Except for medicine, astronomy is the most ancient of the sciences. The night sky, undimmed by electric lighting and thus blazing with at least six thousand visible stars, provided our earliest ancestors with an endless pageant of meteors, comets, eclipses, and recurring patterns to be deciphered.

The history of Western astronomy can be subdivided into four distinct periods. The first, which ran from time immemorial until 1609, when the telescope was invented, was the era of naked-eye astronomy. Despite having only their own vision with which to explore the universe and an incorrect, geocentric paradigm of the solar system, ancient astronomers accumulated an astonishing amount of knowledge. Solar and lunar eclipses, for instance, could be accurately predicted as early as the fifth century B.C. The Gregorian calendar, promulgated in A.D. 1581, is so nearly accurate that it won’t be a day off astronomical reality until the year 4901.

The invention of the telescope hugely expanded the power of the human eye to see farther. For instance, Galileo found the four large moons of Jupiter, known ever since as the Galilean moons, almost as soon as he aimed his telescope at the sky. This era of direct observation, during which the size of the solar system was determined and the planets Uranus and Neptune were discovered, as well as numerous planetary satellites, lasted until the 1890s.

Then the invention of dry plates and the rapidly increasing speed of photographic emulsions allowed hours-long exposures to reveal vast