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

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    $\begingroup$ The first two of Uranus' moons were discovered in 1787. en.wikipedia.org/wiki/… 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. $\endgroup$ – userLTK Jul 29 '19 at 3:18

The high inclination of the Uranus system to its orbit around the Sun has been known since the satellites were discovered. Lassell 1851 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 and Young 1883 measured significant polar flattening. Barnard 1896 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.

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  • $\begingroup$ fyi I've just asked First spectroscopic detection of Uranus' rotation? $\endgroup$ – uhoh Jul 29 '19 at 23:26
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    $\begingroup$ @uhoh Edited again. Will keep looking for when they narrowed down the ecliptic plane. $\endgroup$ – Mike G Jul 30 '19 at 3:15
  • $\begingroup$ looks good, thank you! $\endgroup$ – uhoh Jul 30 '19 at 4:51
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    $\begingroup$ In my previous comment I meant "equatorial" instead of "ecliptic." $\endgroup$ – Mike G Jul 30 '19 at 9:13

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