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Can anyone explain why the star trails make concentric circles? What's at the middle of the circles? I understand why you might see concentric circles in time

Can anyone explain why the star trails make concentric circles? What's at the middle of the circles? I understand why you might see concentric circles in time elapsed photos from the North or South Poles on Earth, but why from space?

ggchappell 14y ago
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.
- adamsmith 14y ago
  I agree that the camera is rotating. I guess the follow up question would be: why isn't the Earth rotating as well about the same axis?
  - narag 14y ago
    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.
    - ggchappell 14y ago
      > 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.
    - adamsmith 14y ago
      Got it. Thanks!
mixmastamyk 14y ago
The station is orbit, rotating with the earth.
- mixmastamyk 14y ago
  in
- deleted 14y ago
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ggchappell 14y ago

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.

adamsmith 14y ago
I agree that the camera is rotating. I guess the follow up question would be: why isn't the Earth rotating as well about the same axis?
- narag 14y ago
  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.
  - ggchappell 14y ago
    > 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.
  - adamsmith 14y ago
    Got it. Thanks!

mixmastamyk 14y ago

The station is orbit, rotating with the earth.

mixmastamyk 14y ago
in
deleted 14y ago
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