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The Italian scientist Galileo Galilei was the first person known to use a telescope to make astronomical observations. His records indicate that he began observing Mars through a telescope in September 1610. [26]
However Cassini's observation did bear out Galileo's argument that sunspots indicated that the Sun was rotating, [62] and Cassini did discover the rotation of Mars and Jupiter, [63] which supported Galileo's contention that both the Earth and the Sun rotated.
Because its orbit takes it between the Earth and the Sun, Venus as seen from Earth exhibits visible phases in much the same manner as the Earth's Moon. Galileo Galilei observed the phases of Venus in December 1610, an observation which supported Copernicus's then-contentious heliocentric description of the Solar System.
Galileo observed the phases of Venus's appearance with the telescope and was able to confirm Kepler's first law of planetary motion and Copernicus's heliocentric model, of which Galileo was an advocate. [76] Galileo claimed that the Solar System is not only made up of the Sun, the Moon and the planets but also comets. [77]
The first observations of the full planetary phases of Venus were by Galileo at the end of 1610 (though not published until 1613 in the Letters on Sunspots).Using a telescope, Galileo was able to observe Venus going through a full set of phases, something prohibited by the Ptolemaic system that assumed Venus to be a perfect celestial body.
Due to Galileo's observations of Venus, Ptolemy's system became highly suspect and the majority of leading astronomers subsequently converted to various heliocentric models, making his discovery one of the most influential in the transition from geocentrism to heliocentrism.
Galileo di Vincenzo Bonaiuti de' Galilei (15 February 1564 – 8 January 1642), commonly referred to as Galileo Galilei (/ ˌ ɡ æ l ɪ ˈ l eɪ oʊ ˌ ɡ æ l ɪ ˈ l eɪ /, US also / ˌ ɡ æ l ɪ ˈ l iː oʊ-/; Italian: [ɡaliˈlɛːo ɡaliˈlɛːi]) or mononymously as Galileo, was a Florentine astronomer, physicist and engineer, sometimes described as a polymath.
Italian polymath Galileo Galilei was an early user and made prolific discoveries, including the phases of Venus, which definitively disproved the arrangement of spheres in the Ptolemaic system. Galileo also discovered that the Moon was cratered, that the Sun was marked with sunspots, and that Jupiter had four satellites in orbit around it. [13]