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Squink
2014-Jul-07, 02:46 PM
Is the ISS naked eye visible when it transits the sun?
ISS transit of the sun (http://spaceweathergallery.com/indiv_upload.php?upload_id=99121), near sunspot groups 2107 and 2104 (http://www.spaceweather.com/images2014/05jul14/hmi1898.gif?PHPSESSID=9r02pgguueobc0uf2sja70t411)

Those are pretty big spots, they look like they might be visible to a protected eye. ISS is similar in size.
So has anyone seen the ISS silhouetted against the sun without magnification, or using a pinhole device?

NEOWatcher
2014-Jul-07, 03:24 PM
Is the ISS naked eye visible when it transits the sun?
OH NO! not with your naked eye. http://www.cosgan.de/images/smilie/verschiedene/s020.gif (http://www.cosgan.de/smilie.php)

(I know you don't really mean that since you explained what you were getting at.)

Squink
2014-Jul-09, 12:42 PM
Looks like ISS is naked eye visible against the sun, but you have to be quick about it:

ISS subtends 42 arcseconds (http://www.perseus.gr/Astro-Sat-ISS-2010-05-29.htm).
Limit of sunspot visibility is 41 arcsec penumbra, 15 arcsec umbra (http://adsabs.harvard.edu/full/1992QJRAS..33...83K).
ISS is umbra-like.

By way of comparison, Venus subtended 58.3 arcseconds (http://sunearthday.nasa.gov/2012/articles/ttt_75.php), and was easily visible, during the 2012 transit.
Mercury subtends 10-12 arcsec (http://books.google.com/books?id=3AODx2z63vEC&pg=PA131&lpg=PA131&dq=mercury+transit+arcseconds&source=bl&ots=0PLqspPbbC&sig=E8xM-sOF-SyMSCR_nc2EfjvTSXY&hl=en&sa=X&ei=5Ay8U6L9Faa78QG41IHQAQ&ved=0CEIQ6AEwBA#v=onepage&q=mercury%20transit%20arcseconds&f=false) during transit, and is not naked eye visible.

The difficulty with eyeballing an ISS transit, is the short duration of the event (http://legault.perso.sfr.fr/iss_endeavour_transit.html), about 0.75 seconds (http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/6451503/Main/5874975).
Still, with an audible countdown, it should be possible to catch a quick glimpse of the ISS against sun or moon. I looked but didn't turn up anyone who claims to have done that.

George
2014-Jul-13, 06:48 PM
Using solar glasses, it does seem likely that some people would be able to see it. Your reference -- a nice paper! -- states that many trained observers miss sunspots that are as much as 1 arcminute in diameter. But others can see about 1/2 arcminute, which is twice what you generally read for acuity.

However, the Sun is bright enough to excite the much higher resolution region (fovea), so 1/2 arcminute sizes, I suppose, should not surprise us.

This looks like another fun-n-the-Sun event worth trying. Thanks!

Hornblower
2014-Jul-15, 01:31 PM
Sunspots have much lower contrast than what we commonly see in photographs in which the contrast has been boosted. That could account for some of the variability in visibility for any given size. A person with 20/20 vision often can see totally black spots much smaller than one arcminute. The common rule of thumb of one arcminute is for resolving details such as double stars.

George
2014-Jul-15, 02:44 PM
Sunspots have much lower contrast than what we commonly see in photographs in which the contrast has been boosted. That could account for some of the variability in visibility for any given size. A person with 20/20 vision often can see totally black spots much smaller than one arcminute. The common rule of thumb of one arcminute is for resolving details such as double stars. Ah, that makes sense. I suppose this contrast would be with a brightness level that excites the fovea (greater acuity) to explain resolution < 1 arcmin.