Eclipse helped astronomers determine sun's radius

May 25, 2012

By SEIJI TANAKA/ Staff Writer

This week's annular eclipse allowed astronomers in Japan to come up with the most precise measurement yet of the sun's radius.

With a margin of error of 20 kilometers, they determined the sun has a radius of 696,010 km.

The finding, announced May 24, was based on the eclipse that occurred three days earlier.

All studies have put the sun's radius at about 696,000 km, give or take 50 or 100 km.

This sizable difference is due to the sun's brightness and the difficulty of determining its precise contour.

The latest estimate was derived from observations of a phenomenon called Baily's beads, whereby beads of sunlight shine through the moon's uneven topography during a solar eclipse.

Astronomers filmed the event with camcorders at 23 observation points scattered across Japan from Kagoshima Prefecture, in the southwest, to Fukushima Prefecture, in the northeast. High-precision data was recorded at 11 observation sites.

Tsutomu Hayamizu, director of the Sendai Space Hall in Satsumasendai, Kagoshima Prefecture, and a leading member of the project, said the network of observations enabled the annular eclipse to be recorded across a wide area, allowing for precise measurements to be made.

The next annular eclipse in Japan will be visible from Hokkaido in 2030.

By SEIJI TANAKA/ Staff Writer
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This X-ray image of the sun under partial eclipse was captured May 21 by the Hinode solar physics satellite. (Provided by the Japan Aerospace Exploration Agency and the National Astronomical Observatory of Japan)

This X-ray image of the sun under partial eclipse was captured May 21 by the Hinode solar physics satellite. (Provided by the Japan Aerospace Exploration Agency and the National Astronomical Observatory of Japan)

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  • This X-ray image of the sun under partial eclipse was captured May 21 by the Hinode solar physics satellite. (Provided by the Japan Aerospace Exploration Agency and the National Astronomical Observatory of Japan)

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