Galileo’s Instruments of Discovery

Galileo’s telescope anchors the exhibit “Galileo, the Medici and the Age of Astronomy,” which is at the Franklin Institute in Philadelphia until September 7, 2009. Though Galileo didn’t invent the telescope (Dutch opticians did), he improved upon it. The first telescope used a combination of two lenses within a tube, and it could magnify items by three times, but showed them upside down. But when Galileo constructed his own telescope in 1609, he added a third lens. His telescope magnified items by eight times and showed them right side up. Over the following years, Galileo built several telescopes, including one that would magnify items by a factor of 30.

At the center of this instrument sits a globe representing the earth. The bands around it pivot on a common center and illustrate the paths of the sun and moon, known planets and important stars. The device was invented sometime in the last few centuries before Christ, but the sphere became widely used in Europe by a thousand years ago. This armillary sphere dates to 1578.

This portable instrument had rings that could be lined up with the equator, the meridian (the plane that holds both the observer and the celestial poles, about which the earth appears to revolve) and various angles in relation to the celestial equator. With such information, the user could make astronomical measurements and tell time.

During the day, a person could tell time with this pocket-size device by unfolding a gnomon, a vertical shaft, and thereby turning the device into a small sundial. Turn it over at night, line it up with the pole star, Ursa Major, and it became a nocturnal. At night, the sky appears to revolve around the pole star, and though the stars’ positions vary by time of year, they can give an indication of time during the night. A nocturnal could be manipulated to help a person determine the local time based on these star positions. This nocturnal and sundial device dates to 1554.

At night, this small device could be used as a nocturnal to tell time. During the day, it could be used as an horary disk to tell time and also to determine when the sun and moon would rise and set. Similar devices went into use in Europe around the eighth century. This one dates to 1647.

An astrolabe is a portable astronomical calculator that could show how the sky would look when standing at a particular place at a particular time. With one, a person could determine the time of day or night, figure out when the sun would rise and set and find the positions of certain stars. This brass astrolabe dates to about A.D. 832 and corresponds to latitudes between the Black Sea and Persia.

This extremely rare sundial, also known as the navicula de Venetiis, or "little ship of Venice," was named for its boat shape. (This one dates from the 15th century.) It is an example of an altitude dial, which can be used to tell time based on the variation in the sun’s altitude during the day. Another example of an altitude dial from the Medici collection is the astronomical ring dial.

Galileo invented the geometric and military compass, his first commercial scientific instrument, in 1597. The device, which resembled two rulers that moved over a third, curved piece, acted as an early calculator. Merchants could use it to work out monetary exchange rates. Shipwrights employed the device when testing hull designs in scale model. And it even found use on the battlefield, where soldiers could use the compass to determine the charge for a cannon.

This instrument is inscribed with the words “to find the distance by means of the surface.” It was a device used in surveying. A user could determine the distance of a faraway point by taking measurements from two locations and then using simple geometry. This example dates from about 1560.

Another instrument used in surveying, the graphometer could be used to measure angles up to 180 degrees and determine, through triangulation, distances at land and sea. The device was introduced in 1597 by a Frenchman and was particularly popular in France. This graphometer is from the 17th century.

This square could be used either vertically, as shown, or horizontally to measure heights and distances through triangulation. It also had a magnetic compass to determine direction. This square is from the 16th century.

The theodolite, used in surveying, consisted of one telescope fixed on one side of a disk and a second telescope that rotated on the opposite side. This theodolite belongs to the Medici collection, but there have been other famous users. Thomas Jefferson purchased one in 1778 and later wrote that "the measure of angles, by the wonderful perfection to which the graduation of instruments has been brought… removes nearly all distrust from that operation."