# Meaning of Uranus's 98-degree tilt? **URL:** \ **Category:** Factual Questions\ **Created:** [July 16, 2015, 2:20am UTC](https://boards.straightdope.com/t/meaning-of-uranuss-98-degree-tilt/725285 "2015-07-16T02:20:09Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Ponch8](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ponch8/32/9883_2.png) [@Ponch8](https://boards.straightdope.com/u/Ponch8)\ **Post date:** [July 16, 2015, 2:20am UTC](https://boards.straightdope.com/t/meaning-of-uranuss-98-degree-tilt/725285/1 "2015-07-16T02:20:09Z")
Websites such as [this one](http://www.universetoday.com/18955/tilt-of-uranus/) say that Uranus’s axis is tilted 98 degrees, as compared to Earth’s tilt of 23 degrees. What I don’t understand is how it can be any more than 90 degrees (i.e., one pole pointing directly at the sun). What is the difference between a 98-degree tilt and an 82-degree tilt? Why don’t we say that the Earth’s axis is tilted 157 degrees? ---
**Author:** ![Northern\_Piper](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/northern_piper/32/5304_2.png) [@Northern\_Piper](https://boards.straightdope.com/u/Northern_Piper)\ **Post date:** [July 16, 2015, 2:25am UTC](https://boards.straightdope.com/t/meaning-of-uranuss-98-degree-tilt/725285/2 "2015-07-16T02:25:24Z")
Isn’t the answer that all the planets rotate in the same direction, so “north” is the same for all? And based on Uranus’ rotation, its North Pole is tipped below the ecliptic? ---
**Author:** ![Ponch8](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ponch8/32/9883_2.png) [@Ponch8](https://boards.straightdope.com/u/Ponch8)\ **Post date:** [July 16, 2015, 2:28am UTC](https://boards.straightdope.com/t/meaning-of-uranuss-98-degree-tilt/725285/3 "2015-07-16T02:28:39Z")
But how are north and south defined? I didn’t think that all the planets have a magnetic north or magnetic south like Earth does. ---
**Author:** ![Northern\_Piper](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/northern_piper/32/5304_2.png) [@Northern\_Piper](https://boards.straightdope.com/u/Northern_Piper)\ **Post date:** [July 16, 2015, 2:32am UTC](https://boards.straightdope.com/t/meaning-of-uranuss-98-degree-tilt/725285/4 "2015-07-16T02:32:41Z")
By the direction of rotation around the axis. ---
**Author:** ![Giles](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/giles/32/60_2.png) [@Giles](https://boards.straightdope.com/u/Giles)\ **Post date:** [July 16, 2015, 2:35am UTC](https://boards.straightdope.com/t/meaning-of-uranuss-98-degree-tilt/725285/5 "2015-07-16T02:35:30Z")
It’s nothing to do with magnetism: it’s the direction of rotation. When you look down at the north pole, the planet is rotating anti-clockwise; at the south pole, it’s rotating clockwise. ---
**Author:** ![Ponch8](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ponch8/32/9883_2.png) [@Ponch8](https://boards.straightdope.com/u/Ponch8)\ **Post date:** [July 16, 2015, 2:35am UTC](https://boards.straightdope.com/t/meaning-of-uranuss-98-degree-tilt/725285/6 "2015-07-16T02:35:53Z")
Oh, now I get the gist. It rotates in a retrograde direction, like Venus (177-degree tilt). ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [July 16, 2015, 2:37am UTC](https://boards.straightdope.com/t/meaning-of-uranuss-98-degree-tilt/725285/7 "2015-07-16T02:37:01Z")
The meaning of the tilt is that it’s mooning us. A 98 degree tilt is an 82 degree tilt that’s rotating backward. There’s (at least) two ways to define north. One is it’s the pole where the planet is rotating counter-clockwise when seen from above it. With this definition, Uranus has a 98 degree tilt. That is, its axis is tilted 98 degrees from being upright in its orbit. The other definition (this is the official IAU definition[sup]1[/sup]), is the pole that points to the northern half of the Universe. That northern half being defined by the Earth’s orbital plane. By this definition, the planet is tilted 82 degrees but is rotating retrograde. And its north and south poles are switched from those in the other definition. Personally, I think the second definition is, er, less than optimal. A lot of astronomers do as well. I have no idea why the IAU adopted it. [sup]1[/sup] Let’s keep argiuments about other IAU definitions to other threads, please. ---
