International Space Station Transits the Sun (30 Jul 2008)

emc · Post by **emc** » Wed Jul 30, 2008 12:34 pm

http://apod.nasa.gov/apod/ap080730.html

ISSSSS (simulates sun spot)

Today, due to the low level of solar activity… a small group of humans create their own!

orin stepanek · Post by **orin stepanek** » Wed Jul 30, 2008 1:58 pm

It looks like a mosquito on a big white ball.

Orin

apodman · Post by **apodman** » Wed Jul 30, 2008 2:11 pm

I have a question about the appearance of the Sun in this photo.

There is a slightly dark region (about the fraction of a sphere equal to the Arctic on Earth) with the "pole" at about 12:30 on the Sun.

More prominently there is a light arc around a region of similar size with the "pole" at about 9:30.

I adjusted the brightness, contrast, and gamma in a copy of the photo. When I decreased the gamma to 0.3, the bright arc stood out even more prominently while the dark region did not.

So what am I looking at?

emc · Post by **emc** » Wed Jul 30, 2008 2:15 pm

orin stepanek wrote:It looks like a mosquito on a big white ball.
Orin

Not enough that mosquitoes want to drain us dry... now their eyeing the sun.

Hey, wouldn't that be cool... if mosquitoes were suddenly attracted to the sun instead of us... hey hey, bye bye! Not likely though, since they don't seem to like the sunny side of my yard.

Thanks Orin for the diversion! 8)

Arramon · Post by **Arramon** » Wed Jul 30, 2008 4:07 pm

Now if the ISS would have just waited for friday's partial eclipse.... d'oh!! =)

orin stepanek · Post by **orin stepanek** » Wed Jul 30, 2008 4:24 pm

apodman wrote:I have a question about the appearance of the Sun in this photo.

There is a slightly dark region (about the fraction of a sphere equal to the Arctic on Earth) with the "pole" at about 12:30 on the Sun.

More prominently there is a light arc around a region of similar size with the "pole" at about 9:30.

I adjusted the brightness, contrast, and gamma in a copy of the photo. When I decreased the gamma to 0.3, the bright arc stood out even more prominently while the dark region did not.

So what am I looking at?

I noticed the discoloration also! I wonder if it is a normal condition on the sun of it is a photograph related condition.

Orin

TwilightSoul · Post by **TwilightSoul** » Wed Jul 30, 2008 5:09 pm

orin stepanek wrote:I noticed the discoloration also! I wonder if it is a normal condition on the sun of it is a photograph related condition.

Orin

Me three.

And why does the sun look like it's been illuminated? That is, the center is slightly brighter than the edges, as I might expect to see from a planet. I would expect the sun to be equally luminous from every angle.

Or should I have taken a lot more physics or astronomy classes than I did?

->A

BMAONE23 · Post by **BMAONE23** » Wed Jul 30, 2008 5:13 pm

TwilightSoul wrote:
orin stepanek wrote:I noticed the discoloration also! I wonder if it is a normal condition on the sun of it is a photograph related condition.

Orin
Me three.

And why does the sun look like it's been illuminated? That is, the center is slightly brighter than the edges, as I might expect to see from a planet. I would expect the sun to be equally luminous from every angle.

Or should I have taken a lot more physics or astronomy classes than I did?

->A

Part of that might be from the angle that the Photons are striking the imager's line of sight. Those coming from the center of the viewpoint will impart the most energy on the imager while those slightly off angle will transfer less energy.

here is another great APOD on the same subject
http://antwrp.gsfc.nasa.gov/apod/ap060921.html

apodman · Post by **apodman** » Wed Jul 30, 2008 5:48 pm

TwilightSoul wrote:the center is slightly brighter than the edges

What you refer to is "limb darkening".

http://en.wikipedia.org/wiki/Limb_darkening

Imagine an object the size of the Sun whose surface is completely covered with illuminated light bulbs.

If the light bulbs are perfectly clear, a viewer at a distance will actually see brighter edges than center, as there are more filaments glowing in a given size field of view. If the light bulbs are frosted, the ones at the edges will seem dimmer, as the light from each filament is obscured by the frosting of multiple bulbs.

I guess the light sources in the Sun and the material in the Sun's photosphere that blocks or absorbs light act more like the frosted bulbs than the clear ones.

TwilightSoul · Post by **TwilightSoul** » Thu Jul 31, 2008 3:36 am

Great explanations! Thanks!

But we still have the mystery of the semicircular discolorations at 12:00 and the arc at 9:30... Ghost images from lenses or something?

->A

neufer · Post by **neufer** » Thu Jul 31, 2008 8:41 am

http://en.wikipedia.org/wiki/Image:Bat- ... n_1989.jpg

NoelC · Post by **NoelC** » Thu Jul 31, 2008 2:06 pm

neufer wrote:http://en.wikipedia.org/wiki/Image:Bat- ... n_1989.jpg

Heh, and some complain about the APOD being not astronomy-related.

I suspect the faint "defects" in the image may be from combination of multiple images to provide a full image of the orb, but that's pure speculation at this point.

-Noel