Tag: oblateness

Why is the Sun so round?

By Phil Plait | September 12, 2012 7:00 am


[NOTE: When I originally wrote this, I made a mistake - I said the Sun was 30 arcseconds across, when it's actually 30 arcminutes. For some reason, that number got stuck in my brain, and the math I did was based on the incorrect number! I have corrected the math in the text below. Usually I keep the original mistake in an article (striking through the text) along with the correction - that's my way of admitting mistakes. But given that this is math, I was afraid that might look a bit confusing, so instead I'll note my brain hiccup here, and keep the math clean by simply fixing it. However, this does change the analogy I used in the text comparing the Sun to a basketball, so in that case I struck through the text and added the correct analogy. I know, it sounds confusing, but it'll be clear when you read the article. My apologies for this!]

A new study has been published that seems very simple yet has some very interesting repercussions: it shows the Sun is the most spherical natural object ever measured.

Measuring the Sun’s diameter is actually rather difficult. For one thing, observations from the ground have to deal with our atmosphere which warbles and waves above us, distorting images of astronomical objects. To get past that, the researchers used a camera on NASA’s Solar Dynamics Observatory, which orbits high above the Earth. The camera is very stable, and gets past a lot of the problems of measurement uncertainty.

Another problem is that the Sun doesn’t have a solid surface. It’s not like a planet – and even that can be tough to measure. Since the Sun is gaseous, it just kind of fades away with height, so if you try to get too precise you find a lot of wiggle room in the size. In fact, the largest variation the researchers found in the solar diameter was due to intrinsic roughness of the Sun’s limb – in other words, on very small scales the Sun isn’t smooth.

Still, there are ways around that. The point here isn’t necessarily to find the actual size, but the ratio of the diameter of the Sun through the poles (up and down, if you like) to the diameter through the equator. That tells you how spherical the Sun is.

What I would expect is that the Sun is slightly larger through the equator than through the poles, because it spins. That creates a centrifugal force, which is 0 at the poles and maximized at the equator. Most planets are slightly squished due to this, with Saturn – the least dense and fastest spinning planet, with a day just over 10 hours long – having a pole to equator ratio of about 90%. It’s noticeably flattened, even looking through a relatively small telescope.

The Sun spins much more slowly, about once a month. That means the centrifugal force at its equator isn’t much, but it should be enough to measure. So the scientists went and measured it.

And what they found is that the polar and equatorial diameters are almost exactly the same. In fact, they found that the equatorial diameter is 5 milliarcseconds wider than the polar diameter. An arcsecond is a measure of the size of an object on the sky (1° = 60 arcminutes = 3600 arcseconds), and the Sun is about 30 arcminutes (1800 arcseconds) across. In other words the equatorial diameter is only 0.0003% wider than the polar diameter!

The Sun is a 99.9997% perfect sphere. Hmmm.

Put another way, if you shrank the Sun to the size of a basketball, the equatorial diameter would be wider than the polar one by about 0.4 microns – the width of a human hair less than the size of an average bacterium! That’s actually pretty cool.

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CATEGORIZED UNDER: Astronomy, Cool stuff
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