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SYFY WIRE Bad Astronomy

The Sun is 1,392,684 +/- 65 km across!

By Phil Plait

You would think that, of all the astronomical measurements we could make, one of the best known would be the diameter of the Sun.

You'd be wrong. It's actually really hard to measure! For one thing, we sit at the bottom of an ocean of air, gas that is constantly moving around, mucking up precise measurements. For another, our telescopes themselves can be difficult to calibrate to the needed accuracy to get a really solid measurement of the size of the Sun.

Nature, however, provides us with a way to measure our nearest star. Naturally! And it involves Mercury, the smallest and, critically, the closest planet to the Sun.

In 2003, and again in 2006, Mercury passed directly across the face of the Sun as seen from Earth (the picture above is a view of the 2006 transit as seen by SOHO; a very short animation was made from this as well). Mercury's orbit is tilted a little bit with respect to Earth's, so these transits don't happen terribly often, occurring only every few years. But because we know the orbit of Mercury so well, and our own distance from the Sun, by precisely timing how long it takes the diminutive world to cross the Sun, we can get a very accurate measurement of the Sun's diameter.

A team of scientists did exactly this, using SOHO, which is a solar observing and solar-orbiting satellite. Because it's in space, it doesn't suffer from the problems of peering through a murky, dancing atmosphere. They were able to measure the timing of Mercury's passage of the Sun to an accuracy of 3 seconds in 2003 and 1 second in 2006. They had to take into account a large number of effects (the Sun's limb is darker than the center, which affects timing; they had to accurately measure the position of Mercury; they had to account for problems internal to SOHO like focus and the way it changes across the detector; and, of course, correct for the fact that Mercury cut a chord across the Sun and didn't go straight across the diameter -- but that only took knowledge of Mercury's orbit and some trig) but when they did, they got the most accurate measure of the Sun's diameter ever made: 1,392,684 +/- 65 km, or 865,374 +/- 40 miles.

That uncertainty of 65 km is quite a it better than what can be done from the ground, amazingly. It may sound like a lot, but it actually represents an accuracy of 99.995%! The Sun is big. Really, really big.

... and they're not done. The authors are going to observe the Transit of Venus coming up in June, hoping it'll improve their measurements. I'll be very curious to see how that goes; Venus has an atmosphere which I would think would confound the observations. They may have ways around that though.

Either way, I think this is completely fascinating. Even thrilling! The Sun is the brightest thing in the sky, the center of our solar system, the basis of light and heat and life on Earth, and the best-studied star in the Universe.

And here we are, just now figuring out how big it is. Sometimes the simplest things can be the hardest, I suppose.

Image credit: NASA/EDA/SOHO

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