Astronomer Jay Pasachoff, Students, and Colleagues to Observe the Last Transit of Mercury until 2032

Media contact: Gregory Shook, director of media relations; tele: 413-597-3401 email: [email protected]

WILLIAMSTOWN, Mass., November 6, 2019—Mercury will make a rare transit across the face of the sun on the morning of Monday, November 11. Mercury will appear as a tiny dot, and viewers would need a safe solar filter to reduce the sun’s brightness by a factor of about a million in order to see the event safely, or use a projection method. Williams College’s Field Memorial Professor of Astronomy Jay Pasachoff, author of the Peterson Field Guide to the Stars and Planets, has expanded his studies of the objects that go in front of the sun from just the moon, making a solar eclipse, to the two interior planets in our solar system: Mercury and Venus.

Pasachoff has observed transits of Mercury that occurred in 1999, 2003, 2006, and 2016; the next transit of Mercury visible from Earth after the 2019 event will not be until 2032. He discusses transits of Mercury and Venus at his web page The transits of Venus seen by Pasachoff, students, and colleagues in 2004 and 2012 will not occur again until 2117. Studying the dimming of the total light from the sun by the silhouetted planets is analogous to how thousands of exoplanets have been discovered around other stars in recent decades.

The whole transit of Mercury will be visible from the eastern half of the United States for a period of approximately 5 1/2 hours, from about 7:30 a.m. to 1 p.m. Eastern Standard Time. In the western United States, the sun will rise with Mercury already on the solar disk. Mercury is so small that it would not be seen without magnification. We are at the minimum of the sunspot cycle, so it is unlikely that there will be any sunspots at all on the disk to compare in size with Mercury.

The whole transit will also be visible throughout South America. In Europe and Africa, the sun will set in mid-transit. The transit will not be visible from Asia or Australia.

Pasachoff will observe the transit from the Big Bear Solar Observatory of the New Jersey Institute of Technology, which is located in Big Bear Lake, Calif., in the mountains above San Bernardino in Southern California at an altitude of 6750 feet, which should give clear skies. With colleagues there, he will use the Goode Solar Telescope to provide high-resolution images of Mercury against the granulation pattern on the sun’s surface. He and colleagues have used past transits of Mercury and Venus observed from the ground and from NASA spacecraft to show that the “black-drop effect” that appears as the silhouettes of those planets enter or exit the solar disk occurs from a combination of the murkiness of the sun’s atmosphere and the effect of the telescopes’ finite size, and not, as had been thought for Venus, from Venus’s atmosphere.

Pasachoff’s students and colleagues will also participate in a worldwide effort to determine the distance of the Earth to the sun by studying the transit from widely separate locations in Europe and the United States. His students and colleagues will be observing, weather permitting, from Williams College in Williamstown, Mass., and from the Embry-Riddle Aeronautical University in Daytona Beach, Fla. Lecturer in Astronomy and Observatory Supervisor Kevin Flaherty has arranged for the solar telescopes on the observing deck atop Williams College’s Thompson Physics and Astronomy Lab to be open from 9:30 a.m. through the end of the transit just after 1 p.m. Williams College senior Christian Lockwood ’20 will represent the Williams College research team at Embry-Riddle, where he and Pasachoff observed with colleagues there for an occultation of a star by Triton, Neptune’s largest moon, in 2017, in the hope that the Florida skies will be clearer than those in Williamstown, where snow was predicted for transit day. Colleague Robert Lucas from Sydney, Australia, a frequent observer with Pasachoff, will join Lockwood in Daytona Beach.

Alumnus Daniel B. Seaton ’01 is now working with NOAA’s GOES-16 and GOES-17 spacecraft to use their solar telescopes that are mounted on the solar panels even while the main cameras are pointed down at the Earth. Seaton hopes to be able to provide a movie of Mercury’s tiny disk going across the solar disk within hours. Alumnus Kevin Reardon ’92 is now at the NSF-funded National Solar Observatory, also based in Boulder, Colo., and is helping obtain studies of sodium in Mercury’s thin atmosphere, known as an exosphere.

At Big Bear Solar Observatory, Pasachoff will again work with Glenn Schneider of the University of Arizona; their joint work at the transits of Mercury of 1999, 2003, 2006, and 2016 has dealt with the black-drop effect that is seen as Mercury (or Venus in 2004 and 2012) enters or leaves the solar disk. Also joining them will be historian of astronomy William Sheehan, who is now studying the history of measurements of the solar irradiance that reaches Earth. Others in the team include Evan Zucker from San Diego, Nancy Kutner from La Jolla, Naomi Pasachoff, Paula Eisenhart, Deb Sheehan, and Eric Kutner. At the Big Bear Solar Observatory, they will work with Claude Plymate and director Wenda Cao. Thomas Puzia of the Pontifical University in Santiago, Chile—for whom Pasachoff gave a lecture just prior to the July 2 total solar eclipse there, will observe and photograph the whole transit from his ideally located position for this event. Not only will the whole 5 1/2 hours of the transit be visible there but also from his southern latitude a long triangle among his site with California and Germany will be valuable for the project of measuring the astronomical unit.

Pasachoff’s work on solar eclipses continues to be funded by a renewal grant as of July 1 from the Solar Terrestrial Program of the Atmospheric and Geospace Sciences Division of the National Science Foundation. Lockwood’s expedition to Daytona Beach is funded by a grant from the NASA-funded Massachusetts Space Grant Consortium, based at MIT. Pasachoff’s fare to California is funded by the Astronomical Society of the Pacific, which will present him with its Klumpke-Roberts Award for astronomical outreach on November 9, bringing him to San Francisco.

For a link to Pasachoff’s Peterson Field Guide to the Stars and Planets, see for a web page of his books.


Founded in 1793, Williams College is the second-oldest institution of higher learning in Massachusetts. The college’s 2,000 students are taught by a faculty noted for the quality of their teaching and research, and the achievement of academic goals includes active participation of students with faculty in their research. Students’ educational experience is enriched by the residential campus environment in Williamstown, Mass., which provides a host of opportunities for interaction with one another and with faculty beyond the classroom. Admission decisions on U.S. applicants are made regardless of a student’s financial ability, and the college provides grants and other assistance to meet the demonstrated needs of all who are admitted.


Published November 6, 2019