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May 1609: Galileo First Hears About the Telescope

The telescope; inset of Galileo Galilei.

Galileo Galilei is justly known for many contributions to science, as well as for his persecution and confinement under the Inquisition. But among his most memorable achievements is his adaptation of a novel instrument, the telescope, with which he observed the Moon, discovered four satellites of Jupiter, resolved nebular patches into stars, and observed the phases of Venus. In the process, he helped lead a revolution in cosmology — along with his fellow astronomers — that conclusively toppled the traditional Aristotelian model in favor of the Copernican system.

Historians generally agree that the telescope originated in the Netherlands, with two simultaneous patent applications appearing in October 1608; a third inventor apparently developed a telescope around the same time and attempted to sell it at the Frankfurt Fair. These designs consisted of a convex and concave lense in a tube, able to magnify objects by two or three times their original size. The news of the invention spread rapidly throughout Europe, and samples of the device soon followed. By April 1609, citizens could purchase three-powered spyglasses in local spectacle makers' shops in Paris; within four months, they were also available in Italy.

News of this marvelous new instrument for "seeing faraway things as though nearby" reached Galileo in May 1609, and he quickly duplicated the invention and constructed his own three-powered telescope that summer, then set about making improvements in the design. He presented an eight-powered instrument to the Venetian Senate in August, and was rewarded with a doubling of his salary and lifetime tenure at the University of Padua. By late October, he had completed a twenty-powered telescope, which is when he first turned it to the heavens to observe celestial bodies.

Galileo initially used the instrument for a series of observations of the Moon, which neared completion at the end of 1609, when Jupiter was at opposition and closest to the Earth, and hence the brightest object in the evening sky, apart from the Moon itself. After making the necessary adjustments, he began observing the planet, noting on 7 January 1610 that Jupiter appeared to have three fixed stars nearby. Intrigued, he returned to the planet the following evening, expecting the then-retrograde planet to have moved from east to west, leaving the three little stars behind. Instead, Jupiter seemed to have moved to the east — an interesting anomaly.

Puzzled by the planet's behavior, Galileo returned to the formation repeatedly, observing several key details. First, the little stars never left Jupiter, but appeared to be carried along with the planet. Second, as they were carried along, they changed their position with respect to each other and to Jupiter. Finally, there were four of these little stars. By the 15th of January, he concluded the objects were not fixed stars, but planetary bodies that revolved around Jupiter. The planet had four moons-strong support for Copernican theory. He published this groundbreaking observation in his book, Sidereus Nuncius, which appeared in Venice in the middle of March 1610, guaranteeing his fame and ensuring his place in scientific history.

Following the publication of the Sidereus Nuncius, Galileo continued to make observations of celestial objects. In July 1610, he first remarked on the strange appearances of Saturn, which sometimes seemed to be oval, sometimes two lateral bodies, and at other times solitary and perfectly spherical-another puzzling enigma. By December, he was able to verify the observations of other astronomers that Venus has phases similar to the Moon, providing additional proof that Venus orbits the Sun, in conformance with the Copernican System.

The product of craftsmen, rather than an invention of scientists, the telescope nonetheless enjoys an important place in history as the prototype of modern scientific instruments. The observations made by Galileo and his scientific colleagues revealed hitherto unsuspected phenomena in the heavens and had a profound impact on the 17th century controversy between followers of the traditional geocentric astronomy and those who favored the heliocentric system of Copernicus.

A Typical Galilean Telescope

Galileo's famous telescope for observing Jupiter's moons had a convex lens with a focal length of about 30-40 inches and a concave ocular lens of about 2 inches, contained in a little tube that could be adjusted for focusing. The instrument's magnification was between 15 and 20, and the aperture of the convex lens was 0.5 to 1 inch, yielding a field of view of about 15 inches in 100 yards (i.e., 15 arc-minutes). This small field of view meant that only a quarter of the full Moon could be accommodated. However, the Galilean telescope could be used for terrestrial and celestial observations interchangeably, unlike the astronomical telescope described by Johannes Kepler in 1611, which employed both a convex objective and a convex ocular, resulting in an inverted image.

—Adapted primarily from information provided by The Galileo Project (http://es.rice.edu/ES/humsoc/Galileo)

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