# A (very) brief history of astronomy

Early astronomers, in different civilizations, used the observed motion of the stars, the Sun, Moon and planets as the basis for clocks, calendars and a navigational compass. The Greeks developed models to account for these celestial motions.

Copernicus, in the 16th century, was the first to explain the observed looping (retrograde) motion of planets, by replacing a geocentric heliocentric model of the Universe with a heliocentric model. Modern planetary astronomy really began in the 17th century with Kepler, who used Tycho Brahe’s very accurate measurements of the planetary positions to develop his three laws.

Galileo contributed to the development of astronomy by teaching the Copernican view, and by devising a telescope which he used to show Jupiter’s moons as a model for the solar system, among other things.

Newton built on earlier insights with his universal law of gravitation and its fruits: predictions or explanations of Kepler’s laws, the motion of comets, the shape of the Earth, tides, precession of the equinoxes and perturbations in the motion of planets which led to the discovery of Neptune. He also had to invent the mathematics to do this: calculus.