Ground-Based Optical Observatories From 1667 to the Present

Opened in 1667 under Louis XIV, this is the oldest continuously operating astronomical observatory in the world and still an active French research institution. It served the problem of finding longitude by timing eclipses of Jupiter's satellites, and its building is the origin of the Paris meridian, once a rival to Greenwich.

Founded in 1675 on the orders of Charles II to improve navigation by mapping the stars, it measured star positions with a transit circle that timed each star's passage across the meridian. Its hill now defines zero degrees longitude and Greenwich Mean Time, and the original Flamsteed House survives as a museum.

Opened in 1839 near Saint Petersburg, it became known for exceptionally precise stellar position measurements, its principal work being the coordinates of stars and constants such as precession, nutation, aberration and refraction. It was heavily damaged in the Second World War and rebuilt after 1945.

Opened in 1888 on Mount Hamilton in California as the first permanently occupied mountain-top observatory, paid for by a bequest from James Lick, who is buried beneath the main telescope pier. Its 36-inch refractor was the largest in the world when completed, and the site now studies exoplanets and supernovae.

Founded in Flagstaff, Arizona in 1894 by Percival Lowell, who used it to study Mars and to search for a ninth planet. That search ended in Clyde Tombaugh's 1930 discovery of Pluto with the observatory's 13-inch astrograph.

Opened in 1897 in Williams Bay, Wisconsin, under the University of Chicago, it houses the 40-inch Yerkes refractor, the largest refracting telescope ever put to regular scientific use. It is often called the birthplace of modern astrophysics, and its telescopes were built beside laboratory space for physics and chemistry.

Begun in 1904 in the San Gabriel Mountains, its 100-inch Hooker Telescope was the largest in the world from 1917 to 1949. Here Edwin Hubble identified galaxies beyond the Milky Way and in 1929 published the distance and redshift relation that revealed the expanding universe.

Opened in 1939 on Mount Locke in the Davis Mountains of West Texas, funded by a bequest from the banker William Johnson McDonald and run by the University of Texas. Its dark skies and the Hobby-Eberly Telescope have made it a major site for spectroscopic surveys.

Operating since 1949 in the Palomar Mountains and run by Caltech, it entered the large-telescope era with its 200-inch Hale telescope. Its 48-inch Samuel Oschin Telescope survey later found the dwarf planet Eris, a discovery that helped prompt the 2006 redefinition of what counts as a planet.

Established in 1958 on land leased from the Tohono O'odham Nation southwest of Tucson, it is the United States national optical observatory and holds the largest collection of optical telescopes on a single mountain in the continental United States. Its Mayall 4-meter telescope was long among the largest in the country.

Its two telescopes near the summit of Mauna Kea saw first light in 1990 as the largest optical reflecting telescopes in the world. Adaptive-optics imaging of stars orbiting Sagittarius A*, led by Andrea Ghez over more than 25 years, helped prove the Milky Way has a central supermassive black hole and shared the 2020 Nobel Prize in Physics.

Operating since 1998 on Cerro Paranal in Chile, this European Southern Observatory facility observes both visible and infrared light. In 2018 its instruments tracked the star S2 past the Milky Way's central black hole and confirmed Schwarzschild precession, a signature of general relativity.