The Dane Tycho Brahe was the last great astronomer of the pre-telescope era. Realizing the importance of trying to record more accurate positions, Tycho built some high-precision instruments for measuring angles. He accumulated an abundance of observations, far superior to those available to Copernicus.

Magnifying the image

The realm of heavenly bodies still seemed remote and inaccessible to astronomers at the time of Tycho’s death in 1601. However, the invention of the telescope around 1608 suddenly brought the distant universe into much closer proximity.

Telescopes have two important advantages over eyes on their own: they have greater light-gathering power, and they can resolve finer detail. The bigger the main lens or mirror, the better the telescope on both counts. Starting in 1610, when Galileo made his first telescopic observations of the planets, the moon’s rugged surface, and the star clouds of the Milky Way, the telescope became the primary tool of astronomy, opening up unimagined vistas.

Planetary dynamics

After Tycho Brahe died, the records of his observations passed to his assistant Johannes Kepler, who was convinced by Copernicus’s arguments that the planets orbit the sun. Armed with Tycho’s data, Kepler applied his mathematical ability and intuition to discover that planetary orbits are elliptical, not circular. By 1619, he had formulated his three laws of planetary motion describing the geometry of how planets move.

Kepler had solved the problem of how planets move, but there remained the problem of why they move as they do. The ancient Greeks had imagined that
the planets were carried on invisible spheres, but Tycho had demonstrated that comets travel unhindered through interplanetary space, seeming to contradict this idea. Kepler thought that some influence from the sun impelled the planets, but he had no scientific means to describe it.

If I have seen further it is by standing on the shoulders of giants.” Isaac Newton

Universal gravitation

It fell to Isaac Newton to describe the force responsible for the movement of the planets, with a theory that remained unchallenged until Einstein. Newton concluded that celestial bodies pull on each other and he showed mathematically that Kepler’s laws follow as a natural consequence if the pulling force between two bodies decreases in proportion to the square of the distance between them. Writing about this force, Newton used the word gravitas, Latin for weight, from which we get the word gravity.

Improving telescopes

Newton not only created a new theoretical framework for astronomers with his mathematical way of describing how objects move, but he also applied his genius to practical matters. Early telescope makers found it impossible to obtain images free from colored distortion with their simple lenses, although it helped to make the telescope enormously long. Giovanni Domenico Cassini, for example, used long “aerial” telescopes without a tube to observe Saturn in the 1670s.

In 1668, Newton designed and made the first working version of a reflecting telescope, which did not suffer from the color problem. Reflecting telescopes of Newton’s design were widely used in the 18th century, after English inventor John Hadley developed methods for making large curved mirrors of precisely the right shape from shiny speculum metal. James Bradley, Oxford professor and later Astronomer Royal, was one astronomer who was impressed and acquired a reflector.

There were also developments in lens-making. In the early-18th century, English inventor Chester Moore Hall designed a two-part lens that greatly reduced color distortion. The optician John Dollond used this invention to build much-improved refracting telescopes. With high-quality telescopes now widely available, practical astronomy was transformed.

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