The center of the circles lies in the direction the camera is rotating about. I imagine the camera was fixed relative to the ISS, and the ISS always keeps the same side toward the Earth. If so, then the center of the circles would be the direction perpendicular to the orbital plane of the ISS.
The most sensible rotation in the photo is the one of the ISS around Earth, not Earth's own rotation. The orbit doesn't need to be equatorial.
(Edit:) About your other question: the middle of the circles is the ISS's orbit pole.
(Edit 2:) OK, I get what you find weird: the fact that stars's circles and land ones don't have exactly the same pole. That's because Earth's rotation.
IIRC, a very low orbit object makes a circle in 90 minutes. I guess IIS is higher so it would orbit in a few hours. In these few hours the traces of land lights result from the composition of orbit and rotation, while the star's one only from the orbit. An angle of, let's say 20% of ISS orbit with equatorial plane would explain the difference.
BTW, I don't think the camera is rotating. Something, maybe an antenna shows in the photo against the star background. The ISS must be rotating in the same period of the orbit, so it always shows the same face to Earth's surface.
> BTW, I don't think the camera is rotating. Something, maybe an antenna shows in the photo against the star background. The ISS must be rotating in the same period of the orbit, so it always shows the same face to Earth's surface.
The camera would not be rotating with respect to the ISS. But the ISS, as you said, rotates with the same period as its orbit. The camera, being fixed to the ISS, therefore rotates with this same period in the same plane.
So, yes, the camera is rotating; hence, the star circles.
The astronaut who took these pictures, Don Pettit, is also an accomplished hardware hacker. He built his own barn door tracker from spare parts at the station. A barn door tracker is basically a device that allows you to make long exposure pictures and by slowly moving the camera, keeps stars as points of light instead of trails (the opposite of these pictures).
“My star trail images are made by taking a time exposure of about 10 to 15 minutes. However, with modern digital cameras, 30 seconds is about the longest exposure possible, due to electronic detector noise effectively snowing out the image. To achieve the longer exposures I do what many amateur astronomers do. I take multiple 30-second exposures, then ‘stack’ them using imaging software, thus producing the longer exposure.”
Thats my guess as well. What's weird though is that either the rotations are going much faster than I thought, or the long-exposure noise reduction or something similar is still turned on. (Without that, a camera can effectively immediately take another 30s exposure, so why the interruption otherwise?)