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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 ' s closest approach to Venus came at 05:58:48 UTC on February 10, 1990, at a range of 16,106 km (10,008 mi). [82] Due to the Doppler effect, the spacecraft's velocity relative to Earth could be computed by measuring the change in carrier frequency of the spacecraft's transmission compared to the nominal frequency. [85]
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 telescopic observation of Mars was by Galileo Galilei in 1610. Within a century, astronomers discovered distinct albedo features on the planet, including the dark patch Syrtis Major Planum and polar ice caps. They were able to determine the planet's rotation period and axial tilt.
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.
Observation of the phases of Venus was inconsistent with this view but was consistent with the Polish astronomer Nicolaus Copernicus’s idea that the solar system is centered on the Sun. Galileo’s observation of the phases of Venus provided the first direct observational evidence for Copernican theory. [14]
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]
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.