How fast does light travel? At first it seemed that the speed of light could not be measured. An Italian scientist, Galileo, made an attempt about 1630, but all he could prove was that light travelled very fast. In 1748 an English astronomer, James Bradley, calculated the speed of light from the direction in which light fell from a star to Earth. He came up with a figure of 176,000 miles per second. In 1849 another scientist, Fizeau, improved on Galileo’s method and came up with a new figure—196,000 miles per second. In 1927 Albert Michaelson changed the figure for the speed of light. He said it was 186,295 miles per second. In 1972 the official speed of light was said to be 186,282.3959 miles per second.

Using this figure, scientists learned that light waves can travel from the Earth to the Moon in 1¼ seconds. They learned that it would take 100,000 years to travel the length of the Milky Way.

Then Albert Einstein came along and the speed of light became a universal speed limit that no one could break. In his usual clear style, Asimov guides the reader to an understanding of why the speed of light is necessary to our knowledge of the Universe.

The first three chapters of this book are a fairly standard (for Asimov) history of the study of the speed of light, such as can be found in Understanding Physics or Asimov’s New Guide to Science. The last two chapters discuss some implications of the speed of light—how it’s used to measure astronomic distances, and how Einstein’s theory of relativity makes it an absolute speed limit for travel. Again, these are topics which Asimov covers at length in other books.

The result, then, is an excellent little book for children to learn some basic facts of science which I would highly recommend. Adults, however, would be just as well off to get this same information less copiously illustrated from one of Asimov’s other books.