January 11th 1787 was a good day for William Herschel. Not many of us get to discover even one major moon in a lifetime, but on this particular day in history Herschel was fortunate enough to find both the largest moon of Uranus, Titania, and the second largest, Oberon.
Both moons were named after characters from Shakespeare’s A Midsummer Nights Dream; they were king and queen of the faeries. They joined the sprites, Ariel (from The Tempest), and Umbriel (from Alexander Pope’s The Rape of the Lock) which had been discovered in October 1851.
Oberon is the outermost major Uranian moon, and the ninth most massive moon in the solar system. It is about 50/50 rock and ice, with the rock probably forming the core. It’s quite “moonish” in appearance, being fairly dark and rather cratered by impacts, much like our own companion. However, unlike our dry Moon, as well as being partly icy, Oberon may well have a layer of liquid water between the core and the mantle.
Titaniais similar in appearance and composition to Oberon, but is less heavily cratered, possibly as a result of internal processes that removed some of the older cratering. Both moons were probably formed in situ from an accretion disc.
Only Voyager 2 has ever been close enough to Uranus to capture good shots of Oberon and Titania. It made its closest approach to the planet on January 26th 1986, but had already got to about 300,000 miles of Titania and 400,000 miles from Oberon two days earlier, at which time it took these photographs.
1865 – German astronomer Johannes Franz Hartmann born.
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.
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.
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, Polydeucesby the Cassini Imaging Science team.
Today in 2003, Cupid and Mab, moons of Uranus, both of which had been too dim to see on Voyager photographs, were discovered by Mark R Showalter and Jack J Lissauer using the Hubble Space Telescope. Cupid was named after the character in William Shakespeare’s rarely performed and possibly incomplete play Timon of Athens, and Mab after the queen of the fairies who is mentioned in Romeo and Juliet (and as a long-time fan I would also refer you to The Fairy Feller’s Master Stroke on the album Queen II). Mab has unusual hobbies. She drives her chariot (which I believe is made from an acorn) up people’s noses to enable her to influence their dreams, and she is thought to decide who gets infected by herpes simplex.
Neither Cupid nor Mab could be described as impressive bodies. Cupid measures about 18 km in diameter, while Mab is thought to be about 24 km.
Also today, asteroid 84 Klio was discovered by R Luther in 1865. Clio (or Klio or Kleio) was the muse of history. A daughter of Zeus and Mnemosyne, she had one son, to whom she gave the butchly masculine name Hyacinth. Her own name is derived from the verb kleô, meaning to celebrate or make famous.
1889 – Asteroid 287 Nephthys discovered by C F H Peters, and is the last of his incredible haul of 48 asteroids. Nephthys is a large, S-type main belt asteroid, and for a change is named after a character from Egyptian mythology, the daughter of Nut and Geb, and sister of Isis.
I’m going to have to do some digging on Peters, because I find it hard to believe that after 28 years of tracking the things, he doesn’t have an asteroid named after him. I’ve found two so far named after people called Peters, and he isn’t either of them.
Finally today,Voyager 2 made its closest approach to Neptune on August 25th, 1989. This was the end of a bit of a purple patch for NASA. Photographs from the outer planets had enthralled the inhabitants of this one for more than a decade, and Neptune didn’t disappoint. Voyager 2 was able to get some great shots of the planet, including the “Great Dark Spot” which seems to have subsequently vanished. There was also time for a visit to Triton, Neptune’s volcanically active largest moon, thought to be a captured Kuiper Belt object.
Although they didn’t know it at the time, this fly-by marked the point at which every planet in the solar system had been visited (back in the day they still had Pluto on the list, but it would eventually be removed).
This discovery was slightly unusual, as the photograph used to discover Perdita was taken in 1986 by Voyager 2, but the tiny moon wasn’t noticed until 1999, which therefore goes down as the year of discovery. Following this, it wasn’t possible to verify Perdita’s existence, so it was stripped of the title “moon” by the International Astronomical Union until the Hubble telescope picked it up in 2003.
The name Perdita is an inspired choice. All the moons of Uranus are named after characters from the plays of William Shakespeare or The Rape of the Lock by Alexander Pope. Perdita, daughter of Laontes in The Winter’s Tale, has a name meaning “lost”. She grows up believing she is a poor shepherd’s daughter, only to find out later that she is a princess.
Despite being just 33km wide, Perdita is by no means the smallest of Uranus’ moons. Indeed she is quite large compared to some of the most recent discoveries. Mab, for example, is only 25km in diameter, Fransisco 22km, and Trinculo and Cupid both a teeny 18km.
ALSO TODAY . . .
1895 — Asteroid 403 Cyane discovered.
1912 — Asteroid 758 Mancunia discovered.
1969 — Apollo 10 launched.
1991 — Helen Sharman becomes the first Briton in space.
Also known as Uranus V, Miranda was discovered by Gerard P Kuiper on February 16th 1948, making it the last Uranian moon to be discovered by Earth-based observing equipment. We have some fairly good shots of Miranda’s southern hemisphere taken by Voyager 2, which paid a visit in 1986. These show this small moon (only one seventh the size of our own Moon) to be an interesting place, crossed by grooves and enormous canyons, some more than ten times deeper than the Grand Canyon. Miranda, in common with the other larger Uranian moons, is thought to be composed of mostly silicates and water ice.
Obviously there must always remain a degree of uncertainty regarding the composition of a place we’ve only been to once, rather fleetingly, for ‘Tis far off, and rather like a dream than an assurance (Act 1, Scene 2). Miranda is the only female character in Shakespeare’s The Tempest.
1880 – Asteroid 213 Lilaea discovered by C H F Peters. Lilaea is approximately 83 km across, has a year lasting four and a half Earth years, a day lasting just over 8 hours, and is named after a Naiad (water nymph).
1891 – 305 Gordonia is a 49 km wide main belt asteroid, discovered by Auguste Charlois and named after his patron, James Gordon Bennett Jr, publisher of the New York Herald, and thought to be the man from whom we get the exclamation “Gordon Bennett!”.
1965 – launch of Pegasus 1, via Saturn I rocket number SA-9 from Cape Kennedy to study the effects of micrometeoroid impact, which it achieved by use of two giant wings, unfurled upon reaching orbit. Pegasus 1 remained operational until it was deactivated on August 29th 1968. It remained in orbit until 1978.