JF Ptak Science Books Post 1730
“When the father and creator saw the creature which he has made moving and living…, he rejoiced, and in his joy determined to...make the universe eternal, so far as might be. …Wherefore he resolved to have a moving image of eternity, and when he set in order the heaven, he made this image eternal but moving according to number, while eternity itself rests in unity; and this image we call time… Such was the mind and thought of God in the creation of time. The sun and moon and five other stars, which are called the planets, were created by him in order to distinguish and preserve the numbers of time… And for this reason the fixed stars were created, to be divine and eternal animals, ever-abiding and\ revolving after the same manner and on the same spot…” Plato, in which he wrote about the formation of the universe, among many other things, (Jowett. v. 2. Timaeus, p. 19), and spotted at the Linda Hall [Science] Library here
Well, not exactly, but this was some pretty good thinking by Plato for his time, and beyond that--actually, the thinking was held for centuries. But it was some very nice thinking by (Danish) Ole Rømer that pieced this part of Plato out, turning it around, and coming to terms with the use of "the order of heaven" to "preserve the numbers" of the speed of light, all through the observation of eclipses of Jupiter's moon Io.
I should first say that today's post came about via Peter Horrebow's (1679-1764) Operum Mathematico-Physicorum... , a three-volume work published in Copenhagen in 1740-41. Horrebow was a very accomplished astronomer in his own right (we'll get to that in a moment), but what is of interest for me right now is the third volume of his book, as it (the Basis Astronomiae...) contains very detailed descriptions of the astronomical instruments and observatory of his fellow Danish astronomer Rømer (1644-1710). Horrebow must have been a very gifted machinist and man-about-the-"lab", as he was able to make his way through the educational system and then to the highest levels of academia even though beginning his life as a fish-seller's son--an extraordinary accomplishment, really, as there were not many opportunities for people without some sort of privilege to succeed on this level. And succeed Horrebow did, serving as the great Rømer 's assistant and charge, living in the man's house for some time. (He didn't stay for very long, as Horrebow was a father of twenty.)
There are some famous illustrations in this book--not the least of which is the depiction of Rømer at work on a transit in his observatory--but the image I like most of all is not one of the instruments, but a beautiful engraving of the observatory built by Charles IV. The massive structure--called the Rundetam--was begun in 1637and finished in 1642; it is as its name suggests a "round tower" that rises 34 meters; it has no stairs using a 7.5-turn walkway instead. (It is a confusing thing, walking 'round to go up, and seems to me to have been much more than just seven turns; in a weird way by the top of the walk it felt as though one was going "down" somehow. Maybe I just got dizzy.) In any event the tower would be an observatory, with Rømer (and then, later, Horrebow) working from the roof as well as from some high windows, and was meant to replace the great Tycho Brahe's demolished Stjeneborg
Rømer's career was remarkable, but what I find particularly beautiful was his determination of the speed of light, and all from basically sitting there with his instruments in a window of a massive stone structure in the middle of a city, figuring out minute differentiations, making detailed observations of eclipses of the moons of a planet that had only been observed telescopically less than 67 years earlier (with the newly-invented instrument, the telescope).
The moons of Jupiter had been observed in 1666-1668 by Cassini and were a subject of intense study by him and Rømer, among others; Rømer in fact would travel to the Paris observatory and be an assistant to Cassini as the two worked together on the moons and eclipse observations. In this Rømer noticed a particular peculiarity in the changes of the times of these eclipses, becoming shorter when the Earth was closer to Jupiter and longer when farther away. From this Rømer concluded that light was taking different times to reach the Earth, and from their calculated its speed. He reported to the Royal Academy of Science on 22 August 1676:
This second inequality appears to be due to light taking some time to reach us from the satellite; light seems to take about ten to eleven minutes [to cross] a distance equal to the half-diameter of the terrestrial orbit.
What a jewel that was to give to the world! It is difficult to imagine the impact of this sort of announcement in the first three-quarters of a century of the telescope, to be able to calculate something as elusive as the idea of the speed of light, and that coming from the observation of moons of another planet. This was the first true quantitative measurement of the speed of light--there were earlier attempts (by Isaac Beekman and Galileo), but while their ideas for measuring were interesting and theoretically workable the instrumentation for recording minute differences in light flashes were not. (For example in the Galileo experiment (measuring the differences in the light of exposed lanterns a mile apart, the time was not measurable. If it could have been measured the answer would've been 10 microseconds; at that time the idea of the "second hand" a clock had not yet come to be.) Rømer established that the speed of light was finite, and gave a value that was within about 25% of the modern standard, or at least 220,000 km/sec. Galileo's earlier best estimate was that light was at least ten times that the speed of sound. (HE was right, of course, light is at least 10x sound, but the capacity for more accurate observation and instrumentation were just beyond his age.) This was a truly remarkable accomplishment.
The observatory had a relatively short shelf-life, what with light pollution and the rumble and vibration of city traffic doing it in by the mid-18th century; so for its size and heavy fullness, the building lasted a little more than a century as a useful observatory.