section_path: Discovery and exploration
section_name: Discovery and exploration
paragraph_text: Discovery and exploration
  
  Humanity's knowledge of the Solar System has grown incrementally over the centuries. Up to the Late Middle Ages – Renaissance, astronomers from Europe to India believed Earth to be stationary at the center of the universe and categorically different from the divine or ethereal objects that moved through the sky. Although the Greek philosopher Aristarchus of Samos had speculated on a heliocentric reordering of the cosmos, Nicolaus Copernicus was the first person known to have developed a mathematically predictive heliocentric system.
  
  Heliocentrism did not triumph immediately over geocentrism, but the work of Copernicus had its champions, notably Johannes Kepler. Using a heliocentric model that improved upon Copernicus by allowing orbits to be elliptical, and the precise observational data of Tycho Brahe, Kepler produced the Rudolphine Tables, which enabled accurate computations of the positions of the then-known planets. Pierre Gassendi used them to predict a transit of Mercury in 1631, and Jeremiah Horrocks did the same for a transit of Venus in 1639. This provided a strong vindication of heliocentrism and Kepler's elliptical orbits.
  
  In the 17th century, Galileo publicized the use of the telescope in astronomy; he and Simon Marius independently discovered that Jupiter had four satellites in orbit around it. Christiaan Huygens followed on from these observations by discovering Saturn's moon Titan and the shape of the rings of Saturn. In 1677, Edmond Halley observed a transit of Mercury across the Sun, leading him to realize that observations of the solar parallax of a planet (more ideally using the transit of Venus) could be used to trigonometrically determine the distances between Earth, Venus, and the Sun. Halley's friend Isaac Newton, in his magisterial Principia Mathematica of 1687, demonstrated that celestial bodies are not quintessentially different from Earthly ones: the same laws of motion and of gravity apply on Earth and in the skies.
  
  The term "Solar System" entered the English language by 1704, when John Locke used it to refer to the Sun, planets, and comets. In 1705, Halley realized that repeated sightings of a comet were of the same object, returning regularly once every 75–76 years. This was the first evidence that anything other than the planets repeatedly orbited the Sun, though Seneca had theorized this about comets in the 1st century. Careful observations of the 1769 transit of Venus allowed astronomers to calculate the average Earth–Sun distance as 93,726,900 miles (150,838,800 km), only 0.8% greater than the modern value.
  
  Uranus, having occasionally been observed since 1690 and possibly from antiquity, was recognized to be a planet orbiting beyond Saturn by 1783. In 1838, Friedrich Bessel successfully measured a stellar parallax, an apparent shift in the position of a star created by Earth's motion around the Sun, providing the first direct, experimental proof of heliocentrism. Neptune was identified as a planet some years later, in 1846, thanks to its gravitational pull causing a slight but detectable variation in the orbit of Uranus. Mercury's orbital anomaly observations led to searches for Vulcan, a planet interior of Mercury, but these attempts were quashed with Albert Einstein 's theory of general relativity in 1915.
  
  In the 20th century, humans began their space exploration around the Solar System, starting with placing telescopes in space since the 1960s. By 1989, all eight planets have been visited by space probes. Probes have returned samples from comets and asteroids, as well as flown through the Sun's corona and visited two dwarf planets (Pluto and Ceres). To save on fuel, some space missions make use of gravity assist maneuvers, such as the two Voyager probes accelerating when flyby planets in the outer Solar System and the Parker Solar Probe decelerating closer towards the Sun after flyby with Venus.
  
  Humans have landed on the Moon during the Apollo program in the 1960s and 1970s and will return to the Moon in the 2020s with the Artemis program. Discoveries in the 20th and 21st century has prompted the redefinition of the term planet in 2006, hence the demotion of Pluto to a dwarf planet, and further interest in trans-Neptunian objects.