---
title: "History of astronomy"
chunk: 9/11
source: "https://en.wikipedia.org/wiki/History_of_astronomy"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T03:59:16.412764+00:00"
instance: "kb-cron"
---

=== Galileo ===
 The invention of the telescope in 1608 revolutionized the study of astronomy. Galileo Galilei was among the first to use a telescope to observe the sky, after constructing a 20x refractor telescope. He discovered the four largest moons of Jupiter in 1610, which are now collectively known as the Galilean moons, in his honor. This discovery was the first known observation of satellites orbiting another planet. He also found that the Moon had craters and observed, and correctly explained sunspots, and that Venus exhibited a full set of phases resembling lunar phases. Galileo argued that these facts demonstrated incompatibility with the Ptolemaic model, which could not explain the phenomenon and would even contradict it. With Jupiter's moons, he demonstrated that the Earth does not have to have everything orbiting it and that other bodies could orbit another planet, such as the Earth orbiting the Sun. In the Ptolemaic system the celestial bodies were supposed to be perfect so such objects should not have craters or sunspots. The phases of Venus could only happen in the event that Venus orbits around the Sun, which did not happen in the Ptolemaic system. He, as the most famous example, had to face challenges from church officials, more specifically the Roman Inquisition. They accused him of heresy because these beliefs went against the teachings of the Roman Catholic Church and were challenging the Catholic church's authority when it was at its weakest. While he was able to avoid punishment for a little while he was eventually tried and pled guilty to heresy in 1633. Although this came at some expense, his book was banned, and he was put under house arrest until he died in 1642. Sir Isaac Newton developed further ties between physics and astronomy through his law of universal gravitation. Realizing that the same force that attracts objects to the surface of the Earth held the Moon in orbit around the Earth, Newton was able to explain – in one theoretical framework – all known gravitational phenomena. In his Philosophiæ Naturalis Principia Mathematica, he derived Kepler's laws from first principles. Those first principles are as follows:
In an inertial frame of reference, an object either remains at rest or continues to move at constant velocity, unless acted upon by a force.
In an inertial reference frame, the vector sum of the forces F on an object is equal to the mass m of that object multiplied by the acceleration a of the object: F = ma. (It is assumed here that the mass m is constant)
When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body.
Thus while Kepler explained how the planets moved, Newton accurately managed to explain why the planets moved the way they do. Newton's theoretical developments laid many of the foundations of modern physics.

=== Completing the Solar System ===

Outside of England, Newton's theory took some time to become established. René Descartes' theory of vortices held sway in France, and Christiaan Huygens, Gottfried Wilhelm Leibniz and Jacques Cassini accepted only parts of Newton's system, preferring their own philosophies. Voltaire published a popular account in 1738. In 1748, the French Academy of Sciences offered a reward for solving the question of the perturbations of Jupiter and Saturn, which was eventually done by Euler and Lagrange. Laplace completed the theory of the planets, publishing from 1798 to 1825. The early origins of the solar nebular model of planetary formation had begun.
Edmond Halley succeeded John Flamsteed as Astronomer Royal in England and succeeded in predicting the return of the comet that bears his name in 1758. William Herschel found the first new planet, Uranus, to be observed in modern times in 1781.
At first, astronomical thought in America was based on Aristotelian philosophy, but interest in the new astronomy began to appear in Almanacs as early as 1659. The gap between the planets Mars and Jupiter disclosed by the Titius–Bode law was filled by the discovery of the asteroids Ceres and Pallas in 1801 and 1802 with many more following.