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Galileo’s Reports about Venus, the Moon and Galilean Moons


Galileo Galilei was an Italian mathematician and physician who contributed tremendously to the growth, as well as, development of modern science. He made exceptional enhancements to the telescope which was used to observe heavenly bodies hence the relevance of this topic. He is the scientist behind the discovery of motion in astronomical features such as the phases of Venus, the four largest satellites of Jupiter also called the Galilean moons, as well as, studying the accelerated motion of the moon. According to his experiments, the sun is the center of the universe and not the earth as other astronomers believed.

 Galileo's reports about Venus

Galileo had observed the outer hemisphere for quite some time and he discovered that Venus was able to exhibit the same full set of phases as those of the moon. The various sets of phases in the entire rotation of Venus were credited to the geo-heliocentric planetary models which did not rely on the earth’s rotation. The changes in shape of Venus were attributed to the varying distance of Venus from the earth in the cause of rotation. This could be true as Venus like the other heavenly bodies gets its light from the sun such that the degree of sight from the earth varies considerably as the position of the planet shifts (Hartmann, 1987).

 The rotation of the planet Venus around the sun and inside the earth’s orbit leads to varying sections of the planet being exposed to the sun’s light. The entire period of time taken by Venus to move from the new phase to the full phase is given by Galileo as 584 days. This is equivalent to the exact time taken by Venus to move around the earth in the earth’s orbit. Contrary to popular belief that the astronomic bodies are brightest when they are fully exposed to the sun, Venus is most brightly when it is at the crescent stage (Drake, 1990).

 Galileo's reports about the Moon

The moon is the innermost of the four Galilean moons found in the planet Jupiter. It has a diameter of 3,642 kilometers and is the fourth largest moon in the entire solar system. Galileo observed that the moon has very many active volcanoes hence it is noted as the most geographically active body in the solar system (Seeger, 1966).

 Galileo used the knowledge of chiaroscuro to establish the patterns of light and shadow on the moons surface. These topological markers were efficient in deducing the cause of uneven waning as light occlusion from lunar mountains and craters. He went ahead to estimate the heights of mountains present in the moon’s surface. From these observations, he was able to come into a conclusion that the moon is not a perfect sphere and neither is it a translucent planet.

The mountains and craters meant that there were solid objects on the moon’s surface. Consequently, ability of the moon to undergo through the same phases as Venus depending on its position in relation to the earth and the sun showed that the moon was part of the solar system which like the other planets relied on the sun for illumination (Drake, 1990).

 The Galilean moons

The four Galilean moons were discovered between 1609 and1610 after Galileo made advancements on his telescope hence he was able to see the our largest moons around Jupiter. Subsequent observations showed that the four moons were not fixed but rather revolving around Jupiter. The largest moon Io was observed to be made of Sulfur dioxide and it had mountains as high as Mt. Everest with some being higher than this. This is the moon which illuminates the earth at night (Hartmann, 1987).

 Europe is the second moon which is closest to Jupiter and being the smallest, it is less visual with the naked eye as compared to the moon of the earth. It has few craters and its surface is seen as being smooth as if made of a sheet of ice (Seeger, 1966).

 Ganymede is the third moon and it is considered as the largest natural satellite as it is larger than Mercury. Ganymede is made up of core ore with a larger part being covered in ice although it has a small percentage of oxygen and the ozone (Drake, 1990).

 Callisto, the second largest moon of the Galilean moons is the only one that is not affected by appreciable tidal heating. It has wide craters than the other moons and Galileo discovered that it may be harboring a subsurface ocean hence making it a perfect choice for a planet that can support human life (Hartmann, 1987).


Galileo Galilei is one of the most celebrated scientists and astronomers of all times. His contribution to modern science was quite notable as it enabled the up coming astronomer to have it easy in their careers. This is because of the fundamental observations of the moon, Jupiter and Venus.


Drake, Stillman. Galileo: Pioneer Scientist. Toronto: The University of Toronto Press. (1990). Pp.23-68
Hartmann, William K. Astronomy 6th edition -the cosmic journey. Wardsworth publishing company. 1987
Seeger, Raymond J. Galileo Galilei, his life and his works. Oxford: Pergamon Press. (1966)
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