APOD: In Motion: Uranus and Moons (2021 Nov 30)

[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.

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[heehaw]

Why, I practically feel I'm there!

[orin stepanek]

260px-PIA00040_Umbrielx2.47.jpg

Titania,

262_Oberon_732.jpg

Oberon,

titania3_voyg2.jpg

Umbriel

Three of Uranus family!

[orin stepanek]

220_PIA00041.jpg

Ariel!

dog-telescope.jpg

wonder what doggy sees!

[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

[neufer]

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.

[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

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?

[Chris Peterson]

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).

[johnnydeep]

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>