Prior to the late 19th century, almost all astronomical observatories throughout history were located at modest elevations, often close to cities and educational institutions for the simple reason of convenience. The British physicist and mathematician
Isaac Newton is credited with the realization that high-altitude sites are superior for observation because they provide the "most serene and quiet Air" above the dense, turbulent atmosphere ("grosser Clouds"), thereby reducing star twinkling. As
air pollution from industrialization and
light pollution from artificial lighting increased during the
Industrial Revolution, astronomers sought observatory sites in remote locations with clear and dark skies, naturally drawing them towards the mountains. The first permanent
mountaintop astronomical observatory was the
Lick Observatory constructed from 1876 to 1887, at the modest elevation of atop
Mount Hamilton in California. The first
high altitude observatory was constructed atop the
Pic du Midi de Bigorre in the French Pyrenees starting in 1878, with its first telescope and dome installed in 1904. Astronomical observations were also made from
Mont Blanc in the late 1800s. A few other high altitude observatories (such as the
Lowell Observatory in Arizona and
Sphinx Observatory in Switzerland) were constructed through the first half of the 20th century. However, the two most important and prominent of the early 20th century observatories,
Mount Wilson Observatory and
Palomar Observatory, were both located on mid-elevation mountaintops of about in southern California. The stunning successes and discoveries made there using the world's largest telescopes, the 100-inch
Hooker Telescope and 200-inch
Hale Telescope, spurred the move to ever higher sites for the new generation of observatories and telescopes after World War II, along with a worldwide search for locations which had the best
astronomical seeing. Since the mid-20th century, an increasing number of high altitude observatory sites have been developed at locations around the world, including numerous sites in Arizona, Hawaii, Chile, and the Canary Islands. The initial wave of high-altitude sites were mostly in the range, but astronomers soon sought even higher sites above . Among the largest, best developed, and most renowned of these high altitude sites is the
Mauna Kea Observatory located near the summit of a volcano on the Island of Hawaii, which has grown to include over a dozen major telescopes during the four decades since it was founded. In the first decade of the 21st century, there has been a new wave of observatory construction at very high altitudes above , with such observatories constructed in India, Mexico, and most notably the Atacama Desert in northern Chile, now the site of several of the world's highest observatories. The scientific benefits of these sites outweigh the numerous logistical and physiological challenges which must be overcome during the construction and operation of observatories in remote mountain locations, even in desert, polar, and tropical island sites which magnify the challenges but confer additional observational advantages. Sites at high altitude are ideal for
optical astronomy and provide optimal seeing, being above a significant portion of the
Earth's atmosphere with its associated weather, turbulence, and diminished clarity. In particular, sites on mountaintops within about of the ocean often have excellent observing conditions above a stable
inversion layer throughout much of the year. High altitude sites are also above most of atmosphere's
water vapor, making them ideal for
infrared astronomy and
submillimeter astronomy as those wavelengths are strongly absorbed by water vapor. On the other hand, high altitude does not offer as significant an advantage for
radio astronomy at longer wavelengths, so relatively few radio telescopes are located at such sites. At the far end of the spectrum, for the extremely short wavelengths of
x-ray and
gamma ray astronomy, along with high-energy
cosmic rays, high altitude observations once again offers significant advantages, enough that many experiments at these wavelengths have been conducted by balloon-borne or even by
space telescopes, although a number of high-altitude ground-based sites have also been used. These include the
Chacaltaya Astrophysical Observatory in Bolivia, which at was the world's highest permanent astronomical observatory from the time of its construction during the 1940s until surpassed in 2009 by the new
University of Tokyo Atacama Observatory, an optical-infrared telescope on a remote mountaintop in Chile. == Highest permanent observatories ==