The Mount Wilson Observatory (MWO) is an astronomical observatory in Los Angeles County, California

Los Angeles County, California

Los Angeles County is a county in the U.S. state of California. As of 2010 U.S. Census, the county had a population of 9,818,605, making it the most populous county in the United States. Los Angeles County alone is more populous than 42 individual U.S. states...

, United States

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

. The MWO is located on Mount Wilson

Mount Wilson (California)

Mount Wilson is one of the better known peaks in the San Gabriel Mountains, part of the Angeles National Forest in Los Angeles County, California. It is the location of the Mount Wilson Observatory and has become the astronomical center of Southern California with and telescopes, and and tall...

, a 5,715 foot (1,742 m) peak in the San Gabriel Mountains

San Gabriel Mountains

The San Gabriel Mountains Range is located in northern Los Angeles County and western San Bernardino County, California, United States. The mountain range lies between the Los Angeles Basin and the Mojave Desert, with Interstate 5 to the west and Interstate 15 to the east...

 near Pasadena

Pasadena, California

Pasadena is a city in Los Angeles County, California, United States. Although famous for hosting the annual Rose Bowl football game and Tournament of Roses Parade, Pasadena is the home to many scientific and cultural institutions, including the California Institute of Technology , the Jet...

, northeast of Los Angeles

Los Ángeles

Los Ángeles is the capital of the province of Biobío, in the commune of the same name, in Region VIII , in the center-south of Chile. It is located between the Laja and Biobío rivers. The population is 123,445 inhabitants...

. The observatory contains two historically important telescopes: the 60 inch (1.5 m) Hale telescope built in 1908, and the 100 inch (2.5 m) Hooker telescope, which was the largest telescope in the world from its completion in 1917 until 1948.

Thanks to the inversion layer

Inversion (meteorology)

In meteorology, an inversion is a deviation from the normal change of an atmospheric property with altitude. It almost always refers to a temperature inversion, i.e...

 that traps smog

Smog

Smog is a type of air pollution; the word "smog" is a portmanteau of smoke and fog. Modern smog is a type of air pollution derived from vehicular emission from internal combustion engines and industrial fumes that react in the atmosphere with sunlight to form secondary pollutants that also combine...

 over Los Angeles, Mount Wilson has naturally steadier air than any other location in North America, making it ideal for astronomy

Astronomy

Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...

 and in particular for interferometry

Interferometry

Interferometry refers to a family of techniques in which electromagnetic waves are superimposed in order to extract information about the waves. An instrument used to interfere waves is called an interferometer. Interferometry is an important investigative technique in the fields of astronomy,...

.