**Author:** ![Saint\_Cad](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/saint_cad/32/18907_2.png) [@Saint\_Cad](https://boards.straightdope.com/u/Saint_Cad)\ **Post date:** [July 16, 2015, 2:40am UTC](https://boards.straightdope.com/t/meaning-of-uranuss-98-degree-tilt/725285/8 "2015-07-16T02:40:09Z")
Right hand rule: hold your right hand as if they are lines of latitude pointing in the direction of spin so for example on a globe of the Earth your palm would sit on the Pacific Ocean and stretch towards S. America and the Atlantic. Your thumb will now point to the North. Another way to think about it. We define the North Pole as the pole where looking down the planet rotates west to east or counter-clockwise. Another way to think about it is that looking at a planet at the equator from the orientation that it rotates left to right, north is to the top Incidently, Venus has a retrograde rotation so it is upside-down with an axial tilt of around 177 degrees. ---
**Author:** ![Duckster](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/duckster/32/1244_2.png) [@Duckster](https://boards.straightdope.com/u/Duckster)\ **Post date:** [July 16, 2015, 2:55am UTC](https://boards.straightdope.com/t/meaning-of-uranuss-98-degree-tilt/725285/9 "2015-07-16T02:55:34Z")
> [@dtilque](#): > > The meaning of the tilt is that it’s mooning us. Well, of course! It is Uranus! ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [July 16, 2015, 3:24am UTC](https://boards.straightdope.com/t/meaning-of-uranuss-98-degree-tilt/725285/10 "2015-07-16T03:24:03Z")
> [@Duckster](#): > > Well, of course! It is Uranus! So it’s required to show us its ass-teroid? ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [July 16, 2015, 4:02am UTC](https://boards.straightdope.com/t/meaning-of-uranuss-98-degree-tilt/725285/11 "2015-07-16T04:02:07Z")
> [@dtilque](#): > > There’s (at least) two ways to define north. One is it’s the pole where the planet is rotating counter-clockwise when seen from above it. With this definition, Uranus has a 98 degree tilt. That is, its axis is tilted 98 degrees from being upright in its orbit. > > The other definition (this is the official IAU definition[sup]1[/sup]), is the pole that points to the northern half of the Universe. That northern half being defined by the Earth’s orbital plane. By this definition, the planet is tilted 82 degrees but is rotating retrograde. And its north and south poles are switched from those in the other definition. > > Personally, I think the second definition is, er, less than optimal. A lot of astronomers do as well. I have no idea why the IAU adopted it. [It’s worse than that.](https://en.wikipedia.org/wiki/Poles_of_astronomical_bodies) The IAU defines the north poles of planets and their satellites the second way, but the poles of dwarf planets, minor planets, and asteroids are defined according according to the first definition (the right hand rule) - but in this case called the “positive” pole instead of north pole. ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [July 16, 2015, 4:07am UTC](https://boards.straightdope.com/t/meaning-of-uranuss-98-degree-tilt/725285/12 "2015-07-16T04:07:31Z")
> [@Saint\_Cad](#): > > Incidently, Venus has a retrograde rotation so it is upside-down with an axial tilt of around 177 degrees. It’s six of one, half dozen of the other as to whether you consider Venus to be rotating backwards or upside down. Venus’s unusual rotation is usually attributed to an enormous collision with another body early in its history. I’ve wondered whether it’s easier for a collision to make a body spin in reverse, or to flip it over so it’s rotating upside down. ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [July 16, 2015, 6:03am UTC](https://boards.straightdope.com/t/meaning-of-uranuss-98-degree-tilt/725285/13 "2015-07-16T06:03:59Z")
