Galileo's Telescopes

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History of Galileo's Telescopes

While the telescope was invented by Hans Lippershey and Zacharias Janssen as a type of spyglass in 1608, Galileo Galilei took their technology and pointed it skywards to study the heavens. The key to Lippershey's spyglass was the use of a concave lens that was stronger than the convex lens used with it. Lippershey was denied a patent for his invention, as it was deemed too easy to replicate, a sentiment that had been proven to be true by the time Galileo heard about it in 1609. With a basic understanding of how the spyglass worked, Galileo set about to improve the invention to use it as he wished. His first telescope had roughly 3x magnification using concave and convex lenses he bought from opticians, and to improve the magnification even more he began grinding his own lenses. The problem he ran into in using store-bought lenses was that he was unable to achieve a desirable difference in the strengths of the concave and convex lenses, as the opticians of his time only made glasses in a narrow range of strengths. By grinding his own lenses Galileo was quickly able to achieve ninefold linear magnification, much stronger than any other available lenses. He approached the Senate of Venice to demonstrate his new instrument, so impressing many of the senators that they would climb the highest belltowers in Venice to look at far off ships at sea. Galileo then wrote a letter to the Doge of Venice:

Galileo Galilei, a most humble servant of Your Serene Highness, being diligently attentive, with all his spirit, not only to discharging the duties pertaining to the lecturing of mathematics at the University of Padua, but also to bringing extraordinary benefit to Your Serene Highness with some useful and remarkable invention, now appear before You with a new contrivance of glasses, drawn from the most recondite speculations of perspective, which render visible objects so close to the eye and represent them so distinctly that those that are distant, for example, nine miles appear as though they were only one mile distant. This is a thing of inestimable benefit for all transactions and undertakings, maritime or terrestrial, allowing us at sea to discover at a much greater distance than usual the hulls and sails of the enemy, so that for two hours or more we can detect him before he detects us...

He concluded his article by asking for tenure from the University of Padua:

....(the telescope is) one of the fruits of the science which he has professed for the past 17 years at the University of Padua, with the hope of carrying on his work in order to present You greater ones, if it shall please the Good Lord and Your Serene Highness that he, according to his desire, will pass the rest of his life in Your service.

He was granted the requested tenure and an increase in salary, but with a proviso that he would not receive any future raises. In addition to observing the night sky, Galileo sold his telescopes to merchants, who found them useful at sea and as items of trade.

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Galileo's Astronomical Discoveries

Galileo's new telescopes allowed him to make observations in the night sky that were previously impossible for the naked eye to discern. His first major astronomical discovery occurred in November 1609 after turning his telescope to Earth's Moon. While Copernicus had introduced his theory that the universe was heliocentric rather than geocentric, at the time this idea was widely denied. Rather, the Aristotelian belief that the every heavenly body revolved around the Earth, and that these heavenly bodies were perfect spheres. Galileo was able to see that the Moon's surface was in fact littered with mountains and craters. In order to prove the existence of these mountains, he sketched the appearance of parts of the Moon's surface at different times of the month. With the change in the angles of lighting during the course of a month he was able to demonstrate that the light and shadow visible on the Moon was a result of the Moon's topography. His theory replaced the previous idea that the variations in light were a result of something within the perfect sphere of the Moon, a theory that, though cumbersome and unappealing, allowed for the geocentric model of the universe to hold true. He was also able to estimate the height of the lunar mountains by measuring how far he could see the bright spots on the surface.

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Galileo's next major discovery took place on January 7, 1610, when he noticed what appeared to be three odd stars fixed in a line across the planet Jupiter. In five nights of observing Jupiter, Galileo noticed that the stars had moved not east to west as would be expected from Jupiter's retrograde motion, but rather from west to east, as would be expected from celestial bodies that were orbiting Jupiter. He also noticed that the stars never left Jupiter, appearing to be carried along with it, that they changed positions in relation to one another each night, and that there were not three, but rather four stars. By January 15 he realized that the stars were not fixed stars but were Jupiter's moons. This discovery had a large impact on the astronomical beliefs of the day, as the ran counter to the idea that the Earth was the only center of motion in the universe and that all other celestial bodies revolved only around the Earth. Regardless of what one's beliefs were, there were now at least two known centers of motion in the universe, the Earth or Sun and Jupiter.
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In addition to discovering the Moon's mountains and the moons of Jupiter, Galileo found that the planet Venus has phases similar to those of our Moon. He began observing Venus in November 1610, Galileo noticed that he could see the crescent, gibbous, and full phases of Venus, an observation which contradicted Ptolemy's theory that it was impossible for any planet to intersect the spherical shell carrying the Sun. Galileo also discovered the rings of Saturn, though was confused by their appearance and reappearance in the various times he observed the planet. When he first observed Saturn he thought the rings were planets, though the next time he looked the rings were directly oriented at Earth, leading him to believe that two of the bodies had disappeared. A later observation of Saturn showed that the bodies had reappeared, which further confused Galileo. He also observed the planet Neptune, though did not realize it was a planet.


Fowler, Michael. "Galileo and the Telescope." Web. <>.

"The Telescope." Web. <>.

Van Heiden, Al. "Satellites of Jupiter." Web. <>.

Wooton, David. Galileo: Watcher of the Skies. November 2010. Yale University Press.

Zumwalt, Ann. "Galileo's Moons-Then and Now." Web. <>.