NASA's Solar and Heliospheric Observatory (SOHO) has provided precise measurements of the Sun's diameter by tracking the time taken for tiny Mercury to transit across its face.

Scientists from Hawaii, Brazil and California calculated the Sun's radius as 696,342 kilometres with an uncertainty of just 65 kilometres using the Michelson Doppler Imager (MDI) aboard SOHO, for transit events in May 2003 and November 2006.

“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,” says Jeff Kuhn of the University of Hawaii.

SOHO is a space-based observatory, so the measurements do not suffer the same distortion effects caused by the Earth's atmosphere that plague ground-based telescopes.

"Ground observations are limited by the spatial resolution with which one can determine the instant Mercury crosses the limb," the authors report in their paper that will appear in a forthcoming issue of the Astrophysical Journal. "This is the first time accurate Mercury’s transit contact times are measured by an instrument in space and improves at least 10 times the accuracy of classical observations."

The team will use observations of the forthcoming transit of Venus to improve the accuracy of their measurements even further.

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