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Space Observations of Mercury Transits Yield Precise Solar Radius

Mercury transit 2006

Mercury’s path as seen from Earth. Courtesy NASA.

A group of scientists from Hawa‘ii, 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 432,687 miles (696,342 km) with an uncertainty of only 40 miles (65 km). 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 IfA scientists Marcelo Emilio (visiting from Ponta Grossa, Brazil), Jeff Kuhn, and Isabelle Scholl, in collaboration with 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. Because Mercury is farther from Earth and much smaller than Venus, transits of Mercury are not visible with the naked eye.

More recently, the team observed 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.