How Big is the Sun, Really?

Sky and Telescope ^ | 3/21/2012 | Kelly Beatty

[U-238]

With all the attention that astronomers have lavished on old Sol over the centuries, you'd think that by now they'd know its diameter to, oh, 10 or 12 significant digits.

During the past 40 years, astronomers have attempted to measure the Sun's sizedozens of times using various methods. The dashed line corresponds to a radius of 696,000 km, the value most often used.

While the Sun's girth has indeed been measured dozens of times over the past 40 years, the results haven't converged on a single value and scatter by as much as ± 0.1%. One big reason is that, though some measurement techniques are extremely precise, their accuracy suffers because of the turbulence induced by Earth's atmosphere. Most often astronomers use a compromise value of 865,000 miles (1,392,000 km).

So Marcelo Emilio (State university of Ponta Grossa, Brazil) has teamed with observers at the University of Hawaii and Stanford to approach this measurement with, literally, space-age techniques. They used images taken by the Michelson Doppler Imager (MDI) aboard NASA’s Solar and Heliospheric Observatory (SOHO), homing in on transits of the planet Mercury across the solar disk in 2003 and 2006.

This makes perfect sense. The spacecraft sits at the L1 Lagrange point, a million miles from Earth, and Mercury has nothing but the barest wisps of atmosphere — a made-to-order combination for crisp images.

[ml/nj]

Did you fly your Lear Jet up there? Geez you’re vain. You probably think this post is about you.

Nope. Didn't have my ticket yet then. (And I never was PIC of anything with more than one propeller.)

Carly Simon was singing about the Nova Scotia eclipse which occurred on July 10, 1972, BEFORE Saratoga opened on July 31 of that year. SHE's so STUPID.

For those who don't know the song's lyrics are in part:

Well I hear you went up to Saratoga and your horse naturally won
Then you flew your lear jet up to Nova Scotia
To see the total eclipse of the sun

ML/NJ

[WillVoteForFood]

Hey, check out that Sun up there. That sucker’s huge.

Tiny E.

[Verginius Rufus]

It was still true in 1999 (I was lucky enough to see the total eclipse that year which was visible in parts of Europe--lucky because the clouds broke just in time to watch the total phase). I think there is a line in Samuel Taylor Coleridge's "Rime of the Ancient Mariner" about the sun appearing no bigger than the moon.

The next total eclipse visible in the US will be on August 21, 2017...the first in the 48 states since 1979.

[brivette]

thank you Carly.

[U-238]

They seem to be assuming that it has a constant diameter. Isn’t it possible that the sun’s diameter changes from time to time?


The Sun expands and contracts because it made from gas. Gravity pulls the sun together and the heat and light of the fusion process expands it. So there is a balance between the forces and has a steady radius. Why scientists want to know?Maybe they want to know how far along the Main Sequence the Sun in evolving. See Hydrostatic equilibrium

[SunkenCiv]

Thanks U-238.

Also, if memory serves, the Sun's cornea ('surface') pulses on some interval, I think it's about 45 minutes, so the diameter varies between two values.


Thanks .

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[Happy Rain]

There was a total solar eclipse visible to parts of the South Eastern United States in 1969—to the best of my knowledge—maybe it was 1970 or 1968—but I remember my motther—irrationally protective—fussed at me because I watched it without vision filters after it went total—it was so weird cuz the night creatures started singing and the day creatures went silent and the corolla was awesome—a cosmic ring of flashing electric emanations that unlike lightning were straight and variable in length—to me on the ground it was like a horizon to horizon dark glass soup bowl was put down on my back yard.

[ml/nj]

There was a total solar eclipse visible to parts of the South Eastern United States in 1969—to the best of my knowledge—maybe it was 1970 or 1968—but I remember my motther—irrationally protective—fussed at me because I watched it without vision filters after it went total

I think the eclipse you are referring to was the same one as the one I saw on Nantucket on March 7, 1970. Here's a map of the path it took:

There are always a lot of warnings before an eclipse about the danger of looking directly at the sun during the eclipse. This is because almost everyone will be looking at a partial eclipse. Even those in the path of totality usually get about an hour before and after totality of waxing and waning partial eclipse. During the short period of totality though (and not a second before or a second after) looking directly at the sun is no more dangerous than looking at the full moon. I actually looked directly at the eclipse through an unfiltered telescope for about 30 seconds. Seeing the red-orange solar prominences was quite spectacular. Something like this:

This isn't my picture, and my recollection is that I saw more orange than appears in this picture.

ML/NJ

[Happy Rain]

That must have been it—I was living in Goose Creek, SC at the time—which is about halfway between Charleston and Summerville- almost smack dab on the redline.

One of those very intense moments of my youth—I was awestruck.