<p>The high inclination of the Uranus system to its orbit around the Sun has been known since the satellites were discovered.
<a href="https://ui.adsabs.harvard.edu/abs/1851AJ......2...70L/abstract" rel="nofollow noreferrer">Lassell 1851</a> shows Ariel and Umbriel moving at least as far north and south of the planet as they do east and west of it.
William Herschel probably noticed the same thing about Titania and Oberon in 1787.</p>

<p>When Uranus is at an equinox, its equator appears edge-on to the Sun, making good opportunities to observe the planet's oblateness from Earth.
<a href="https://ui.adsabs.harvard.edu/abs/1842AN.....20...61M/abstract" rel="nofollow noreferrer">M&auml;dler 1842</a>
and <a href="https://ui.adsabs.harvard.edu/abs/1883Obs.....6..331Y/abstract" rel="nofollow noreferrer">Young 1883</a> measured significant polar flattening.
<a href="https://ui.adsabs.harvard.edu/abs/1896AJ.....16...73B/abstract" rel="nofollow noreferrer">Barnard 1896</a> thought the satellite orbits might be inclined 20&deg;-30&deg; to the equator.
It took longer to determine that the equator was within 1&deg; of the four largest satellites' orbital planes.</p>

CC BY-SA 4.0
2019-07-29T23:26:04.750Z
fyi I've just asked [First spectroscopic detection of Uranus' rotation?](https://astronomy.stackexchange.com/q/32841/7982)
uhoh
CC BY-SA 4.0
2019-07-30T03:15:25.177Z
@uhoh Edited again. Will keep looking for when they narrowed down the ecliptic plane.
Mike G
CC BY-SA 4.0
2019-07-30T04:51:21.370Z
looks good, thank you!
uhoh
CC BY-SA 4.0
2019-07-30T09:13:08.737Z
In my previous comment I meant "equatorial" instead of "ecliptic."
Mike G
CC BY-SA 4.0
2023-04-21T14:29:39.000Z
"It took longer to determine that the equator was within 1° of the four largest satellites' orbital planes." How much longer? When was it conclusively determined, and by whom?
Outis Nemo
CC BY-SA 4.0
2019-07-29T06:17:42.613Z
2019-07-30T03:14:31.793Z
2019-07-30T03:14:31.793Z
Mike G
<p>Who discovered it, and how was that accomplished.  I had assumed it was known before Voyager 2 arrived at the planet since it isn't mentioned in  <a href="https://voyager.jpl.nasa.gov/mission/science/uranus/" rel="nofollow noreferrer">JPL voyager 2 Uranus Approach</a></p>

CC BY-SA 4.0
2019-07-29T03:18:31.330Z
The first two of Uranus' moons were discovered in 1787.   https://en.wikipedia.org/wiki/Timeline_of_discovery_of_Solar_System_planets_and_their_moons#18th_century by William Hershal.   Hershal was also the first to measure Mars' axial tilt (before Mars' 2 moons were discovered).   If they assumed that Moons orbit the planets' equator, then suspicion of Uranus axis may have been as early as 1787, but that's just speculation on my part.
userLTK
<rotation><history><uranus>
Uranus' axis of rotation-when discovered?
Bob516
# Uranus' axis of rotation-when discovered?

(Asked by: username_0 on 2019-07-28)

Who discovered it, and how was that accomplished. I had assumed it was known before Voyager 2 arrived at the planet since it isn't mentioned in [JPL voyager 2 Uranus Approach](https://voyager.jpl.nasa.gov/mission/science/uranus/)

**Tags:** rotation, history, uranus
> **Comments:**
> **username_1**: The first two of Uranus' moons were discovered in 1787.   https://en.wikipedia.org/wiki/Timeline_of_discovery_of_Solar_System_planets_and_their_moons#18th_century by William Hershal.   Hershal was also the first to measure Mars' axial tilt (before Mars' 2 moons were discovered).   If they assumed that Moons orbit the planets' equator, then suspicion of Uranus axis may have been as early as 1787, but that's just speculation on my part.  

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## 1 answers

### Answer by username_2 on 2019-07-29. Score: 6

The high inclination of the Uranus system to its orbit around the Sun has been known since the satellites were discovered.
[Lassell 1851](https://ui.adsabs.harvard.edu/abs/1851AJ......2...70L/abstract) shows Ariel and Umbriel moving at least as far north and south of the planet as they do east and west of it.
William Herschel probably noticed the same thing about Titania and Oberon in 1787.

When Uranus is at an equinox, its equator appears edge-on to the Sun, making good opportunities to observe the planet's oblateness from Earth.
[Mädler 1842](https://ui.adsabs.harvard.edu/abs/1842AN.....20...61M/abstract)
and [Young 1883](https://ui.adsabs.harvard.edu/abs/1883Obs.....6..331Y/abstract) measured significant polar flattening.
[Barnard 1896](https://ui.adsabs.harvard.edu/abs/1896AJ.....16...73B/abstract) thought the satellite orbits might be inclined 20°-30° to the equator.
It took longer to determine that the equator was within 1° of the four largest satellites' orbital planes.

> **Comments:**
> **username_3**: fyi I've just asked [First spectroscopic detection of Uranus' rotation?](https://astronomy.stackexchange.com/q/32841/7982)  
> **username_2**: @uhoh Edited again. Will keep looking for when they narrowed down the ecliptic plane.  
> **username_3**: looks good, thank you!  
> **username_2**: In my previous comment I meant "equatorial" instead of "ecliptic."  
> **username_4**: "It took longer to determine that the equator was within 1° of the four largest satellites' orbital planes." How much longer? When was it conclusively determined, and by whom?  

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