The Telescopes Galileo Made

How He Built Them – An Original and Replicas Now Touring Museums

Aug 29, 2009

By 1609 Galileo had learned of the invention of the spyglass, soon to be called the telescope. As a skilled maker of scientific instruments, he decided to construct one.

He was professor of mathematics at the University of Padua in Italy. In June or July 1609 he made at least one telescope that magnified objects by the power of three. By August he had succeeded in making one that magnified objects by the power of eight.

Demonstrates the Telescope at St. Mark's in Venice

Late that month he took this newly improved telescope to Venice where he showed it to government officials and other dignitaries. From atop the bell tower of St. Mark's Basilica in St. Mark's Square, the men looked at ships at sea. With the telescope, they saw ships two hours before they would have seen them with the naked eye.

Galileo let the assembled men keep the coveted telescope. They gave him lifetime tenure at the university and an increase in salary. Most gathered there were interested in the possible military applications of the telescope. However, it soon became clear that the narrow field of view of Galileo's telescope would make it ineffective in battle. Furthermore, without the adjustments that would come later, it presented objects upside down. Nonetheless, when Galileo turned his telescopes to the skies he made revolutionary discoveries.

Galileo made many telescopes in his lifetime. In the last half of 1609 he made nearly 100. About 10 of those were of very good quality and Galileo used them to make his first significant astronomical discoveries in the fall of 1609. Galileo was a prolific maker of telescopes from 1610 to 1640 because he made them not only for himself but for others who wanted to study the skies.

How He Made Telescopes -– Some on Exhibit in the U. S. A.

To make a telescope he fashioned a long, thin tube made variously of lead, tin or cardboard with a glass lens at each end. Sometimes he did the grinding of the lenses and sometimes his skilled assistant Marcantonio Mazzoleni did the grinding.

The objective lens — the lens nearer the object being observed — was convex, the center being thicker than the edges. The eyepiece housed a concave lens characterized by edges that are thicker than the center. The convex lens "bends" the light, directing it to the eyepiece where it is refocused and the object magnified. The telescopes Galileo made are examples of refracting telescopes.

One of two original telescopes attributed to Galileo is on display at the Franklin Institute in Philadelphia until September 7, 2009. Also on display are a replica of another telescope made by Galileo and a disassembled, modern-day mock-up that allows visitors to see how one telescope Galileo used was constructed. The original and the replicas are on loan from Florence, Italy's Institute and Museum of the History of Science (IMSS), creator of the exhibit. The replicas were made by Jim and Rhoda Morris of Massachusetts.

Dimensions of Some of the Telescopes

The original telescope on display at the Franklin Institute was made about 1610 in Padua or Florence. It is about 1273 mm (4.1 feet) long. Two halves of wood form the tube. Special housings for the lenses are attached at each end. Varnished paper covers the tube, which is held together by thin bands of copper wire.

The other telescope is a replica of the ornate telescope Galileo presented to Cosimo II, the Grand Duke of Tuscany, a member of the rich and powerful Medici family and one of his patrons. It is about 920 mm (36.22 inches) long and 60 mm (2.36 inches) in diameter. When originally made, the tube was formed from 20 joined strips of wood glued onto paper. Separate sections holding the lenses are attached. The telescope is covered in leather, which is decorated with gold accents.

From Philadelphia the telescopes go to the Nobel Museum in Stockholm, where the exhibits opens October 10, 2009. Since this year marks the 400th anniversary of Galileo's first astronomical discoveries, a simple, low-cost telescope kit called the Galileoscope that allows the viewer to see what Galileo saw is available. It costs about US$15.

The copyright of the article The Telescopes Galileo Made in Astronomy History is owned by Linda N. Riggins. Permission to republish The Telescopes Galileo Made in print or online must be granted by the author in writing.