From Mysterious “Spyglass” to Reliable Telescope

Once upon a time, in 1610, Galileo pointed a telescope at Jupiter and discovered that Jupiter had four moons. This event is commonly regarded as the first real triumph for the Copernican theory—the theory that the Sun, and not the Earth, sat at the central point of the Universe. The discovery of Jupiter’s moons was revolutionary, unexpected and mysterious. It turned thousands of years of cosmology on its head. 

Indeed, the discovery was so unexpected that we have reason to wonder: why on earth were Galileo’s observations taken seriously at all? After all, almost no-one else apart from Galileo himself could see the moons, and it seemed as though you had to use a newfangled and prohibitively expensive contraption called a “spyglass” to see the moons anyway. Galileo might as well have said that he had an invisible dragon in his kitchen, but that you could only see it with the aid of diamond spectacles or through a perfectly circular peep-hole.

Just think about it: no-one else could see these moons, and Galileo’s telescope was the only detection tool that yielded the observation. Galileo’s contemporaries had only one alleged method of collecting positive evidence for the existence of Jupiter’s moons—his bloody telescope. Could anyone else cross-check Galileo’s observations? If the telescopic observations couldn’t be cross-checked, then why were his results taken seriously?

Well, the fact is, they weren’t. When Galileo published Sidereus nuncius in 1610, most other scientists were skeptical that Galileo’s observations were reliable, since they were obtained by way of a mysterious spyglass contraption which was regarded as error-prone for celestial observations. Part of the reason for this widespread skepticism undoubtedly lay in the fact that the deliverances of telescopic observation flatly contradicted naked eye observation. “Just go look at Jupiter with your damned eyes, there ain’t no moons there!”  Furthermore, the telescopic observations were very often, shall we say, “anti-Ptolemaic”. Each new discovery made with the telescope undermined geocentrism. For these two reasons, telescopes were considered to be giving misleading information, or as one of Galileo’s contemporaries, Martin Horky, put it: ‘On Earth, it works miracles; in the heavens it deceives.’

Galileo was apparently either deceived or a deceiver.

This “telescope-skepticism” was almost entirely abandoned within a year or two of the publication of Sidereus nuncius, after the growing corroboration of Galileo’s claims by independent observers, including the mathematicians at the official Collegio Romano. In August of 1610, Johannes Kepler was gifted one of Galileo’s telescopes and he made observations of Jupiter’s moons in the presence of a young astronomer fried, Benjamin Ursinus. Together, the pair attempted to show that the telescope’s images were reliable by following the following procedure: ‘what one observed he secretly drew on the wall with chalk, without its being seen by the other. Afterwards, we passed together from one picture to the other to see if they agreed.’

Following this method, Kepler and Ursinus agreed on the relative positions of three of Jupiter’s moons, yet disagreed over a fourth. This surprising, nay miraculous, intersubjective agreement about the positions of the moons was one of the first experimental verifications of the reliability of telescopic observations. Kepler’s aim was to demonstrate that the agreement generated between himself and Ursinus was not due to any suspicious interference, preconceived plan or fraudulent tricks. To that end, Kepler went so far as to withhold all contact from Galileo until publishing his observations, so that none could allege that Kepler was merely a Galilean stooge, acting under his instructions.

Given the rapid pace of telescopic advance in the 17th century, it was in the 1630s, only twenty years or so after Galileo’s initial claim, that his observations could be corroborated by telescopes that worked according to different physical principles. A new type of “astronomical telescope” that made use of a convex rather than a concave ocular lens was first described by Kepler in 1611. Astronomical telescopes became widespread around the middle part of the 17th century, and observations made with these “next generation spyglasses” corroborated what Galileo had said.

One further 17th century advance in telescope technology deserves mention, since the technology is similar to that used in most large telescopes today. Although the Jesuit astronomer Nicolas Zucchi experimented with replacing one of his lenses with a mirror in his telescope of 1616, it was not until 1668 that Newton developed a functioning example of a telescope that worked on the principle of reflection rather than refraction—the telescope used a curved mirror to reflect the incoming image towards the observer. Reflecting, or “Newtonian”, telescopes once again corroborated all that Galileo had said. Their chief improvement on earlier telescopes lay in the elimination of the chromatic aberrations that had plagued earlier observations made with lens-only telescopes. Diffuse halos—artifacts of the lenses—had surrounded the objects of telescopic observations up until Newton came along. This was an immense leap forward that took much technical, theoretical and practical skill to develop. Newtonian telescopes were welcomed for yet other reasons too: they were far shorter than the best refracting telescopes, some of which had grown to inconvenient lengths of 10 metres or more!

Thus, within about a year or two of their discovery, Galileo’s moons had been independently corroborated by other investigators, notably Kepler. Around 20 years afterwards, the observations were corroborated by investigators using “astronomical” telescopes, with entirely different lenses. Within 60 years of Galileo’s first observations, the moons could be observed by telescopes that were totally unlike any that had been used before. All these different telescopes, all operating according to different physical principles, all observed the very same thing: moons. The moons could no longer be argued to be trickery or an artifact of Galileo’s telescope. They were not the result of a deceiving spyglass. Galileo was neither deceived nor deceiver. He was simply, and quite exceptionally, correct.


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