Clyde Tombaugh, a young American research student, made the last discovery of a planet while working in 1930 at the Lowell Observatory, Arizona State College. This planet is Pluto, the ninth one in order of distance from the sun, 3,670 million miles away.
Although Tombaugh, who was 26 at the time, was the first astronomer to see Pluto, its existence had been suspected by Percival Lowell, builder of the observatory at Flagstaff, Arizona. Lowell began searching for the planet in 1905, the year before Tombaugh was born. He observed that there was a difference between the predicted and actual positions of Uranus, and this led him to conclude that there must be another planet. His final calculations about "Planet X" were published in 1914, but he had still not found the planet when he died two years later.
Another American, W.H. Pickering, took up the search, concentrating on the irregular movements of the planet Neptune. He saw a clue in the movement of comets, which seem to be attracted by large planets. There were 16 known comets whose paths took them millions of miles beyond Neptune, which is 2,811 million miles from the sun, and Pickering was convinced that they were being attracted by a still more distant planet.
In 1919 yet another hunt was begun by Milton Humason at Mount Wilson Observatory, Pasadena, California. Instead of mathematical calculations, Humason tried photography. He took two pictures of a series of stretches of the sky, with a gap of one or two days between exposures. In such photographs stars stay still, but planets change position.
When Tombaugh discovered Pluto, it became clear that Humason had photographed the planet twice. Once it had been masked by a star, and the second time its image had coincided with a flaw in the photographic plate. The main difficulty in the search had been that Pluto was extraordinarily faint. Pickering formed the opinion that it was not Lowell's Planet X, but that a huge planet remains to be discovered.
Men have always been fascinated by the stars. Centuries ago, as people looked up at the sky, they saw that some stars did not twinkle but wandered across the sky as bright points of light. These "wandering" stars were what we now call the planets.
There were five planets that could be seen by ancient men - Mercury, Venus, Mars, Jupiter and Saturn.
The ancient Greeks were great watchers of the sky and also great thinkers. As they watched the stars night after night, it was natural for them to think that the earth stood still and the stars, planets, sun and moon were going round and round the earth in space. They thought the sun was between Venus and Mars.
For centuries, men believed this was how the stars moved. To explain the wandering of the planets, however, was very difficult.
Then one day at Cracow University in Poland, about the year A. D. 1500, a young scientist named Nicolaus Copernicus began thinking about the way in which the stars and planets moved.
Suppose, Copernicus said, the earth was not at the centre of the stars and the planets, but that the sun was instead. Suppose that the earth itself was a planet just like Mars and Venus and that the earth and all the other planets were round and round the sun at different distances from it. "After all," he said, "since light comes from the sun, it is only proper that the sun and not the earth should be at the centre of everything."
The ancient Greeks had made the mistake of thinking that because the stars and planets seemed to move as they looked at the sky, the earth must be still. If you have sat in a train and looked out at the trees rushing by, it is easy to understand their mistake.
The trees seem to be moving backwards, but really it is the train that is moving forwards.
Not all of Copernicus' ideas were right. Although he thought, correctly, that the moon went round and round the earth, he also thought the stars were fixed on a large ball outside where the planets moved. He thought the stars did not move at all, but only the earth, moon and planets.
Copernicus was so frightened of what everyone would think of his new ideas
that he did not write them down in a book until he was almost dying. Yet he was the first person to explain properly our solar system.
Copernicus was born at Torun. He studied mathematics at Cracow, canon law and astronomy at Bologna and medicine at Padua. His treatise has the title in Latin "De Revolutionibus Orbium Coelestium".
(from "Finding Out")
Johannes Kepler, German astronomer and mathematician, was the founder of modern astronomy.He was born on December 27, 1571 in the village of Weil-der-Stadt in the Duchy of Wurttemberg, Swabia. He studied mathematics, philosophy, theology and astronomy at the University of Tubingen, earning his M. A. in 1591.
Kepler became a teacher of mathematics and astronomy at Gratz, the Austrian province of Styria from 1594 to 1600. His writings on celestial orbits impressed the famous Danish astronomer Tycho Brahe who invited Kepler to join him at Prague. Kepler accepted and assisted Tycho in preparing new planetary tables. When Brahe died in 1601, Kepler succeeded him as Imperial Mathematician. He had access to all of Tycho Brahe's papers and 20 years of precise observations which he used to form the foundation of his three laws of planetary motion (Kepler's laws) published between 1609 and 1618.
They are: (1) the path of a planet is an ellipse with the sun at one focus; (2) a line from the sun to a planet sweeps out equal areas in equal time periods; and (3) the square of an orbital period of a planet is proportional to the cube of its average distance from the sun.
Kepler spent the latter part of his life as a professor of mathematics at Linz, Austria. He died in Regensburg, Bavaria, on November 15, 1630.