NavList:

Thanks for the calsky link, Greg. I didn't realize that that site included historical orbit data. I've confirmed that their orbit data is, in fact, the correct historical orbit for the ISS on the date in question. So the mean altitude that day was about 219 miles, luckily close to what I suggested as a good approximate height.

I am more convinced than ever that the visible horizon in the photo is the top of a cloud deck, and it's probably a very high cloud deck. Typically that smooth continuous line indicates a cloud deck at the tropopause. So call it an altitude of eight miles. This is the "effective" surface of the Earth in the photo, so you can calculate dip based on an altitude of 211 miles. Note that we're above a large fraction of the atmosphere at the tropopause so the issue of refraction is significantly reduced, maybe 4' of refraction.

By the way, you all should be sure to grab a copy of the large version of the original image from the NASA site. There are more stars visible, and iota Ceti (if I've identified it correctly) should be especially useful since it gives us a wider azimuth range on the available altitudes.

One more thought: the Moon's position among the stars of Pisces in the direction parallel to the Moon's limb (left-right in the photo), will determine the latitude to within about one degree. I don't know yet if the altitudes will give us greater accuracy.

-FER

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