Uranus

When an amateur astronomer named William Herschel discovered the planet Uranus in 1781 the known size of the solar system was instantly doubled. Herschel initially thought he had found a comet, but when its planetary status was confirmed Herschel became world famous. This was the first new planet to be found in recorded history. There was some controversy over what this new discovery should be called. Herschel wanted to name it the "Georgian Planet" in honour of the King George III, but eventually it was named Uranus after the Greek god of the heavens, and father of Saturn.

After the discovery of Uranus, astronomers were able to use Newton's law of gravitation to precisely calculate its orbit. It was revealed that Uranus had been observed on more than twenty occasions in the century prior to its discovery, but had always been mistaken for a star. The planet was even found on many of the star charts of the period.

Uranus has a uniquely tilted spin axis, with one pole directed towards the Sun, that lead to unusual seasonal changes. During the northern summer one pole is lit for 42 years and then plunged into darkness for the next 42 years. Scientists had expected to see large differences in temperature in the two hemispheres but apparently atmospheric circulation results in a remarkably uniform surface. The atmosphere consists mainly of hydrogen and helium, with traces of methane giving Uranus its distinctive blue/green colour.

Uranus also has a tenuous ring circling its equator, but is very hard to detect because it is made from extremely dark rock, which reflects only 2% of the sunlight that it receives. It may be that an asteroid strayed too close to the planet and was torn apart by the gravity of the planet, eventually settling to form the rings. The rings are extremely narrow, less than 10 kilometres across, and are held in place by shepherd moons in a similar manner to Saturn's F ring.

Uranus' ring system was discovered due to a piece of good fortune on the part of astronomers. In March 1977 the planet was scheduled to pass in front of a faint star, and because of uncertainties in the predicted time of the star's disappearance one telescope began making observations 45 minutes early. Soon the astronomers noticed that the starlight was varying in intensity, getting brighter and fainter as the rings passed across the position of the star. At first the variation in signal was attributed to clouds passing in front of the telescope, but when a symmetric signal was measured on the other side of the planet, the existence of the rings was confirmed.