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    Re: NASA photo
    From: Frank Reed
    Date: 2019 Oct 27, 14:52 -0700

    Thanks, Dave! Looks like we were posting at exactly the same time. Yes, that's the bowl of the Big Dipper. 

    I found links to some descriptions of the image which say it's an 11-minute time lapse. That doesn't sound right. In 11 minutes, the stars would move nearly 45° relative to the sidereal pole of the orbit --that fixed "pole" near Groombridge 1830 in the image. The image appears to have been constructed by stacking individual frames from a video, and I could imagine that some version of the original video was 11 minutes, but we're only seeing five minutes worth in the image. The descriptions also confirm that the view is looking across Africa though they say that ISS was "over" Africa at the time. The view at a shallow angle to port was Africa, but the station was directly over the western Indian Ocean in order to see that view.

    It's interesting to consider that the "pole" of relative motion of the stars would remain quite close to that spot below the Big Dipper for a rather long time. Precession of the orbital nodes eventually moves it, but that's around 5° per day. So for that entire day when the time-lapse was taken, totalling some 16 orbits around the Earth, the view of the stars to port would have been nearly the same.

    By the way, I mentioned that the pole of the rotation was near 38° N in the image. And of course it was! The orbital plane of the ISS is inclined at 51.6° to the equator, so the poles of the orbit are located at the complement of that angle relative to the celestial equator: Dec = 38.4°. The RA/SHA of those poles makes a complete circuit around the sky roughly every sixty days (that's nodal precession caused by the oblate/spheroidal distribution of the mass of the Earth). Vega has nearly that declination. Once every two months, Vega would hang there nearly motionless directly to port for a few orbits.

    Frank Reed

       
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