And soon, sure enough, Galileo went to work making bigger and better telescopes. Large light-gathering lenses were not yet available, so he concentrated on making longer telescopes, which produced higher magnifying powers and reduced the halos of spurious colors that afflicted glass lenses in those days. Subsequent observers took the design of glass-lensed, refracting telescopes to great lengths, sometimes literally so. In Danzig, Johannes Hevelius deployed a telescope 150 feet long; hung by ropes from a pole, it undulated in the slightest breeze. In the Netherlands, the Huygens brothers unveiled lanky telescopes that had no tubes at all: The objective lens was perched on a high platform in a field, while an observer up to 200 feet away aligned a magnifying eyepiece and peered through it. Such instruments proffered fleeting glimpses of planets and stars that, like the dance of the seven veils, only aroused a burning desire to see more.

The reflecting telescope, pioneered by Isaac Newton, made it practical to gratify such desires: Mirrors required that only one surface be ground to gather and reflect starlight to a focal point, and since the mirror was supported from behind, it could be quite large without sagging under its own weight, as large lenses tended to do. William Herschel discovered the planet Uranus with a handmade reflecting telescope—he cast his metal mirrors in his garden and basement, and once had to flee from a coursing river of molten metal after the horse-dung mold fractured. Spiral-armed galaxies were first glimpsed through a massive reflecting telescope, with a six-foot diameter primary mirror, that Lord Rosse constructed on his estate in Ireland.

Today's largest telescopes have mirrors up to some ten meters (33 feet) in diameter, with quadruple the light-gathering power of the legendary five-meter Hale Telescope at Palomar Observatory in southern California. Looming large as office buildings, some of these giants are so highly automated that they can dust off their optics at sundown, open the dome, sequence and carry out observations throughout the night, and shut down come threatening weather, all with little or no human intervention. Yet humans, being human, still intervene a lot, if only to make sure nothing goes awry: To lose just one night's work at a big telescope these days is to squander as much as $100,000 in operating costs.

Three of today's largest telescopes—Gemini North, Subaru, and Keck—stand within hailing distance of one another atop the nearly 14,000-foot peak of Hawaii's Mauna Kea, an inactive volcano. The altitude puts them above 40 percent of Earth's atmosphere—and most of its water vapor, which is opaque to the infrared wavelengths the astronomers like to study—but also makes it difficult for the astronomers and engineers who work there to breathe and think. Many wear clear-plastic oxygen tubes in their nostrils as routinely as we might wear eyeglasses. Others rely on the body's ability to adapt but worry about making what they call a CLM, or "career-limiting mistake." "At altitude, we don't improvise; that would be a disaster," says Gemini astronomer Scott Fisher. "We're kind of trained monkeys up here. The real thinking goes on at sea level."