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

By Phil Plait | March 21, 2012 7:00 am

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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CATEGORIZED UNDER: Astronomy, Cool stuff, Science, Top Post

Comments (26)

  1. Matt

    Oh great, another number. When will you astronomers stop with the numbers?! If my kid ends up having to memorize this for a high school science final in a decade I’ll be back, and I won’t be happy.

  2. What an accuracy!!!
    But I think the correct statement is that Sun’s photosphere is 1,392,684 +/- 65 km. What’s your opinion?

  3. Other Paul

    Not exactly sure what constitutes the actual surface of the sphere in question. Given sol’s size, 65km seems well within the ‘feathered’ region, by an order of magnitude at least.

  4. I’m currently 8,872.64 km from my home country, which felt like quite a lot until I read this. But it’s only about 0.637 per cent the diameter of the sun (give or take 65 km). I am a gnat.

  5. Daniel J. Andrews

    I wouldn’t have thought they could get something this accurate (difficulty in measuring the boundaries of a gas ball, pulsations, etc), but shows what I know. It occurs to me if I were ideologically motivated to believe the sun was a certain size and this study contradicted it, I’d be primed to automatically reject it as it contradicts my (flawed) understanding of the sun. Thus my ideology is safe. The Dunning-Kruger lurks within us all waiting for a gap in the critical thinking bars to escape. /bad metaphor.

  6. Most of us are wrong about the sizes of our solar system bodies and their distances by a factor of 10-100! It is difficult to comprehend the reality when most diagrams you see of our solar system show them as concentric circles in a small space. gives a good indication of what those really are.

  7. Messier Tidy Upper

    Is that counting the prominences and flares?

    Or the corona or the solar wind in which we’re all embedded and that stretches all the way beyond Pluto to the heliopause?

    Where does our Daytime Star really end? As #1 Vossinakis Andreas has already hinted at what exactly are we measuring here and how well can it be said to be our Sun or just part thereof?

    Still great precise measuring news & I’m really not complaining. Nicely done. :-)

  8. Chris

    I looked at the paper and I was expecting to see much larger error bars for earlier measurements. Amazingly many of the earlier measurements have error bars much smaller than the current value as far back as 1970. That of course doesn’t mean they are right, but it’d be interesting to see why they perhaps over estimated their accuracy and precision.

  9. Jeff

    it might mean the photosphere is sharper in diameter than I was thinking; they probably have a pretty sharp boundary with the chromosphere to get it this precise within 65 km; or maybe it has a “fuzziness” of around that.

  10. That is amazingly huge. Having just taken a road trip, I wondered just how long it would take to drive a Hypothetical Sol Diameter Highway. At 80mph (about the fastest that I feel safe driving), going 865,374 miles would take almost 1 year and 86 days. Better pack some snacks.

    If we take the car that set the land speed record of 763 mph, though, it will “only” take us 47.25 days.

    Maybe we should take the fastest plane, the X-43. (It’s unmanned but let’s not worry about that right now.) This goes 7,000 mph so the trip will take us over 5 days.

    Maybe we could use one of the old Space Shuttles. They went 17,500 mph. This would mean the trip would take just over 2 days.

  11. vel

    of course you could trust a solar panel manufacturer and go wtih the sun is only 50 times the size of the earth: part of the changing messages beside the logo. yep, really makes me want to trust *that* company……

  12. Eric

    Does this take into account the equatorial bulging due to centripetal forces?

  13. Nerdy Chuckles

    This guy seems to have been able to figure it out. And i bet he’s never looked through a telescope either.

    “Q: “What about the stars, sun and moon and other planets? Are they flat too? What are they made of?”
    A: The sun and moon, each 32 miles in diameter, rotate at a height of 3,000 miles above sea level”

  14. Jeff

    I don’t know if anyone has done this sunspot measurement using solar diameter , but I’ve done it for years and it can be done in a very simple way: this year the sunspot groups have been great. Just project the solar image from a small binoc or scope on paper and measure the sunspot group size in mm, and the solar disk size in mm on the paper. I just got a small hundred buck desktop Newtonian 100 mm scope that is fantastic and it even has a small aperature on the opening for this activity, a piece of cake. (they stop down the aperature because the manufacturer is afraid if they open it wide , the sun will cook the scope, maybe they are right, but it never happened to me, it’s possible)

    The true sunspot size in km = (sunspot in mm/solar disk in mm) X number he has above for solar diameter in km.

    Compare this with the diameter of earth 12,000 km, you’ll be astonished unless you did this before how big these are.

  15. Chris

    @10 TechyDad
    Are we there yet? Are we there yet?

    Actually it wouldn’t be the diameter unless you go through the center. It would be another factor of (pi/2) to go across the face of the sun.

  16. @Chris,

    I figured that the Hypothetical Sol Diameter Highway would go through the center. Better crank that AC up some more.

    Oh and be quiet back there or I *WILL* turn this rocket car around!

    Knowing my kids, they’d decide they needed to use the restroom only after we left the rest area and passed a sign saying “Next Rest Stop in 17,000 miles.”

  17. Thopter

    @11 Vel, not only that, but they couldn’t even spell Antares correctly.

  18. Molnar

    Query: If we don’t know the diameter of the sun, how accurately can we measure our distance from it? Or does that statement refer to something like the center of mass of each body?

  19. Digitalaxis

    @16 TechyDad:

    That’s “Next Sun Spot in 17,000 miles”

  20. MadScientist

    “Venus has an atmosphere which I would think would confound the observations.” …

    Not really. Look at an image of the earth from a geostationary satellite – how thick is the glowing atmosphere compared to the solid disk? In the case of the earth, about 95% or so of the atmosphere is in the lower 30km while the earth has a radius of ~6300km – so the atmosphere is a mere ~0.5% of the radius (less even, since the sky looks pretty black even at 20km altitude). Scattering of the light etc. is also fairly straightforward to model so you can make other compensations. I suspect the number of pixels representing the diameter of Venus would be far more relevant to calculations than the atmosphere.

  21. Chris A.

    @vel (#11)

    “of course you could trust a solar panel manufacturer and go wtih the sun is only 50 times the size of the earth”

    My guess is that someone mixed up radius and diameter, given that the sun spans 109x Earth’s diameter. That being said, I’ve always hated comparisons like “planet A is X times larger than Earth,” because of the ambiguity of “larger.” Are we comparing diameters or volumes? Can we be sure our reader/listener is thinking the same thing we are?

  22. KaoS

    Attn. Phil: ” That uncertainty of 65 km is quite a it better…”

  23. Chet Twarog

    Astronomy Now article had a “calculated the Sun’s radius as 696,342 kilometres with an uncertainty of just 65 kilometres”. So, wouldn’t the diameter have an uncertainty of +/- 130 km?

  24. Dave Jerrard

    Curse you Wikipedia!

    In my model, I had a diameter of 1,391,000 km. Now I gotta do the whole thing over. Thanks, science.


  25. Neal Z.

    Our Astronomy club recently had a talk about the up coming Venus transit of the Sun. The speaker mentioned the “drop” effect that was observed in late 1800’s when the first measurements were attempted. Does Mercury exhibit this also or has the cause been found and remedy developed?

  26. Nemo

    @Eric #12:

    Does this take into account the equatorial bulging due to centripetal forces?

    My thoughts exactly. Surely the Sun deviates from sphericity by more than 65 km?


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