Scientific Observations Planned for Monday's Transit of Mercury

Media contact: Jay Pasachoff at [email protected] or 617 285 6351

WILLIAMSTOWN, Mass., May 5, 2016—The planet Mercury will cross the face of the Sun on Monday, May 9, and Williams College professor Jay Pasachoff will be observing it from the Big Bear Solar Observatory in California. “At the 1999 transit of Mercury,” Pasachoff reports, “Glenn Schneider of the University of Arizona and I used spacecraft observations to show in detail how the merger of Mercury’s edge and the Sun’s edge appears. The dark silhouette of Mercury interacts with the unsharp edge of the Sun to give optical effects that hundreds of years ago for a similar event with Venus were confused with the discovery of Venus’s atmosphere.”

Note that professional and amateur astronomers use very dense filters that let through only about 1 millionth of the sunlight, in order to view the Sun safely. Do not look directly at the Sun without any such filter. Ordinary “sunglasses” are not at all dark enough to allow you to gaze directly at the Sun with them without causing permanent eye damage.

There has not been a transit of Mercury visible from Earth for 10 years, and we will be observing Monday’s event from the giant New Solar Telescope of the Big Bear Solar Observatory in the mountains a couple of hours’ drive east of Los Angeles. The 1.6-m New Solar Telescope at the observatory, owned and operated by the New Jersey Institute of Technology, will give a high-resolution set of images. We will be joined at the site by NJIT professor Dale Gary, who is Director of Solar Observatories at NJIT, and other colleagues. We will be studying especially the details of Mercury appearance as it reaches the edge of the solar disk at 11:12 Pacific Daylight Time. Earlier, Andrew Potter of the National Solar Observatory, who first detected the element sodium in a faint exosphere around Mercury, will make observations in earlier phases of the transit to understand the processes by which these sodium atoms are released from the surface of the planet. Others with us include Robert Lucas of Sydney, Australia, and Evan Zucker of San Diego, along with Professor Bin Chen of NJIT.

We are coordinating observations with those made by Kevin Reardon of the National Solar Observatory in Boulder, Colorado, and Ron Dantowitz of the Clay Center Observatory at Dexter Southfield, in Brookline, Massachusetts. They will use the giant tower Dunn Solar Telescope at the Sacramento Peak Observatory in Sunspot, New Mexico. They will use an Italian-built spectrometer called IBIS for high-resolution observations of Mercury’s exosphere and also record the event with an IMAX-quality camera, a RED Epic.  Reardon, Potter, and Rosemary Killen (NASA’s Goddard Space Flight Center)  previously used the Dunn telescope during the 2006 transit of Mercury for similar observations.

The Dunn Solar Telescope at Sac Peak is a vacuum telescope, while the New Solar Telescope at Big Bear is an open design needed to allow for far-infrared observations. In a vacuum telescope, sunlight is propagated in a closed system from which air has been pumped, to prevent solar heating from distorting the images.

Pasachoff, Field Memorial Professor of Astronomy at Williams College and a visitor at the California Institute of Technology’s Department of Planetary Science, where he is on sabbatical, is also coordinating with Greg Kopp of the University of Colorado. Kopp runs the Total Irradiance Monitors operating on Air Force and NASA satellites to measure, from above the Earth’s atmosphere, the total energy input to our Earth’s atmosphere that is coming from the Sun.

While the 2004 and 2012 transits of Venus showed up as easily detectable decreases in the solar brightness, the decreases for the 2003 and 2006 transits of Mercury were too small to be detected, something that may help understand the limits of detection of exoplanets with the transit method used by, for example, NASA’s planet-hunting Kepler and the soon-to-be-launches TESS satellites.

Pasachoff, along with Alphonse Sterling of NASA’s Marshall Space Flight Center, is coordinating with the Hinode international science team to obtain observations of the transit with Japan’s Hinode solar satellite. NASA’s Solar Dynamics Observatory will also provide a series of observations of the transit.

Pasachoff’s historical work on the discovery of the atmosphere of Venus has been conducted in part with Dr. William Sheehan of Flagstaff, Arizona.

For images from past transits of Mercury and of Venus, see the Williams College transit website at Images there show how small the silhouette of Mercury looks compared with the silhouette of Venus and even compared with the size of a sunspot.

See the magazine Sky & Telescope’s information about the transit at:

Sky and Telescope Magazine will have a live HD webcast with expert commentary, including an interview with Prof. Pasachoff, on May 9, 7 a.m. to 3 p.m. EDT:

Information on how to observe the Sun safely appears in Pasachoff’s website for the International Astronomical Union at Instructions on observing the Sun safely appear at:

NASA’s Goddard Space Flight Center in Maryland will provide a feed of the transit’s progress at:

Another webcast of the transit will come from at:

Images from NASA’s Solar Dynamics Observatory will be posted at

At Williams College, Dr. Steven Souza will have a solar telescope open to observe the transit of Mercury, if the weather is clear. Opening from 10 a.m. to 1 p.m. EDT on Monday, May 9, is expected, if the weather is clear. You can check if the telescope is open by calling 413-597-2165.

One of the collaborators observing at the Sacramento Peak Observatory, Kevin Reardon of the U.S. National Solar Observatory, is a Class of 1992 alumnus of Williams College.


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.