Mercury Makes Precise Measurement of the Sun

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In what sounds like an astronomical “David and Goliath” tale, the smallest planet in the solar system (no, not Pluto, the other one) has helped astronomers precisely measure the width of the largest thing in the solar system — the sun.

Wait. I hear you cry. Do astronomers have nothing better than to measure the girth of our nearest star? Besides, we should know that already… shouldn’t we?

In the sage words of Bad Astronomer Phil Plait: “Sometimes the simplest things can be the hardest, I suppose.” And he’d be right.

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When we hear about incredible observations of black holes guzzling dinky asteroids, tiny worlds being detected around distant stars and the age of the Universe being measured to a precision of 0.8 percent, it sounds absurd that we don’t know the size of the sun. You know, that thing I can see out of my window. Every day! Come on, it can’t be that hard. Can it?

Yes, yes it can. And as planet-dwelling creatures, our atmosphere really doesn’t help.

As announced by scientists at the lovely University of Hawaii Institute for Astronomy (IfA) in Honolulu plus collaborators from Brazil and Stanford University on Wednesday, the sun’s radius was measured by carefully timing two transits of the tiny planet Mercury. A transit is when a planet orbits in front of the sun, from the observer’s perspective.

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Mercury transits occur 12-13 times per century, and in recent years the joint NASA/ESA Solar and Heliospheric Observatory (SoHO) was able to observe two transits — once in 2003 and once in 2006.

Already, you may have realized that SoHO has a distinct advantage. Not only does the observatory have an uninterrupted view of our nearest star — hovering around the Earth-sun L1 point, 1.5 million miles from our planet in the direction of the sun — it doesn’t suffer the obscuring effects of the Earth’s atmosphere.

Our atmosphere bends, distorts, refracts and reflects incoming light, and although the sun may seem very bright and easy to observe, making precision measurements of its size have proven difficult for ground based observatories. As noted by the researchers, “the true solar radius is still a matter of debate” and that the solar radius as noted by other studies vary as much as 500 kilometers (310 miles). Their publication, accepted for publication in the Astrophysical Journal, can be read in its entirety online (PDF).

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But using space-based observations of Mercury transits, the time taken for Mercury to complete its dash across the disk of the sun could be accurately measured to an accuracy of a couple of seconds. As the orbital period of Mercury is very well known, and after using a little trigonometry, the little planet had become a very useful (and very accurate) tape measure.

So, what’s the sun’s waistline measurement?

The sun’s radius is 696,342 (plus or minus 65) kilometers — that’s 432,687 (plus or minus 40) miles.

Therefore, the sun’s diameter is 1,392,684 +/- 65 kilometers or 865,374 +/- 40 miles.

But what I really want to draw attention to is the “+/-” number. This represents the error in the measurements made. “Error” doesn’t mean the astronomers made a mistake; it means that once all the systematic aberrations in the instrumentation, any distortions by looking through the sun’s atmosphere and any slight transit time uncertainties have been considered, this is the precision that the astronomers can be confident their observations achieved.

This precision suggests that their solar diameter measurements are 99.995 percent accurate. So if you could physically go to the sun and pull a tape measure through the solar body, your measurement of the sun’s diameter would be accurate to within 0.005 percent of this figure.

Thank you Mercury, you’re a great help!

Sources: IfA, Bad Astronomy

Image: The Mercury Transit on Nov. 8, 2006. Credit: NASA/ESA/SOHO

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