mountain profile Institute for Astronomy University of Hawaii

Space Observations of Mercury Transits Yield Precise Solar Radius

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March 20, 2012


Dr. Jeff Kuhn
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Mercury Transit 2006

Mercury's path across the solar disk as seen from SOHO on November 8, 2006. Credit: NASA.

A group of scientists from Hawaii, Brazil and California has measured the diameter of the Sun with unprecedented accuracy by using a spacecraft to time the transits of the planet Mercury across the face of the Sun in 2003 and 2006.

They measured the Sun’s radius as 696,342 km (432,687 miles) with an uncertainty of only 65 km (40 miles). This was achieved by using the solar telescope aboard a NASA satellite, thereby bypassing the blurring caused by Earth’s atmosphere that occurs when observations are made from the ground.

The measurements of the Sun’s size were made by University of Hawaii Institute for Astronomy scientists Drs. Marcelo Emilio (visiting from Ponta Grossa, Brazil), Jeff Kuhn and Isabelle Scholl in collaboration with Dr. Rock Bush of Stanford University. They used the Michelson Doppler Imager (MDI) aboard NASA’s Solar and Heliospheric Observatory (SOHO) to make the measurements.

“Transits of Mercury occur 12-13 times per century, so observations like this allow us to refine our understanding of the Sun’s inner structure, and the connections between the Sun’s output and Earth’s climate,” said Kuhn.

The team is preparing to observe the transit of Venus across the Sun on June 5. They expect these observations will improve the accuracy of their solar size measurement even further.

Their scientific paper, which has been accepted by the Astrophysical Journal, is available here.

This work was supported in part by the National Aeronautics and Space Administration under Grant No. 14708980-26967-F issued through the NASA Science Mission Directorate. Any opinions, findings, and conclusions or recommendations expressed in this article are those of the author(s), and do not necessarily reflect the views of the National Aeronautics and Space Administration..

Additional Figures

Larger version of the above figure.

Images from the 2003 Mercury transit observation taken one minute apart show Mercury moving across the face of the Sun. Credit: NASA SOHO/MIDI (1.2 MB).

High-resolution TIFF illustration of Mercury's path as seen from Earth. Courtesy NASA.

A movie of the November 2006 transit is available on the SOHO website.

Founded in 1967, the Institute for Astronomy at the University of Hawaii at Manoa conducts research into galaxies, cosmology, stars, planets, and the sun. Its faculty and staff are also involved in astronomy education, deep space missions, and in the development and management of the observatories on Haleakala and Mauna Kea.

Established in 1907 and fully accredited by the Western Association of Schools and Colleges, the University of Hawaii is the state's sole public system of higher education. The UH System provides an array of undergraduate, graduate, and professional degrees and community programs on 10 campuses and through educational, training, and research centers across the state. UH enrolls more than 50,000 students from Hawaii, the U.S. mainland, and around the world.