope or one side of a pair of binoculars) and an obscuring shade collar, you can produce a large image of the Sun on a background screen. (The direct sunlight may damage your telescope/binocular optics.) A simple pinhole camera will also work, but your baseline to the screen will need to be very long to produce enough contrast to see a silhouette that’s just 1/194th the diameter of the Sun’s disk. (EUROPEAN SPACE AGENCY)
4.) A quality “homemade projector” is sufficient. Using one (disposable) binocular lens or a pinhole camera, a sufficiently large projected image of the Sun will reveal Mercury.
By using either a special filter to remove all the light not in a particular wavelength (L) or setting up an appropriate solar filter on the outside lens(es) of a telescope or binoculars (R), you can safely view the Sun during a transit with a high-magnification eyepiece, revealing the disk of Mercury. (PHILIPP SALZGEBER (L) / DAVID BROSSARD (R), CC-BY-SA-2.0)
5.) A telescope with a solar filter is ideal. Go for 50–100x magnification viewing, put the filters over the front lenses (not the eyepieces), and enjoy!
Because Mercury’s orbit around the Sun is tilted with respect to the plane in which Earth orbits the Sun by about 7°, while the Sun appears to be only 0.5° in angular diameter from Earth, most inferior conjunctions of Mercury (where it passes between the Earth and the Sun) do not result in a transit. But November 11, 2019 will be one such uncommon instance where a transit does occur. (EUROPEAN SOUTHERN OBSERVATORY)
6.) Only 1-in-23 inferior conjunctions result in Mercury’s transit. Mercury, Earth, and Sol rarely align; Mercury’s orbit is inclined 7.005° to Earth’s.
Although more than 4,000 confirmed exoplanets are known, with more than half of them uncovered by Kepler, finding a Mercury-like world around a star like our Sun is well beyond the capabilities of our current planet-finding technology. As viewed by Kepler, Mercury would appear to be 1/285th the size of the Sun, making it even more difficult than the 1/194th size we see from Earth’s point of view. (NASA/AMES RESEARCH CENTER/JESSIE DOTSON AND WENDY STENZEL; MISSING WORLDS BY E. SIEGEL)
7.) NASA’s Kepler discovered zero Mercury-like planets around Sun-like stars. Transiting Mercury imperceptibly dims the Sun, reducing its brightness by merely 0.0027%.
When Mercury (upper) first begins transiting across the Sun, there is no hint of an atmospheric ‘arc’ that would reveal the presence of sunlight filtering through its atmosphere. By contrast, Venus’ atmosphere (lower) displays a clearly defined arc during transits, and did as far back as the 18th century. (NASA/TRACE (TOP); JAXA/NASA/HINODE (BOTTOM))
8.) Mercury has no atmosphere. Unlike Venus, where atmospherically filtered sunlight appears during transits, Mercury is utterly barren.
Global mosaic of the planet Mercury by NASA’s Messenger spacecraft. Messenger (superseding Mariner 10) has confirmed what humans were only able to conclude provisionally from afar: that Mercury is indeed a planet without an atmosphere. (NASA-APL)
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