The Telescope

Optical instruments expand our perception and help us determine what is true and what is not.

The telescope and the microscope changed our awareness of the universe and ourselves, showing that objective truth is available through careful observation and reasoning.

Galileo's discoveries helped him envision Earth not at the center of the universe but as a component of a dynamic system. He demonstrated that truth could be determined by reasoning, not by authority or dictate, and is accessible by all.

This original telescope is one of several Galileo created and is preserved in Museo Galileo in Florence, Italy. Unfortunately, it is unknown which of the instruments he used when observing Jupiter's moons.

The telescopic design incorporated a convergent (plano-convex) lens as the objective and a divergent (plano-concave) lens as the eyepiece. This basic configuration was sufficient for Galileo to make his essential observations and was later improved by Kepler to produce even more precise images.

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Aristotle believed that the heavens and all heavenly bodies are perfect spheres, and everything revolves around Earth. This official view aligns with what we see and feel. Our senses don't tell us our planet is spinning on its axis, traveling thousands of miles per hour through space. And we can see the sun, planets, and stars revolving around us.

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Galileo's first view of the moon convinced him the Aristotelian argument and our common perceptions were incorrect. He saw the moon covered with craters and knew it was not a perfect sphere. Measuring the shadows on the surface, he accurately determined that some of the mountains were four miles high.

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On January 7, 1610, Galileo observed Jupiter, with two stars on the left and one on the right.

On January 8th, three stars appeared on the right.

January 9th was cloudy, and on the 10th, two stars appeared on the left.

What he observed with his instrument was not as significant as what he saw in his mind.

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Galileo concluded that these "stars" were moons that orbit Jupiter, much as our Moon circles Earth.

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The prevailing view held Earth at the center of the universe and that everything revolves around us. This supposition was shattered forever by Galileo's discovery of moons orbiting Jupiter.

This got him in serious trouble and in 1693 Galileo was taken to Rome to stand trial before the Inquisition for heresy.

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His publication of Dialogue Concerning the Two Chief World Systems was a direct challenge to the church's 1616 edict forbidding teaching the Copernican theory of the Earth in motion around the sun. The Inquisition found him "vehemently suspected of heresy" and sentenced him to life imprisonment. In addition, Galileo was forced on his knees to deliver a handwritten confession renouncing his beliefs.


According to legend, as Galileo rose to his feet, he uttered, under his breath, "Eppur si muove" -- "And yet, it moves." In 1979, Pope John Paul II stated that the Roman Catholic Church may have mistakenly condemned Galileo. The pope endorsed this opinion in 1992.

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The Galileoscope is a quality instrument that follows the refractor design of Galileo's telescope. This duplicate of the original design is available in kit form and is easily assembled. With this telescope, you can view the Moon's craters, Venus phases, Europa, Lo, Ganymede, and Cassisto in orbit around Jupiter.

Galileoscope - Amazon


"Finocchiaro's new and revised translations have done what the Inquisition could not: they have captured an exceptional range of Galileo's career while also letting him speak--in clear English. No other volume offers more convenient or more reliable access to Galileo's own words, whether on the telescope, the Dialogue, the trial, or the mature theory of motion."

Michael H. Shank, Professor of the History of  Science, University of Wisconsin–Madison

The Essential Galileo Amazon

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