I often grouse here about the difficulty of finding good books about faith and science for younger readers. The weak spots of such books often seem to lie in the area of history of science. This week I want to highlight another book for younger readers, one that is just about science and history (no faith aspect). It is not a bad book, but it also has trouble with history.
The book is Finding the Speed of Light: The 1676 Discovery that Dazzled the World, by Mark Weston, illustrated by Rebecca Evans, published in 2019 by Tilbury House Publishers. When I saw this book on display in a branch of my local library, I was amazed! A book about Ole Romer, who discovered that light has a speed! How cool! And how surprising—most people have never heard of Romer (here I am using the spelling of his name used in the book—there are other versions; Wikipedia, for example, has the spelling as “Rømer”).
Romer discovered light’s speed by studying the moons of Jupiter. In the seventeenth century those moons, which either Galileo or Simon Marius first discovered in early 1610, were the subject of careful study, because they could serve as a sort of world-wide clock at a time when clock technology had not advanced to the point where a clock could be moved from one place to another and still keep time. No clock could keep accurate time when carried aboard ship or carriage. This made it very difficult to synchronize clocks between one place and another. If you want to determine your position on the Earth using the stars, you need to know what the time is at your location, because of course the stars that are visible are continually wheeling overhead, changing with the time of night. Not knowing time at different locations created all sorts of problems for astronomy, surveying, map-making, and navigation. In the case of navigation, this was a matter of life and death and economics, for miscalculation of position could and did result in terrible shipwrecks. Jupiter’s moons, wheeling regularly about Jupiter, could serve as a universal clock, visible from all points on Earth.
Bernard le Bouyer de Fontenelle (1657–1757) wrote in 1740:
Were there no other use of astronomy than that drawn from Jupiter’s satellites, it would justify well enough these huge calculations, these diligent and scrupulous observations, this large ensemble of instruments built with so much care; [and] this superb building [the Paris Observatory] raised for our science.
Careful study of these moons—diligent and scrupulous observations carried out to know exactly how the moons moved—showed that the speed at which they seemed to move around Jupiter varied slightly. The variation was connected to the distance between Jupiter and Earth. Romer figured out that the motion of the moons actually did not vary. Rather, the variation was caused by the fact that light has a finite speed, and takes time to cross the distance between Jupiter and the Earth. The resulting delay—greater when the distance between Earth and Jupiter was greater, lesser when the distance was lesser—caused the apparent variations in the speeds of the moons. As Fontenelle put it:
The observations of Jupiter’s satellites made by the Academy from 1670 to 1675 led to the discovery in their motion of an inequality not previously known…. M. Cassini and M. Roëmer, then a member of the Academy, after scrutinizing this anomaly, found that it depended of the distance of Jupiter from the Earth…. A very ingenious conjecture on the cause of this inequality first came to the mind of the two astronomers. They imagined that the motion of light was not instantaneous as all previous philosophers believed, but that it took some time to spread….
And as Fontenelle notes, prior to this, people thought that light travelled instantly.
Finding the Speed gets the story outline right. Romer observes Jupiter. He finds that the speeds of Jupiter’s moons vary. He notes that the variation occurs in roughly six-month cycles. He figures out that the variation is caused by the speed of light. The illustrations are very nice.
Nevertheless, there is a really big boo-boo in the book: Weston pays no heed to the timekeeping question at all. Timekeeping is the reason people were studying Jupiter’s moons in the first place. Weston just says that “Ole particularly liked to watch Jupiter and its moons…. [H]e loved to look through his telescope and see Jupiter’s brown and white stripes…. Ole liked trying to predict the exact minute when [Jupiter’s moon] Io would pop out from behind Jupiter”. From reading this book, you would think that Romer just liked to watch Io go around Jupiter.
By contrast, Cassini in 1692 wrote that
It is not by curiosity alone that the most famous astronomers of the present century have observed with so much care the planet Jupiter; they mainly did it in order to obtain an exact knowledge of longitudes, on which the perfection of geography and navigation depends.
In the book, Ole finds that “sometimes Io seemed to slow down and take more time to orbit Jupiter…. [or] speed up and circle the planet more quickly”. He is puzzled by this. Trying to understand it, he cleans his telescope (?) and takes apart his clock. Eventually he hits on the light speed and the distance between Earth and the Jupiter varying over the course of a year. Still the book makes no mention of timekeeping and navigation.
To be fair, Finding the Speed does not aim to be a serious portrayal of history. In it, astronomers find that the ruler they built that measures 40 million miles in length is insufficient to reach the sun; a family drives a car through space. There are a lot of goofy jokes about ice cream, which had become popular in Europe during Romer’s youth. Also, the author Weston includes illustrated stories about the “childhood” of Romer, but on the last page he notes “the autobiography of Danish philosopher Soren Kierkegaard, which proved invaluable when writing about Romer’s childhood (about which we know nothing)”—apparently Weston used Kierkegaard’s childhood as a model for Romer’s (even though Kierkegaard was born in 1813, and Romer in 1644).
Still, the first page of the book features a couple of sailing ships! Finding the Speed could have easily worked in the timekeeping and navigation story, avoided that big boo-boo, and given its readers a much better idea of how science works and why an astronomer in the late seventeenth century might have been obsessed with Jupiter’s moons. But this just goes to show that the history problems in books for younger readers are not just confined to stuff involving religion.