October 24 – Ariel and Umbriel

We have a double-header today.  Two moons of Uranus, Ariel and Umbriel, were discovered on October 24th 1851 by Bolton-born William Lassell, brewer and keen amateur astronomer, who was in the fortunate position of being able to use his not inconsiderable wealth to fund the construction of impressive telescopes.

Let’s take a quick look at Ariel first, a reasonably large moon of just over 579 km radius, orbiting its parent at a distance of about 190,000 km.  It travels at a fantastic pace, completing an orbit of the vast planet below it in two and a half Earth days.

Ariel viewed from Voyager 2 (image credit: NASA)

Ariel viewed from Voyager 2 (image credit: NASA)

Of all the moons of Uranus, the surface of Ariel appears to be the youngest, evidenced by the lack of older, larger impact craters, and the presence of recent geologic activity.  It is also the brightest of the large Uranian moons, and is tidally locked (keeps the same face to the planet at all times).  Ariel has only been visited up close and personal by one mission, Voyager 2, and as far as I know there are no plans to make a return journey just yet.

If you’ discover a moon of Uranus you have two choices from which to choose a suitable name: Alexander Pope’s The Rape of the Lock, or the plays of William Shakespeare.  At the sugestion of Lassell, Ariel was chosen by Sir John Herschel, son of Sir William Herschel, the discoverer of Uranus.  It’s a good choice, as it falls into both categories.

Now we move outwards slightly to Umbriel, which gets its name from that of a sprite in Pope’s narrative poem.  It too was chosen by John Herschel.

View of Umbriel from Voyager 2, 1986 (image credit: NASA)

View of Umbriel from Voyager 2, 1986 (image credit: NASA)

Umbriel is the third farthest major moon from Uranus (Ariel is the second), and is the darkest of the major Uranian satellites.  We don’t yet know why Umbriel is so much darker than similarly sized bodies in orbit around Uranus, but I’m sure somebody’s working on it somewhere.  With a radius of 584 km, it is only slightly larger than Ariel, and, like it’s neighbour, it’s no slouch, travelling at the impressive rate of 16,800 km/hour, meaning a year on Umbriel takes about four and a half Earth days.

For, that sad moment, when the Sylphs withdrew
And Ariel weeping from Belinda flew,
Umbriel, a dusky, melancholy sprite,
As ever sully’d the fair face of light,
Down to the central earth, his proper scene,  
Repair’d to search the gloomy Cave of Spleen.

(The Rape of the Lock, Alexander Pope, 1714)

1601   –   Death of the Danish astronomer, Tycho Brahe, whose use of the title De Nova Stella to announce the supernova of 1572 led to the word nova being applied to all such phenomena.

1890   –   First meeting of the British Astronomical Association at the Hall of the Society of Arts in London. Unlike the already established Royal Astronomical Society, the BAA was open to female members, and even allowed them onto its council. One can only speculate of course, but I assume the RAS had somehow decided that night vision was improved by ownership of testicles.

1964   –   Launch of Kosmos 49 (DS-MG 2) to test orientation systems and study the magnetosphere.  The launch was from the Kapustin Yar site near Volgograd (previously known as Stalingrad). The mission was a short one, and the satellite’s orbit decayed on August21st the year after launch.

2004   –   Discovery of Saturn’s extremely tiny moon, Polydeuces by the Cassini Imaging Science team.


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