Only six planets, including the Earth, were known until the 18th Century.
The planet Uranus was discovered by the noted British astronomer, Sir William Herschel, on March 13, 1781. Actually, the planet had been observed numerous times by other astronomers as early as 1690, but it was thought to be another star.
The planet was discovered accidentally while Herschel was surveying all stars down to magnitude eight — those that are about ten times dimmer than can be seen by the naked eye. One "star" seemed different and within a year was shown to have a planetary orbit 18 times farther from the Sun than Earth. The new planet was named Uranus after the father of Saturn in Roman mythology.
William Herschel built his own telescopes, including one which had a focal length of 6 meters (20 feet) and a larger telescope of the same design with a 12 meter (40 foot) focal length.
In the course of preparing a star catalog in 1801, the Sicilian astronomer Guiseppi Piazzi accidentally discovered a small planet between the orbits of Mars and Jupiter. He named the planet Ceres after the Sicilian god of the harvest.
Ceres is now known to be the largest of thousands of minor planets, or asteroids, most of which have orbits in the region of the solar system between Mars and Jupiter.
Unlike Uranus and Ceres, Neptune was not discovered by accident. It was proposed that a planet beyond Uranus could account for irregularities in Uranus' orbit. Independently, two astronomers, John Couch Adams in England and Urbain-Jean-Joseph Le Verrier in France, calculated the position of this yet unknown planet.
The search began. The British astronomer James Challis, using Adams' predictions, observed the planet on the night of August 4, 1846, but failed to compare his observations with those of the previous night and did not recognize the planet. On September 23, 1846, the planet was finally found on the first try by the German astronomer Johann Galle using Le Verrier's predictions.
Who Discovered Neptune?
Who should receive credit for discovering Neptune? Adams or Le Verrier who predicted its position? Challis who saw it but did not know it? Galle who found it? Controversy raged.
It was fortunate that Neptune was discovered in the mid-19th century. Just 30 years later the planet was far from the positions predicted by both Adams and Le Verrier.
Several astronomers interpreted irregularities in the orbits of Uranus and Neptune as being caused by a more distant planet. Among these astronomers was the American Percival Lowell who is credited with the successful prediction of the planet's orbit. Lowell also started the search for the planet which was ultimately found in 1930 by his successors at the Lowell Observatory in Arizona.
The photographic plate on the left is a copy of one of the plates used to discover Pluto. The image was collected on Jan. 23, 1930 and was compared with other plates collected that month.
The Blink Comparator Used to Discover Pluto
Percival Lowell purchased this Carl Zeiss blink comparator in 1911. While using it to search for a new planet in 1930, Clyde Tombaugh discovered Pluto.
The instrument allows the operator to switch the view back and forth between two photographic plates taken several days apart. The plates are exactly aligned so the distant stars remain stationary, but a planet-like object will appear to move from side to side among them.
The blinking technique Clyde Tombaugh used to discover Pluto is similar to the method used by Henrietta Leavitt and others a century ago to identify distant Cepheid variable stars. Cepheids are still important "astronomical yardsticks" for measuring distances to galaxies.