Eccentric Uranus
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Fact File:
Diameter: 49,000 km
Distance to Sun: 19 AU
Solar Orbit: 84 years
No of Moons: 10
In March 1781, an amateur astronomer in the quiet English country town of Bath discovered the seventh planet. William Herschel had been an oboe player in the Hanoverian army, but had settled in England and was making his living by teaching music. He had patiently ground his own lenses for the telescope in his back-garden. He believed that he had found a comet and duly reported it. Only months later did the truth slowly dawn upon astronomers, that the first-ever new planet had been found. There is therefore no moment when Uranus as a planet was 'discovered.' Herschel lived for 84 years, the planet's orbit period, and discovered it in his 42nd year, his life's mid-point. The planet was called Herschel through much of the nineteenth century. Its period of 84.01 years is fairly precise multiples of seven and twelve, and it is in addition half the orbit-period of Neptune, one-third that of Pluto and (roughly) seven times that of Jupiter.
In 1986 the Voyager spacecraft arrived at Uranus, obtaining pictures of this eccentric and mysterious sphere. Its surface glowed ultra-violet, and nine dark rings surrounded it. It rotates in a retrograde direction, lolling on its side as if it had never heard of the ecliptic plane. Its magnetic axis protrudes at about latitude 40°, and does not pass through anywhere near the planet's centre. Its moons were found to have a strangely patched-up appearance. own lenses for the telescope in his back-garden. He believed that he had found a comet and duly reported it. Only months later did
The moon Miranda reflected as one scientist said, 'all the strange places of the solar system rolled into one.' It looked like 'a pile of spare parts', and then, 'If Miranda was bizarre, the other large moons of Uranus upheld the family's eccentricity and lack of harmony.' The head of Voyager's imaging team Brad Smith concluded, 'To create a historical scenario for what Voyager saw at Uranus, we need more miracles than any thinking person will accept.' (1). Others felt that the strange scars on Miranda indicated that it had been repeatedly reassembled. (2). Note the 'bite' taken out of Miranda seen at the bottom of the image, where huge cliffs are jutting.
More recently, the Hubble telescope has shown us Uranus' fragile ring system. There is one fairly bright ring, which glows more at one end than the other. It wobbles like an unbalanced wagon wheel, as well as glowing in an asymmetric manner. A swarm of eight moons are jostling around this. Astronomers are puzzled that they manage to avoid bumping into each other - or perhaps they do, now and then! These moons of Uranus are chaotic, so that their paths cannot be predicted.
The diagram on the left shows the retrograde loops of Uranus, Earth being at the centre of this big circle. While moving retrograde, approximately 5 to 6 months each cycle, Uranus draws nearest at its solar opposition - this happens yearly, as the Sun opposes Uranus. It then grows brightest in the sky, because it's reflecting the Sun's light, so it is easiest to see.
Neptune spends an even greater period of time being retrograde, so that its retrograde loops are actually overlapping.
Notes
(1) - National Geographic, Aug 1986 'Uranus' p.229.
(2) - N.Booth 1995, p.134
(3) - This picture of Uranus was taken by the Hubble space telescope using infra-red, and it is therefore a false-colour image. Some storms can be seen at the edge of the planet, and, as well as Uranus' rings, several moons are also visible.
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