Viewing a single comment thread. View all comments

karantza t1_ir6aub0 wrote

The answer is a little complicated.

  • Any object in orbit will pass through the same points in its orbit every time, approximately.
  • The Earth is rotating under this path, so except for the special case when the length of the orbit is exactly 24 hours, a different part of the Earth will be under the object each pass.
  • The orbit is not perfect. Orbits precess (probably the word you're looking for) for various reasons, because the Earth pulls on them a little differently due to it not being a perfect sphere, because of the sun and moon's gravity, etc. So while it'll pass through just about the same point (in space, not over the ground) each orbit, that can drift over time.
4

Passeride OP t1_ir6pkda wrote

OMG, lightbulb moment. OFC the orbit would only orbit the same place "on earth" if they sync up.

I would assume if the satellites orbit time is divisible by earths rotation time, it syncs up.

​

Thank you kind and gifted stranger

2

jurc11 t1_ir7gtdb wrote

>I would assume if the satellites orbit time is divisible by earths rotation time, it syncs up.

Right, that's how you get to SSO - sun-synchronous orbits. In terms of Starlink, that's Group 3 at 97.6° inclination. If the orbit is inclined just right for its altitude, it will be sun-synchronous and will appear over the same area at the same time every day. It will precess just right to keep up with the planet below. This is used by imaging sats to have consistent shadows and will be used by Starlink Group 3 to ensure additional peak time coverage.

Here's a plot of altitude vs. inclination, you can see Group 3 at 560km should be inclined to 97.6° to be SSO.

3