3rd century B.C. : Heliocentrism (Earth and the planets revolving around the Sun) by Aristarchus of Samos 3rd century B.C. : The calculation of the circumference of the Earth by Eratosthenes 2nd century B.C. : Discovery of tides being caused by the Moon, by Seleucus of Seleucia 16th century : Explicit heliocentric model by Nicolaus Copernicus 17th century : Moons of Jupiter by Galileo Galilei 17th century : Sunspots by Johannes and David Fabricius, Christoph Scheiner and Galileo Galilei 17th century: Titan is discovered by Christiaan Huygens 17th century : Discovery of four satellites of Saturn, by Giovanni Domenico Cassini 18th and 19th century : The outer planets, Uranus and Neptune discovered. 19th century: Ceres, first asteroid discovered by Giuseppe Piazzi 20th century: Pluto is discovered by Clyde Tombaugh 20th century : Universe beyond our galaxy 20th century : Expansion and age of the universe, based on Hubble's law 20th century : Cosmic microwave background radiation 20th century: Accelerating universe 20th century: confirmed existence of exoplanets 21st century: confirmed existence of gravitational waves