Making the far near

The invention that set the stage for the telescope was the eyeglass, which appeared in the mid-13th century. The first spectacles, designed to correct farsightedness, bore glass magnifiers that were biconvex, meaning they curved outward on both sides. (Each resembled a lentil, or lens in Latin.) Mostly used to help older people read, these lenses focused on objects between 12 and 20 inches away from the eye.



Nearsightedness, a more common affliction, proved more difficult to correct. It required biconcave lenses—those curving inward on each surface—that had to bring objects into focus at the specific distance at which one's eyesight failed. The poorer one's vision, the greater the distance the lenses needed to provide focus.



In 1608, someone in Europe—it's not clear who—figured out that if you placed a lens for the farsighted about 12 to 14 inches away from a lens for the nearsighted, and then peered through the latter lens, distant objects would miraculously appear as if close by. (Oh, to have seen that pioneer's expression upon first realizing this!) Place those lenses in a tube and voilà, you have a spyglass.



Within months, Galileo had not only learned of the new device but was well on his way to improving its design. In his workshop in Padua, Italy, he discovered that plano-convex and plano-concave lenses worked best—that is, lenses with a plane on one side and curved surfaces on the other. Then, drawing on his skills as a professor of mathematics at the University of Padua, he determined the mathematical relationship that governed the instrument's ability to magnify. A spyglass with a plano-convex lens that focuses at 12 inches and a plano-concave lens that focuses at four inches, he found, magnifies images three times (12 divided by four). Galileo played with this formula until, by the late fall of 1609, he'd made a spyglass that could magnify what is seen by 20 times. No other spyglass maker could match that.



That fall, Galileo also did what apparently no one else had ever done with a spyglass before: train the instrument on the heavens. In short order he began making astronomical findings that would shake our understanding of our place in the universe to its foundations. Among them was the discovery of four moons orbiting Jupiter. To Galileo, the moons proved that not everything in space circled the Earth, and therefore our planet was not the absolute center of the universe, as the Church maintained the Bible had it.

Source: http://www.pbs.org/wgbh/nova/galileo/telescope.html