Galileo and the Telescope

The science of astronomy took a huge leap forward in the first decade of the 1600s with the invention of the optical telescope and its use to study the night sky. Galileo Galilei did not invent the telescope but was the first to use it systematically to observe celestial objects and record his discoveries. His book, Sidereus nuncius or The Starry Messenger was first published in 1610 and made him famous. In it he reported on his observations of the Moon, Jupiter and the Milky Way. These and subsequent observations and his interpretations of them eventually led to the demise of the geocentric Ptolemaic model of the universe and the adoption of a heliocentric model as proposed in 1543 by Copernicus.

Galileo's drawings of the Moon
Galileo's drawings of phases of the Moon.
Question: What features are visible here that cannot be seen with the unaided eye?

Galileo's Telescopes

The basic tool that Galileo used was a crude refracting telescope. His initial version only magnified 8x but was soon refined to the 20x magnification he used for his observations for Sidereus nuncius. It had a convex objective lens and a concave eyepiece in a long tube. The main problem with his telescopes was their very narrow field of view, typically about half the width of the Moon.

Galileo's drawing of the optical path of his telescope
The earliest known sketch of a telescope, August 1609.
One of Galileo's telescopes. The focal length is 1330 mm with a 26 mm aperture, it magnifies 14x. It has an objective bi-convex lens and a plano-concave eyepiece.

Galileo's Observations

Galileo made several key discoveries through his systematic use and refinement of the telescope.

The Moon

According to Aristotelian principles the Moon was above the sub-lunary sphere and in the heavens, hence should be perfect. He found the "surface of the moon to be not smooth, even and perfectly spherical,...,but on the contrary, to be uneven, rough, and crowded with depressions and bulges. And it is like the face of the earth itself, which is marked here and there with chains of mountains and depths of valleys." He calculated the heights of the mountains by measuring the lengths of their shadows and applying geometry.

A lunar drawing by Galileo
One of Galileo's lunar drawings.
Note the craters, mountains and mare or "seas". The terminator between lunar day and night is clearly seen down the centre.

Moons of Jupiter

Observations of the planet Jupiter over successive night revealed four star-like objects in a line with it. The objects moved from night to night, sometimes disappearing behind or in front of the planet. Galileo correctly inferred that these objects were moons of Jupiter and orbited it just as our Moon orbits Earth. For the first time, objects had been observed orbiting another planet, thus weakening the hold of the Ptolemaic model. Today these four moons are known as the Galilean satellites; Io, Europa, Ganymede and Callisto.

Galileo's drawings of the moons of Jupiter of successive nights

The Phases of Venus

Venus was observed to go through a sequence of phases similar to the Moon. This could not be explained in the Ptolemaic model but could be accounted for by either the Sun-centered Copernican model or the Earth-centered Tychonic model that had the other planets orbiting the Sun as it orbited the Earth. Galileo rejected Tycho's model as an unnecessary hybrid and used the discovery to consolidate his support of the Copernican model.


Along with contemporaries such as Thomas Harriot, David Frabicius and Christoph Scheiner, Galileo observed dark regions that appeared to move across the surface of the Sun. Debate centered on whether these were satellites of the Sun or actual spots on its surface. Galileo, in his Letters on Sunspots supported the sunspot interpretation and used it to show that the Sun was rotating. Its blemishes and imperfections again undermined the Aristotelian ideal of a perfect cosmos.

"Appendages" on Saturn

Galileo noted two appendages from the sides of Saturn. These disappeared then later reappeared. It was not until 1656 that the Dutch scientist, Christiaan Huygens correctly described them as rings.

Stars in the Milky Way

Even through a telescope the stars still appeared as points of light. Galileo suggested that this was due to their immense distance from Earth. This then eased the problem posed by the failure of astronomers to detect stellar parallax that was a consequence of Copernicus' model. On turning his telescope to the band of the Milky Way Galileo saw it resolved into thousands of hitherto unseen stars. This posed the question as to why there were invisible objects in the night sky?

More stars are resolved in this drawing by Galileo of the Pleiades than are visible to the unaided eye.

Further Information

  1. An excellent online source for all things related to Galileo is: The Galileo Project. It is hosted by Rice University and includes his writings, details on his experiments and observations and links. It is also the source for the image of Galileo at the top of this page.
  2. Another worthwhile site is The Art of Renaissance Science: Galileo and Perspective. It has a wealth of diagrams matched with clear. concise text. There are some animations of his experiments.
  3. The Institute and Museum of the History of Science in Florence has a wealth of detail on the history of astronomy including Galileo's work. Their new website is worth exploring. Much of it is in English although some sections, including an excellent simulation of early telescopes, is only available in Italian at present. They also have an excellent new site: Galileo's Telescope, the Instrument that Changed the World.
  4. Afocal CCD Images Through a Galilean Telescope is an excellent resource that provides CCD images that approximate what the human eye would see through a Galilean telescope. It has images of the Sun, Moon, Venus, stars and nebulae. The site provides historical background and technical details.
  5. Galileo's Sidereus nuncius or The Sidereal messenger is available in translation by A. van Helden from University of Chicago Press, 1989.
  6. The Cambridge Illustrated History of Astronomy, ed Michael Hoskin, Cambridge University Press, 1997 provides a wealth of information and diagrams and is an authoritative yet readable guide to the topic.
  7. The Sleepwalkers by Arthur Koestler is a classic book dealing with the development of astronomical thought up to the time of Newton.

There are numerous other books and web sites covering Galileo's work and the history of astronomy.


  1. Why was the telescope an advance over naked-eye astronomy?
  2. What did Galileo's observations of the Moon reveal?
  3. What was the significance of Galileo observing phases of Venus?
  4. How did Galileo's observations help undermine the existing paradigm of the Ptolemaic model of the Universe and Aristotle's physics?