by Chris Peterson » Thu Dec 02, 2021 1:37 pm
johnnydeep wrote: ↑Thu Dec 02, 2021 1:18 pm
neufer wrote: ↑Wed Dec 01, 2021 10:10 pm
johnnydeep wrote: ↑Wed Dec 01, 2021 3:20 pm
So, I presume that "
Uranus' apparent motion past background stars is really dominated by Earth's own orbital motion around our Sun." is because the Earth is moving much faster in its orbit than Uranus is.
But I'm kind of surprised that over a "mere" four hours, Uranus moved so visibly. After all, this is only 4 hours in Earth's 365 day orbit, or 1 part in 365*(24/4)=2190. So, that's 360/2190=.164 degrees, or about a third of a full moon width (at .5 degrees)? Is that about right (ignoring Uranus' motion entirely)? If so, maybe that's not so surprising after all :)
0.164 degrees is how much the Sun moves.
Uranus is 17 times more distant so it is more like 0.01 degrees.
Huh? Isn't it Earth's orbit (around the Sun) that is causing Uranus to move against the background stars in this APOD?
Yes, but the apparent shift of Uranus against the background stars has to take into account both the distance of the Earth from the Sun, as well as the distance of the Earth from Uranus. By your logic, you'd expect to see the background stars moving massively with respect to each other!
Instead of thinking about the angle of Earth's orbit, put it into linear terms. In four hours, the Earth moves in nearly a line, with a length of about 425,000 km. Uranus is about 3 billion kilometers away. So the angular shift will be about 0.008° against the (essentially infinitely distant) background. Or 30 arcseconds...about 8 times the diameter of Uranus. That's pretty consistent with the video (even though we don't see Uranus resolved).
[quote=johnnydeep post_id=318712 time=1638451133 user_id=132061]
[quote=neufer post_id=318700 time=1638396600 user_id=124483]
[quote=johnnydeep post_id=318694 time=1638372039 user_id=132061]
So, I presume that "[b][i]Uranus' apparent motion past background stars is really dominated by Earth's own orbital motion around our Sun.[/i][/b]" is because the Earth is moving much faster in its orbit than Uranus is.
But I'm kind of surprised that over a "mere" four hours, Uranus moved so visibly. After all, this is only 4 hours in Earth's 365 day orbit, or 1 part in 365*(24/4)=2190. So, that's 360/2190=.164 degrees, or about a third of a full moon width (at .5 degrees)? Is that about right (ignoring Uranus' motion entirely)? If so, maybe that's not so surprising after all :)
[/quote]
0.164 degrees is how much the Sun moves.
Uranus is 17 times more distant so it is more like 0.01 degrees.
[/quote]
Huh? Isn't it Earth's orbit (around the Sun) that is causing Uranus to move against the background stars in this APOD?
[/quote]
Yes, but the apparent shift of Uranus against the background stars has to take into account both the distance of the Earth from the Sun, as well as the distance of the Earth from Uranus. By your logic, you'd expect to see the background stars moving massively with respect to each other!
Instead of thinking about the angle of Earth's orbit, put it into linear terms. In four hours, the Earth moves in nearly a line, with a length of about 425,000 km. Uranus is about 3 billion kilometers away. So the angular shift will be about 0.008° against the (essentially infinitely distant) background. Or 30 arcseconds...about 8 times the diameter of Uranus. That's pretty consistent with the video (even though we don't see Uranus resolved).