The first astronomical light detector—and the only one for the greater part of human history—is the most complex, mysterious, and beautiful of them all: the human eye. No manmade product of our technological age can equal the eye's remarkable combination of properties. It is a sensitive, high-resolution, full-color detector, with autofocus and aperture control, superb dynamic range, and a direct connection to the brain—an image processor next to which our most powerful computers look like toys.

The eye served astronomers for centuries, enabling them to record the brightnesses, colors, positions, and motions of the sun, moon, stars, and planets. Yet for all its remarkable qualities, the eye is only able to record a tiny fraction of the universe. Its shortcomings as an astronomical detector lie in its physiology. The small size of a human pupil—only 7 or 8 millimeters in diameter when fully dilated—limits the amount of light that can enter the eye, hampering its performance at the low light levels common in astronomical work. The eye's refresh rate—like the frame rate of a movie camera—is necessarily rapid. Were it not, images would be blurred and smeared by any motion. This mechanism is essential for our survival, but limits the brightness of an image formed by eye to the quantity of light that can be gathered in a fraction of a second.

Despite these obstacles, astronomers succeeded in learning much about the positions and motions of celestial bodies with their naked eyes, aided by increasingly sophisticated pre-telescopic instruments, but by the end of the sixteenth century, naked eye observation had been pushed to its technical and physiological limits. Learn more.

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