  We report the results of the stellar occultation by (UII) Umbriel on
September 21st, 2020. The shadow crossed the USA and Canada, and 19 positive
chords were obtained. A limb parameter accounted for putative topographic
features in the limb fittings. Ellipse fittings were not robust - only upper
limits were derived for the true size/shape of a putative Umbriel ellipsoid.
The adopted spherical solution gives radius = 582.4 +/- 0.8 km, smaller/close
to 584.7 +/- 2.8 km from Voyager II. The apparent ellipse fit results in a true
semi-major axis of 584.9 +/- 3.8 km, semi-minor axes of 582.3 +/- 0.6 km and
true oblateness of 0.004 +/- 0.008 for a putative ellipsoid. The geometric
albedo was pV = 0.26 +/- 0.01. The density was rho = 1.54 +/- 0.04 g cm-3. The
surface gravity was 0.251 +/- 0.006 m s-2 and the escape velocity 0.541 +/-
0.006 km s-1 . Upper limits of 13 and 72 nbar (at 1 sigma and 3 sigma levels,
respectively) were obtained for the surface pressure of a putative isothermal
CO2 atmosphere at T = 70 K. A milliarcsecond precision position was derived: RA
= 02h 30m 28.84556s +/- 0.1 mas, DE = 14o 19' 36.5836" +/- 0.2 mas. A large
limb parameter of 4.2 km was obtained, in striking agreement with opposite
southern hemisphere measurements by Voyager II in 1986. Occultation and Voyager
results indicate that the same strong topography variation in the surface of
Umbriel is present on both hemispheres.
