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APOD: In Motion: Uranus and Moons (2021 Nov 30)
Posted: Tue Nov 30, 2021 5:06 am
by APOD Robot
In Motion: Uranus and Moons
Explanation: What's that moving across the sky? A planet just a bit too faint to see with the unaided eye:
Uranus. The gas giant out
past Saturn was tracked earlier this month near
opposition -- when it was closest to Earth and at its brightest. The
featured video captured by the
Bayfordbury Observatory in
Hertfordshire,
UK is a four-hour time-lapse
showing Uranus with its four largest moons in tow:
Titania,
Oberon,
Umbriel and
Ariel. Uranus' apparent motion past background stars is
really dominated by Earth's own orbital motion around our Sun. The cross seen centered on
Uranus is called a
diffraction spike and is caused by light
diffracting around the four arms that hold one of
the telescope's mirrors in place. The rotation of the diffraction spikes is not caused by the
rotation of Uranus but, essentially, by the
rotation of the Earth. During the next few months
Uranus itself will be visible with binoculars, but, as always, to see its moons will require
a telescope.
Re: APOD: In Motion: Uranus and Moons (2021 Nov 30)
Posted: Tue Nov 30, 2021 11:49 am
by heehaw
Why, I practically feel I'm there!
Re: APOD: In Motion: Uranus and Moons (2021 Nov 30)
Posted: Tue Nov 30, 2021 3:32 pm
by orin stepanek
- 260px-PIA00040_Umbrielx2.47.jpg (11.42 KiB) Viewed 2622 times
Titania,
Oberon,
Umbriel
Three of Uranus family!
Re: APOD: In Motion: Uranus and Moons (2021 Nov 30)
Posted: Tue Nov 30, 2021 3:35 pm
by orin stepanek
Ariel!
wonder what doggy sees!
Re: APOD: In Motion: Uranus and Moons (2021 Nov 30)
Posted: Wed Dec 01, 2021 3:20 pm
by johnnydeep
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
Re: APOD: In Motion: Uranus and Moons (2021 Nov 30)
Posted: Wed Dec 01, 2021 10:10 pm
by neufer
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.
Re: APOD: In Motion: Uranus and Moons (2021 Nov 30)
Posted: Thu Dec 02, 2021 1:18 pm
by johnnydeep
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?
Re: APOD: In Motion: Uranus and Moons (2021 Nov 30)
Posted: Thu Dec 02, 2021 1:37 pm
by Chris Peterson
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).
Re: APOD: In Motion: Uranus and Moons (2021 Nov 30)
Posted: Thu Dec 02, 2021 9:05 pm
by johnnydeep
Chris Peterson wrote: ↑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
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).
Ok, thanks - your math checks out ( arctan(4.25e5/3e9) = .0081 degrees, or 3600*.0081 = 29 arc seconds ). And that's about 1/60 the angular diameter of the full moon. So I was rather off
And I even totally understand it from a diagram I drew. It seems I have yet to learn to think about these planetary geometry problems the right way. <sigh>