Sun is the most perfect sphere ever observed in nature

Scaled to the size of a beach ball, say scientists, the sun’s equatorial bulge would be less than the width of a human hair

As a spinning ball of gas, astronomers had expected the sun to bulge slightly, but the difference in diameter between equator and poles is only 10km. Photograph: PA

The Sun is the most perfectly round natural object known in the universe, say scientists who have conducted precise measurements of its dimensions.

As a spinning ball of gas, astronomers had always expected our nearest star to bulge slightly at its equator, making it very slightly flying-saucer shaped. The planet Jupiter demonstrates this effect well. Its high rate of spin – once every 10 hours – means that it is almost 7% wider across its equator than the distance from pole to pole.

Now a team led by the University of Hawaii’s Dr Jeffrey Kuhn have made the first precise measurement of the sun’s equatorial bulge, or its “oblateness”. The results were a big surprise. “We were shocked,” says Kuhn. The sun doesn’t bulge much at all. It is 1.4m kilometres across, but the difference between its diameter at the equator and between the poles is only 10 kilometres.

Scaled to the size of a beachball, that difference is less than the width of a human hair. Only an artificial sphere of silicon that was created as a standard for weights is known to be more perfectly spherical.

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