Long before telescopes, Greek scientists such as Aristotle (384–322 B.C.) and Ptolemy (A.D. 100–165) argued that the Sun traveled around Earth.
This seemed so obvious to most people that few questioned it. Then, in 1543, Polish astronomer Nicolaus Copernicus proposed the theory that Earth actually traveled around the Sun. This set off a decades-long effort to find out if Copernicus was right or wrong. The telescope became key to finding the answer. Telescopes have since become more sophisticated, expanding our knowledge beyond Copernicus’s dreams.

1609
◀ Italian scientist Galileo Galilei hears about the new Dutch instrument for seeing far—so he builds one and points it skyward. He quickly finds evidence that Copernicus was right. For instance, champions of an Earth-centered universe argue that no other planet has a moon. This, they say, shows that Earth is unique. But looking through a telescope, Galileo finds four moons orbiting Jupiter (today we know it has many more!). His discoveries make telescopes popular throughout Europe but also upset popular religious beliefs. In 1633, the Roman Catholic Church forces him to reject his own scientific conclusions.
1663
Scottish astronomer James Gregory (right) assembles a reflecting telescope. The device uses mirrors instead of lenses. Five years later, Isaac Newton improves the design, creating the Newtonian telescope. ▶


1773
◀ English mathematician Chester Moor Hall makes refracting telescopes better. He places two different types of glass together to make a lens that prevents color separation. This eliminates the rainbow ring around images.
1781
William Herschel, an astronomer in England, discovers the planet Uranus using his own telescope. He soon builds the world’s first giant reflector, with a mirror 48 inches wide. His sister Caroline discovers eight comets while using it. ▶


1814
◀ German lens maker Joseph von Fraunhofer notices something while looking through a telescope. He sees dark lines within the spectrum of visible light. Later, scientists realize these lines allow them to determine the temperature and chemical makeup of stars.
1835
German chemist Baron Justus von Liebig makes larger reflecting telescopes possible by inventing the modern mirror. It is a piece of glass with a thin silver coating. Before this, mirrors were made of shiny metal and were very heavy. ▶


1845
◀ William Parsons, the Third Earl of Rosse, builds a reflecting telescope on his estate in Ireland. His telescope has a 72-inch mirror. It remains the world’s biggest reflecting telescope until the 100-inch Hooker telescope is built near Los Angeles in 1917.
1889
George Ellery Hale invents the spectroheliograph. This instrument reveals details of the Sun’s surface. He later builds some of the world’s biggest telescopes, including the largest refractor in the world, the Hale Telescope, on Palomar Mountain in California. ▶


1932
◀ Bell Labs engineer Karl Jansky is asked to track down whatever’s causing static interference that’s blocking radio signals. When he finds it is coming from space, he accidentally invents radio astronomy.
1957
The Soviet Union (now Russia) launches Sputnik, the first human-made satellite, into space. This leads to satellites and probes carrying telescopes. In 1959, Russia’s Luna 3 gives scientists the first look at the far side of the Moon. ▶


1960s
◀ Breakthroughs happen in detecting infrared, ultraviolet, X-rays, and other invisible types of radiation. These developments create new ways for astronomers to study the universe. Pictured here is Earth shown in ultraviolet.
1980s
Mirrors larger than 15 feet wide sag under their own weight, distorting an image. Engineers find two ways to solve this. They use a honeycomb structure to support the mirror, or they make one mirror out of many smaller mirrors. ▶


1990s
◀ NASA launches the Hubble Space Telescope, one of the largest observatories in space, named for astronomer Edwin Hubble (left). NASA also launches the Chandra X-Ray Observatory.
2000s
The Spitzer Space Telescope (right), used to detect infrared light, is launched. The Swift telescope is also sent into orbit, to scan the skies for mysterious, ultrabright flashes known as gamma-ray bursts. These bursts are the most powerful explosions in the universe. ▶


2010s
◀ The Hisaki ultraviolet telescope is launched. Its mission is to study the atmospheres of Venus, Mars, and Jupiter’s moon Io. The Giai observatory (left) is also launched. Its mission is to map up to a billion stars in the Milky Way galaxy.
2020s
The James Webb Space Telescope is launched. Its images provide the deepest and sharpest pictures of the universe ever seen. These pictures include light from one galaxy that traveled 13.1 billion years before it was observed. ▶
