This morning somebody on Twitter tweeted that William Herschel discovered the planet Uranus on this day in 1781. A typical tweet amongst history of science fans on Twitter, who like to acknowledge and celebrate births, deaths, inventions and discoveries in what amounts to a rolling history of science calendar. On this occasion my history of science soul sisterTM, Rebekah “Becky” Higgitt, who’s quite knowledgeable about eighteenth-century astronomy, tweeted, quite correctly, that Herschel initially thought he had discovered a comet and it was Nevil Maskelyne, who first suggested that he had in fact observed a new planet and not a comet. She then asked if we should not then say that it was Nevil Maskelyne who discovered Uranus and not Herschel? Becky could be considered a bit biased having fairly recently devoted several years of her life to the study of the life and work of Maskelyne and also having edited a, highly recommended, book on the man. Herschel fans might thus feel justified in dismissing her comment and maintain their position than it was the Hanoverian musician turned amateur astronomer who discovered the first new planet to be observed since antiquity. Rather than trying to stoke the fires of a discovery priority dispute, of which there are all too many in the history of science, I think this an opportunity to look critically at what the term discovery actually means in the history of science.

For some reason we love to hang a specific date, even better the exact time, when a discovery of science was made in the history of science. In fact I have about a running metre of books within arms reach of this computer full of such information. William Herschel discovered Uranus on 13 March 1781, Galileo Galilei discovered the moons of Jupiter on 7 January 1610, Simon Marius did the same just one day later, Johannes Kepler discovered his third law of planetary motion on 8 March 1618 and so on and so forth. However this accurate pinning of scientific or technological discoveries onto the ribbon of time creates a very false impression of what discovery is and this was exactly the point that Becky was trying to make on Twitter, which in turn led to me writing this post. Discovery is not a single act by a single person for which it is possible to give a stopwatch accurate moment of discovery but is rather a process spread over a period of time, which can in fact take several years and which almost always involves quite a large number of people.

To illustrate what this means let us take a closer look at Galileo’s epoch making discovery of the four largest (actually it was only three on the first day) moons of Jupiter. On 7 January 1610 whilst observing the planet Jupiter Galileo noted three stars that roughly formed a line with the middle axis or equator of the planet. When he observed again on the following evening they were still there. You might ask so what? Stars belong to the sphere of fixed stars, which are so called because they ‘always’ remain in the same place, whereas planets are called planets (the Greek for wanderer) because they move around with reference to the fixed stars. This being the case Galileo’s three new stars that he had recorded should have changed their position relative to Jupiter, or more accurately Jupiter should have changed its position relative to the three stars. Galileo was astute enough to realise that he was on to something and continued to observe and record the now four new stars and Jupiter over the following nights. The new stars did change their positions relative to Jupiter but not in the way he would have expected if they were fixed stars plus they always stayed in the vicinity of the planet. With time and enough observations Galileo realised that the four new objects were in fact orbiting Jupiter. He had discovered Jupiter’s four largest moons, or had he?

Science requires that new discoveries can be repeated by other independent practitioners/observers and discoveries are only confirmed and thus accepted when this has taken place. Now as stated above Simon Marius in Ansbach had also first observed the moons of Jupiter just one day later on 8 January 1610 and like Galileo had continued to observe them and had also reached the conclusion that they were orbiting the planet. This would have been the necessary confirmation that Galileo required but Marius only published his observations four years later, in 1614, leading Galileo, who by this time had long been acknowledged as the discoverer to denounce Marius as a plagiarist. Back in 1610 when Galileo fist published his observations on 14 March, in his Sidereus Nuncius, people were, not surprisingly, rather sceptical about his claims.

As I have recorded on several occasions on this blog it was the Jesuit mathematician astronomers under Christoph Clavius at the Collegio Romano who provide the necessary independent confirmation of his observations but this was not a simple process. At first the Jesuits did not have a telescope powerful enough to resolve the moons of Jupiter and their initial attempts to construct one failed. However Grienberger and Lembo persevered with assistance from Galileo, from afar by post, and in the end they were able to confirm all of Galileo’s observations. Another aspect of this discovery was to prove that they were actually moons orbiting Jupiter the four new objects needed to be observed consistently and accurately in order to determine their orbits so that one could predict their positions at any given time. Both Galileo and Marius undertook this task, Marius’ results were more accurate than those of his Tuscan rival, but it was first Cassini several decades later who, with much superior telescopes at his disposal, was able to produce tables of the orbits accurate enough to truly satisfy the requirements of the astronomical community.

It would now seem that we are finished with our tale of the discovery of the four moons of Jupiter but there is another extremely important factor that needs to be addressed. New discoveries often involve new methods and/or new scientific instruments, without which the discovery would not have been possible. This was very much the case with the discovery of the moons of Jupiter, which was only made possible by the very recently invented, September 1608, telescope. Any such new methodology or instrumentation must be clearly and convincingly shown to provide objective verifiable facts based on solid scientific theory. No such demonstration of objective scientific reliability existed at this point in time for the telescope. In fact all those in 1610, who doubted the telescopes ability to deliver objective verifiable scientific facts, and who tend to get ridiculed by the cheerleaders of scientism today, were perfectly correct to do so. Galileo, who when it came to optics was a tinkerer rather than a theorist, was not in the position to deliver the very necessary scientific theory of the telescope. Enter Johannes Kepler.

Kepler had already ready written extensively on theoretical optics including one of the earliest scientific analysis of how lenses functions. He was also an unabashed cheerleader for Galileo’s telescopic discoveries, sight unseen, writing the first positive, rather gushing in fact, review of Sidereus Nuncius, which Galileo used for his own propaganda purposes. Kepler realised at once that in order to confirm those discoveries a theoretical description of how the telescope functions was necessary and he sat down and wrote one. His Dioptrice, which explains the science of single lenses, the convex/concave two lens Dutch telescope used by Galileo, the convex/convex two lens astronomical or Keplerian telescope, the three lens terrestrial telescope and even the telephoto lens, was published in 1611. Galileo, arrogant and egoistical as ever, dismissed it as unreadable but it successfully silenced those who doubted the scientific objectivity of the telescope.

All of the factors that I have described above played an important and indispensible part in the discovery of the four largest moons of Jupiter. What we have here is not the act of one person at a specific point in time, in this case Galileo’s first observation of those three stars, but a chain of intertwined events or a process spread over a period of several years. There is nothing exceptional in the discovery of the moons of Jupiter but all scientific and technological discoveries involve a similar complex process carried out by a group of people over a period of time. Discovery is not the single act of a single person but a process involving several and sometimes many people spread over a period of time. The anniversaries that we like to celebrate are mostly just the starting point to that process.