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What is the solar diameter in SI units?

Google tells me this is 1.3927 Gm.

How accurate is that figure?

I am looking for estimates with referenced data and calculations, and the uncertainties:

  • due to measurement
  • due to variation in shape
  • due to short term variations in time over different time periods
  • due to definition of extent of the solar body.

An historical perspective may also be interesting, but I am asking about the modern day.

In summary I am looking for the sun's diameter based on referenced observations and the number of significant figures it is reasonable to quote.

David
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    The main problem would be definition: https://www.aanda.org/articles/aa/full_html/2018/08/aa32159-17/aa32159-17.html – Leos Ondra Jan 21 '24 at 10:43
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    The nominal solar radius is exactly 695700 km. But yes, the size depends on how you want to define the extent of the solar body. https://en.wikipedia.org/wiki/Sun#Atmosphere has some info on the approximate sizes of the various layers in the solar atmosphere. – PM 2Ring Jan 21 '24 at 12:24
  • Jupiter = 10 * Earth. Sun = 10 * Jupiter. So ~200 Earth Radii? – Eric Duminil Jan 21 '24 at 19:19
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    Google doesn't know what the diameter of the sun is; it's just reporting a value from one particular site. ("what is the solar diameter", for example, reports a value from http://science.nasa.gov/sun/facts). You can always look at multiple Google results to see if they are consistent with each other, instead of just taking the first result at face value, if you don't trust it. – chepner Jan 22 '24 at 14:04
  • @chepner I agree. That's why I'm here asking for referenced data. – David Jan 22 '24 at 14:06
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    It seems there is a thermal minimum close to the bottom of the chromosphere. The radial thermal minimum would itself define a size although it might not correspond to anything physically except a thermal minimum. – David Jan 22 '24 at 14:11
  • Why are you not first suggesting what you think the diameter should, or should not be, based on whatever observations you've seen or even read of, and the number of significant figures you think it reasonable to quote?

    To put that another way, how does this Question really mean anything other than 'What is the solar diameter?'

     – Robbie Goodwin Jan 23 '24 at 23:27
  • @RobbieGoodwin I asked the question because there didn't seem to be any easy to find answer with the qualities I was looking for. – David Jan 25 '24 at 04:29
  • Thanks, David. I'm sorry. I hope if the Question had been mine, I'd have included that extra info in the first place, because it makes much of the difference between what you were legitimately Asking, and what I wrongly thought you were Asking, that being 'please do my homework for me!' – Robbie Goodwin Jan 29 '24 at 19:06
  • My interest was sparked because I was looking at the figures and 1.3927 seemed unreasonably close to $1+\frac{π}{8}$ but I couldn't find the link to any definite observations. – David Jan 30 '24 at 11:30

1 Answers1

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This is a difficult measurement to make with accuracy. There are issues with the "fuzzy" edge of the sun, and the challenges of pointing a telescope at something so bright. If the observations are made from the Earth, there is also refraction in the atmosphere.

The sun is observed to be extremely spherical. It has a low rotation speed and so the polar diameter should be very close to the equatorial diameter (to within 10km). Observations support this.

The most consistent series of observations I could find were in a paper Variation of the diameter of the Sun as measured by the Solar Disk Sextant (SDS). The paper describes a series of balloon observations. For practical reasons, these observations can only be made in September, and there are a series of observations. I have converted the solar radius values from arcseconds to solar diameter values in km.

Flight number Epoch (year) R⊙@ 1 au (arcsec) D (km) (± 29km)
6 1992.82 959.638 ± 0.020 1392005
7 1994.81 959.675 ± 0.020 1392059
8 1995.82 959.681 ± 0.020 1392068
9 1996.85 959.818 ± 0.020 1392266
10 2001.83 959.882 ± 0.040 1392359 (±58km)
11 2009.87 959.750 ± 0.020 1392168
12 2011.86 959.856 ± 0.020 1392321

The "edge" of the sun was defined as the point of inflection in brightness as one moves from the disk to the background (using a wideband filter centred on 615nm), and this is why this value may differ from other quoted values. However it is consistent between flights, so one is comparing like-with-like.

The variation observed here is greater than instrumental uncertainty and doesn't correlate with the 11 year solar cycle. The researchers state that there was no periodicty observed in these data, but that the sparse nature of the observations made it not possible to determine if the results were cyclic or not.

James K
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