> [@Colibri](#): > > Venus’s unusual rotation is usually attributed to an enormous collision with another body early in its history. I’ve wondered whether it’s easier for a collision to make a body spin in reverse, or to flip it over so it’s rotating upside down. A collision is not necessary to get Venus to rotate retrograde. [Atmospheric tides](http://www.nature.com/nature/journal/v275/n5675/abs/275037a0.html) or [other effects](http://www.nature.com/nature/journal/v411/n6839/abs/411767a0.html) could have caused it. I’ll admit I don’t understand all this theory, so you’ll have to read the papers yourself if you want nitty-gritty details. A lot more papers can be found [here](http://scholar.google.com/scholar?q=atmospheric+Tides+Venus&hl=en&as_sdt=0&as_vis=1&oi=scholart) ---
**Author:** ![Your\_Great\_Darsh\_Face](https://avatars.discourse-cdn.com/v4/letter/y/ac8455/32.png) [@Your\_Great\_Darsh\_Face](https://boards.straightdope.com/u/Your_Great_Darsh_Face)\ **Post date:** [July 16, 2015, 9:12am UTC](https://boards.straightdope.com/t/meaning-of-uranuss-98-degree-tilt/725285/14 "2015-07-16T09:12:16Z")
> [@Duckster](#): > > Well, of course! It is Uranus! :smack: ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [July 16, 2015, 11:35am UTC](https://boards.straightdope.com/t/meaning-of-uranuss-98-degree-tilt/725285/15 "2015-07-16T11:35:40Z")
It means somebody shorted Slartibartfart when they paid the bill. ---
**Author:** ![BobLibDem](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/boblibdem/32/3149_2.png) [@BobLibDem](https://boards.straightdope.com/u/BobLibDem)\ **Post date:** [July 16, 2015, 12:07pm UTC](https://boards.straightdope.com/t/meaning-of-uranuss-98-degree-tilt/725285/16 "2015-07-16T12:07:58Z")
But why the tilt? Does that indicate that Uranus was captured from another system? If it originated from the same spinning disk of dust that the other planets were formed from, one would think that its tilt would be similar. ---
**Author:** ![Folacin](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/folacin/32/3195_2.png) [@Folacin](https://boards.straightdope.com/u/Folacin)\ **Post date:** [July 16, 2015, 1:33pm UTC](https://boards.straightdope.com/t/meaning-of-uranuss-98-degree-tilt/725285/17 "2015-07-16T13:33:34Z")
I believe one popular theory is that it collided with something. ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [July 16, 2015, 2:26pm UTC](https://boards.straightdope.com/t/meaning-of-uranuss-98-degree-tilt/725285/18 "2015-07-16T14:26:30Z")
> [@BobLibDem](#): > > But why the tilt? Does that indicate that Uranus was captured from another system? If it originated from the same spinning disk of dust that the other planets were formed from, one would think that its tilt would be similar. As has been said, it’s usually attributed to a collision. If it were a capture, I would presume that it wouldn’t necessarily be in the same plane as the other planets (although I don’t know if orbital mechanics would eventually cause it to move into it). ---
**Author:** ![Doctor\_Jackson](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/doctor_jackson/32/32_2.png) [@Doctor\_Jackson](https://boards.straightdope.com/u/Doctor_Jackson)\ **Post date:** [July 16, 2015, 8:53pm UTC](https://boards.straightdope.com/t/meaning-of-uranuss-98-degree-tilt/725285/19 "2015-07-16T20:53:13Z")
It means that I am drunk and have fallen into a shallow hole. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [July 16, 2015, 11:41pm UTC](https://boards.straightdope.com/t/meaning-of-uranuss-98-degree-tilt/725285/20 "2015-07-16T23:41:58Z")
Come to think of it, for most unexplained irregularities in the Solar System, “there was a collision” seems to usually be the favored explanation. And with such a small sample size, well, everything in the Solar System is irregular. [Next page](https://boards.straightdope.com/t/meaning-of-uranuss-98-degree-tilt/725285.md?page=2)