The moon of Uranus – Perdita (XXV) – was previously temporarily designated as S/1986 U10. It was discovered in photos sent from the →Voyager-2, which flew through the Uranian system on January 24, 1986. The one to draw the right conclusions from these photos was E. Karkoschka in 1999. However, Earth-based telescopes failed to confirm it in follow-up observations and soon afterwards astronomers concluded that it must have been one of the 9 moons closest to Uranus which were already known. Later on, in December 2001, the →International Astronomical Union, made a final decision on withdrawing its entry from the list. Similar mistakes happened regarding the moons of Saturn when soon after the Voyager flyby 28 known moons have been announced and soon 10 of them proved to be images of objects that were already discovered.
    This time it was different since astronomers have finally found out that it does exist and being substantialy fainter then other neighbouring moons was the main cause of its elusive nature experienced by astronomers until...
    ...Until late August 2003 when M. Showalter and J. Lissauer, astronomers from Stanford University using the Hubble Space Telescope definitely confirmed its existence!
    It is intersting to note that it is the first inner moon of Uranus discovered with a ground-based telescope since Miranda was found 55 years earlier! Over the last few years Earth-based telescopes discovered five previously unknown moons but all of them orbit millions of kilometers from the planet! This moon however is in a prograde elliptical (→eccentricity = 0.0116) orbit around Uranus with a →semimajor axis = 76,417 km. At pericenter (closest to the planet) Perdita is separated from the Uranus a distance of q = 75,531 km. At apocenter (furthest from the planet) this moon is separated from the Uranus a distance of Q = 77,303 km.

    An interesting feature of Perdita is that it revolves around Uranus slightly faster than Uranus spins on its axis!

    The orbital period is 0.6 day, which means that the average speed of its celestial motion is 564° (1.6 revolutions).
    The main obstacle in discovery of that moon was its small angular distance from Uranus and also its (remembered above) low brightness of only 23.6m (almost 11 million times fainter than the faintest stars visible to naked eye). For comparison: Uranus' brightness is about 5.5m, so Perdita is shining fainter by about 18.1m. The corresponding difference in brightness is almost 17.4 million times!!

I have given some crucial data of Perdita below. Table one contains the basic information. Whereas table two gives more detailed parametrs of its orbit (calculated using the following formulae).

Translated by Karol Pankowski


Mean distance from the planet  [×103 km] 76.4
Mean distance from the planet  [planetary R] 3.0
Orbital period  [days] 0.64
Orbital eccentricity  [e] 0.0116
Orbit inclination  [degrees] 0.5
Mean diameter  [km] 20.0
Main discoverer and year of the discovery E. Karkoschka   1999
Visual magnitude  [mag] 24.0
Mass  [kg]


Orbital Parameters

Distance from the planet  [×103 km] 75.5 77.3
Distance from the planet  [planetary R] 3.0 3.0
Angular size of the moon's orbit observed from the Earth*  [degrees] 0°00'05.73'' 0°00'05.86''
Angular diameter of the planet's disc as observed from the moon  [degrees] 39°33'48.34'' 38°37'10.16''
Brightness of the planet as observed from the moon**  [mag] –17.3 –17.2
Diameter of the moon's disc as observed from the planet's "surface"  [degrees] 0°01'22.55'' 0°01'19.72''
Brightness of the moon as observed from the planet's "surface"**  [mag] 1.2 1.3
Orbital velocity  [km/sec] 8.81 8.61
  * This value is calculated for Uranus at opposition (distance 2.7 billion km = 18.2 AU)
** The given value of magnitude is not corrected for some decreasing factors (e.g. the changing phase of illumination)

See other related links:
Monde des MonatsPerdita
Views of the Solar System – Photo Archives

One Moon for Neptune, Another for Uranus

The Astronomy Workshop – Satellite Viewer
Planetary Satellite Mean Orbital Parameters

Natural Satellite Physical Parameters

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