# Sunrise on Uranus

1. Nov 20, 2007

### ppatter1

How often does the sun rise at the south pole of Uranus?

2. Nov 20, 2007

### Staff: Mentor

Is this homework? What do you know about Uranus's rotation? Hint: that's partly a trick question.

3. Nov 20, 2007

### mgb_phys

Oh, nasty one ......

4. Nov 20, 2007

### LURCH

I don't see the trick (which probably means I fell for it); Neptune has an axial tilt of about 30o, and an orbital period of about 150yrs. So sunrise would be once every 150yrs, wouldn't it?

5. Nov 20, 2007

### mgb_phys

Yes, but the question is about uranus which has an axial tilt of 98deg

6. Nov 20, 2007

### cesiumfrog

Go on, tell us.

7. Nov 20, 2007

### MrJB

I'm not sure if I want to hear the punchline...

8. Nov 20, 2007

### Staff: Mentor

Heh, yeah, you're right - not so much of a trick (or maybe I tricked myself with it?).

Last edited: Nov 20, 2007
9. Nov 20, 2007

### marcus

Uranus' period is about 84 years
Axis tilts 98 degrees, basic idea right but substitute 84 for 150

Last edited: Nov 20, 2007
10. Nov 20, 2007

### mgb_phys

But if the axis tilt is 98 deg isn't the period somewhat irrelevant?

11. Nov 24, 2007

### LURCH

No, the rotational period becomes irrelevant, but the sun rises once every time the planet orbits, so the orbital period is the time between sunrises. But it does confound me some, to think about whether or not one pole stays pointed at the Sun always. I'm told this is not the case, but I haven't been able to put the question to rest in my own mind. I mean, some sattelites put into orbit around the Earth have their longitudinal axis lined up with their tangiential path, and then get spun around that axis (for even distribution of heat, I suppose). These sattelites will orbit so that their axis of rotation stays aligned with their orbital path as they go through 360o of orbit. Yet, as the Earth goes round the Sun, the tilt of the polar axes does not change with the orbital position. One pole does not constantly point toward the Sun while the other points constantly away. I don't really undesrtand why.

I also don't really understand why I suddenly thought this question was about Neptune. Those two really do look alot alike, you know.

12. Nov 24, 2007

### mgb_phys

I assumed one pole pointed at the sun. So if the pole is pointed in a constant direct (relatice ot distant stars ) what would you call this rotation ?

13. Dec 2, 2007

### pixel01

It's rather tricky. Every planet's south pole or north pole has one sunrise (sunset) every year (planet's year), unless its axis is perpendicular to the orbital plane

14. Dec 2, 2007

### mgb_phys

That's the point, Uranus is slightly odd in that it's axis points towards the sun.

15. Dec 2, 2007

### pixel01

98 degrees is not 90 degrees. So in one Uranus year, its south pole has only one sunrise.

edit:not taking the wobbling into account, anyway, it's very long compared to a complete orbit.

Last edited: Dec 2, 2007
16. Dec 2, 2007

Umm. This guy has 1 post from this morning and is not responding.
Something tells me that someone just wanted to see if us dorks would overlook the joke and carry on for hours about degrees of tilt and axes of rotation.

Pretty good but it would have been way funnier in a Friday night imo!

Casey

17. Dec 2, 2007

### sysreset

Well what the heck, it's Sunday. With regard to Lurch's post above about the idea of Uranus spinning on it's 90 degree axis AND maintaining a polar orientation towards the sun (which apparently, it does not). Has a single planet or moon ever been discovered that spins relative to two axes simultaneously?? That's what it would take. I seem to recall that various asteriods tumble erratically, so that one could say that more than one axis is involved. But precisely two??

18. Dec 3, 2007

### LURCH

I don't think there is such an object yet discovered, but I still can't figure out why. Sattelites in earth orbit do it, so why not natural sattelites around their host bodies? I was thinking the answer would have something to do with tidal forces, but that doesn't really sound right; te tidal forces between the top and bottom of a com sattelite are negligable.

19. Dec 3, 2007

### sysreset

I read that the obliquity (tilt) of both Mars and Earth oscillates over a period of many hundreds of thousands of years, in the case of Earth between 22-24 degrees and in the case of Mars from 15 to 60 degrees! Maybe that qualifies as a multi-axis spin?

20. Dec 3, 2007

### cesiumfrog

You would call it ordinary because, as already mentioned, that assumption was false (not just "out by 8 degrees" either).

What makes you think satellites can do such a thing (except perhaps by powering flywheels then ignoring them when measuring the "satellite rotation")?