Astronomy. What's the first thing you think of when you read that word? The many beautiful vistas returned from Hubble? A family in a backyard huddled around a small telescope? Giant research observatories such as the Keck? Whatever may come to mind, it usually involves a telescope. But the very nature of telescopes has changed over the centuries, with different arrangements of lenses dominating for decades before succumbing to some combination of basic physics or technical limitations. We'll (somewhat belatedly) celebrate the 400th anniversary of Galileo's telescope by taking you on a quick tour of four centuries of progress and frustration.

The birth of the lens

As early as the thirteenth century, artisans in Venice and Florence were producing seeing glasses for elderly presbyopic (long-sightedness caused by age) scholars. Magnifying glasses were common among universities and monasteries at the dawn of the fourteenth century, where they became a symbol of wisdom and respect. These glasses were held against the eye, where, due to their small size and rounded shape, they were named "lentils of glass" or, in the latin, "lenses."

The glass wasn't good by modern standards, having inclusions of air bubbles and a notably greenish tint from iron. By 1350, when we see them appear in illustrations, the two-lensed spectacle had been invented in Venice and rapidly became a sign of education and wisdom among the scholars and noble elite of the day.

The fifteenth century saw the convex lens' opposite appear: the concave lens for myopia, a vision impairment of the young. We have documentation of these being available as early as 1450. The problem solved, nobody seemed to hold any further interest in optics until Thomas Digges, the first astronomer to attempt a measurement of parallax.

Digges (or his son Leonard, or both—it isn't clear) used a convex lens and a concave mirror to magnify distant objects, but the shortcomings of such an arrangement were obvious: the observer was in the way of the mirror. His device was forgotten, and Digges was best remembered for his popularization of the heliocentric system.

In 1608, Hans Lipperhey in the Netherlands applied for a patent on a pair of lenses, one with a much shorter focal length than the other, arranged in a tube. He called it a "spyglass" as it allowed the observation of greatly distant events from a secluded retreat—Lipperhey noted that counting coins from afar was a suitable use. The patent was denied because the device was so very easily constructed. Lipperhey's design was biconcave, two concave lenses arranged so as to compress incoming light into a smaller area, where it would appear magnified.

By early 1609, small spyglasses of two to three powers were being produced by the spectacle makers of Paris and Venice. By mid-1609, they were dotted around France and Italy. In August 1609, Thomas Harriot is known to have turned such a spyglass on the Moon, but with only three powers, he was unable to correctly discern craters and mountains. Such a feat would require at least five to six powers.

The spyglass meets astronomy

We need to go back further, though, to see why the telescope was even necessary for astronomy, as obvious as it may seem to the modern reader. Tycho Brahe and his contemporaries didn't have telescopes, but they did have similar devices to enhance the precision of the eye. A wall quadrant such as the one Brahe invented (though 'built' may be the better word) could be used to derive the position of a celestial object with great accuracy using no more than one's naked eye—greater accuracy than a 5 power telescope could manage alone.

The positions that allowed Kepler to nearly derive Newton's law of universal gravitation were taken using such a device. Even "almost derive" was good enough; Kepler's Laws (strictly a special case of Newton's Universal Gravitation where one mass is irrelevant) were enough to give him a place in the history books, even though he never had the benefit of a telescope.

It was this wall quadrants device that gave birth to the astronomical telescope. One does not compare the car to the industrial steam engine, even though they share great commonality—one instead compares it to the horse and coach. Astronomy's horse and coach were the quadrants, its car was the telescope; they may be entirely unrelated devices in operation, but they were put to use for the same purpose.

The big breakthrough came when Galileo was informed of Lipperhey's failure to secure a patent. He was certainly aware of the Venetian prowess in lens grinding, as well as work in optics that Kepler had done. Galileo decided to make such a device for himself, inspired by a mixture of Renaissance gung-ho and a desire to make his name. Presumably, he reasoned that a device able to magnify distant objects would also minimize the uncertainty in their position, providing an improved version of the wall quadrant.

Galilean telescope

What his telescope gave him was nothing short of astonishing: far from simple positions, Galileo discovered mountains on the moon, librations that proved the Moon was spherical and not a flat disc, moons of Jupiter that flew in the face of contemporary Catholic thought, and a view of the Milky Way that indicated it wasn't an "atmospheric phenomena," but rather was made of countless faint stars. The unquestionably true dogma of many Christians was now unquestionably false. Galileo began to question the knowledge of Ptolemy, Plato, Pythagoras and Aristotle, setting into motion the golden age of astronomy and its ascendance to being a true science.