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For most of human history, everything people knew about the universe came from what they could see with the naked eye. That changed in 1609, when the Italian astronomer Galileo Galilei pointed a small telescope at the night sky. His instrument magnified objects only about twenty times, yet it revealed mountains on the Moon, four moons circling Jupiter, and countless stars invisible to unaided vision. The telescope did not merely extend human sight; it transformed humanity's understanding of its place in the cosmos.
Galileo's telescope was a refractor, using glass lenses to bend and focus light. Refractors suffer from a flaw called chromatic aberration, in which different colours of light focus at slightly different points, producing blurry, rainbow-edged images. Isaac Newton offered a solution in 1668 with the reflecting telescope, which gathers light with a curved mirror instead of a lens. Because mirrors reflect all colours identically, reflectors avoid chromatic aberration entirely, and they can be built far larger than lenses, which sag under their own weight. Nearly every major research telescope built since has been a reflector.
The twentieth century became an age of giant mirrors. The Hooker telescope at Mount Wilson Observatory in California, with a mirror 100 inches (2.5 metres) across, began operating in 1917; within a decade, Edwin Hubble had used it to show that faint spiral smudges were in fact entire galaxies beyond our own, and that the universe is expanding. In 1948 the 200-inch Hale telescope opened at Palomar Mountain, and it remained the world's largest effective telescope for more than four decades.
Size alone, however, is not enough; location matters enormously. Astronomers build observatories on remote mountaintops for several reasons. High altitude places the telescope above much of the atmosphere, which absorbs and distorts starlight. Dry climates, such as the Atacama Desert in Chile or the summit of Mauna Kea in Hawaii, which rises about 4,200 metres above the Pacific, offer clear skies for most of the year. Distance from cities limits light pollution, the artificial glow that washes out faint objects.
Even from the best sites, turbulence in the air makes stars twinkle, blurring fine detail. Modern observatories counter this with adaptive optics, a technology that flexes a thin deformable mirror hundreds of times per second to cancel out atmospheric distortion. Because there is not always a bright natural star near the target to serve as a reference, many telescopes fire a laser into the sky to create an artificial guide star, allowing computers to measure and correct the shimmering in real time.
The ultimate escape from the atmosphere is to leave it altogether. The Hubble Space Telescope, launched in 1990, carries a mirror only 2.4 metres across, modest by ground-based standards, yet its position above the atmosphere gives it exquisitely sharp vision. After astronauts repaired a flaw in its optics in 1993, Hubble produced some of the most celebrated images in astronomy, from the towering dust columns of the Eagle Nebula to the Hubble Deep Field, which revealed thousands of galaxies in a patch of sky no larger than a grain of sand held at arm's length.
Hubble's successor, the James Webb Space Telescope, was launched in December 2021. Its primary mirror, 6.5 metres wide and made of gold-coated beryllium segments, had to fold up to fit inside its rocket and then unfold in space. Webb observes in infrared light, which lets it peer through dust clouds and detect light from the earliest galaxies, stretched by the expansion of the universe into wavelengths our eyes cannot see. It operates about 1.5 million kilometres from Earth, farther than any human has ever travelled.
The next generation of ground-based giants is already under construction. The Extremely Large Telescope, now being assembled on a flattened mountaintop in Chile, will gather light with a mirror 39 metres in diameter, collecting more light than all existing professional telescopes combined and hunting for signs of life on planets around other stars. Four centuries after Galileo, the basic bargain remains unchanged: larger and better-placed telescopes keep turning faint points of light into answers about how the universe began, how it works, and whether we are alone in it.