Samuel Pierpont Langley (August 22, 1834 – February 27, 1906) was an American aviation pioneer, astronomer and physicist who invented the bolometer. He was the third secretary of the Smithsonian Institution and a professor of astronomy at the University of Pittsburgh, where he was the director of the Allegheny Observatory.

Langley was born in Roxbury, Boston, on August 22, 1834.^[3]

Langley attended Boston Latin School and graduated from English High School of Boston, after which he became an assistant in the Harvard College Observatory. He then moved to a job at the United States Naval Academy, nominally as a professor of mathematics. However, he was actually sent there to restore the Academy's small observatory. In 1867, he became the director of the Allegheny Observatory and a professor of astronomy at the University of Pittsburgh (then known as the Western University of Pennsylvania), a post he kept until 1891 even while he became the third Secretary of the Smithsonian Institution in 1887. In same year (1887) the Manchester Literary and Philosophical Society awarded Langley honorary membership of the Society.^[4]

Langley was the founder of the Smithsonian Astrophysical Observatory. In 1875, he was elected as a member to the American Philosophical Society.^[5] In 1888 Langley was elected a member of the American Antiquarian Society.^[6] In 1898, he received the Prix Jules Janssen, the highest award of the Société astronomique de France, the French astronomical society.

Langley arrived in Pittsburgh in 1867 to become the first director of the Allegheny Observatory, after the institution had fallen into hard times and been given to the Western University of Pennsylvania. By then, the department was in disarray – equipment was broken, there was no library and the building needed repairs. Through the friendship and aid of William Thaw Sr., a Pittsburgh industrial leader, Langley was able to improve the observatory equipment and build additional apparatuses. One of the new instruments was a small transit telescope used to observe the position of the stars as they cross the celestial meridian.^[7]

He raised money for the department in large part by distributing standard time to cities and railroads. Up until then, correct time had only occasionally been sent from American observatories for public use. Clocks were manually wound in those days and time tended to be imprecise. Exact time had not been especially necessary. It was enough to know that at noon the sun was at its highest elevation for the day. That changed with the arrival of railroads, which made the lack of standard time dangerous. Trains ran by a published schedule, but scheduling was chaotic. If the timepieces of an engineer and a switch operator differed by even a minute or two, trains could be on the same track at the same time and collide.

Using astronomical observations obtained from the new telescope, Langley devised a precise time standard, including time zones, that became known as the Allegheny Time System. Initially he distributed time signals to Allegheny city businesses and the Pennsylvania Railroad. Eventually, twice a day, the Allegheny time signals gave the correct time via 4,713 miles of telegraph lines to all railroads in the US and Canada. Langley used the money from the railroads to finance the observatory. From about 1868 revenues from Allegheny Time continued to fund the observatory, until the US Naval Observatory provided the signals via taxpayer funding in 1883.

Once funding was secure, Langley devoted his time at the Observatory initially in researching the sun. He used his draftsman skills—from his first job out of high school—to produce hundreds of drawings of solar phenomena, many of which were the first the world had seen. His remarkably detailed 1873 illustration of a sunspot, observed while using the observatory's 13-inch Fitz-Clark refractor, became a classic. It is featured on page 21 of his book, The New Astronomy, and was also widely reprinted in the Americas and Europe.

In 1886, Langley received the inaugural Henry Draper Medal from the National Academy of Sciences for his contributions to solar physics.^[8] His publication in 1890 of infrared observations at the Allegheny Observatory in Pittsburgh together with Frank Washington Very along with the data he collected from his invention, the bolometer, was used by Svante Arrhenius to make the first calculations on the greenhouse effect. In 1898, Langley received the Prix Jules Janssen, the highest award of the Société astronomique de France (the French astronomical society).

Langley attempted to make a working piloted heavier-than-air aircraft. His models flew, but his two attempts at piloted flight were not successful. Langley began experimenting with rubber-band powered models and gliders in 1887. (According to one book, he was not able to reproduce Alphonse Pénaud's time aloft with rubber power but persisted anyway.) He built a rotating arm that functioned like a wind tunnel, and made larger flying models powered by miniature steam engines. Langley realised that sustained powered flight was possible when he found that a 1 lb. brass plate, suspended from the rotating arm by a spring, could be kept aloft by a spring tension of less than 1 oz.

Langley understood that aircraft need thrust to overcome drag from forward speed, observed higher aspect ratio flat plates had higher lift and lower drag, and stated in 1902 "A plane of fixed size and weight would need less propulsive power the faster it flew", the counter-intuitive effect of induced drag.^[9]

He met the writer Rudyard Kipling around this time, who described one of Langley's experiments in his autobiography:

In 1880 Langley invented the bolometer, an instrument initially used for measuring far infrared radiation.^[18] The bolometer has enabled scientists to detect a change of temperature of less than 1/100,000 of a degree Celsius.^[19] It laid the foundation for the measurements of the amount of solar energy on the Earth. He published an 1881 paper on it, "The Bolometer and Radiant Energy".^[20] He made one of the first attempts to measure the surface temperature of the Moon, and his measurement of interference of the infrared radiation by carbon dioxide in Earth's atmosphere was used by Svante Arrhenius in 1896 to make the first calculation of how climate would change from a future doubling of carbon dioxide levels.^[21]

Starting with his tenure at Allegheny Observatory in the Pittsburgh area in the late 1860s, Langley was a major player in the development of astronomically derived and regulated time distribution services in America through the later half of the 19th century. His work with the railroads in this area is often cited as central to the establishment of the Standard Time Zones system. His very successful and profitable time sales to the Pennsylvania Railroad stood out among the many non-government-based observatories of the day who were largely subsidizing their research by time-service sales to regional railroads and the cities they served. The United States Naval Observatory's increasing dominance in this field threatened these regional observatories' livelihoods and Langley became a leader in efforts to preserve the viability of their commercial programs.

Langley held himself responsible for the loss of funds after the June 1905 discovery that Smithsonian accountant William Karr was embezzling from the Institution. Langley refused his salary in the aftermath. In November he suffered a stroke. In February 1906 he moved to Aiken, South Carolina to recuperate, but had another stroke and died on February 27. He was buried in Forest Hills Cemetery in Boston.^[22]

Air and sea craft, facilities, a unit of solar radiation, and an award have been named in Langley's honor, including:

• Langley Gold Medal by the Smithsonian Institution^[23]
• NASA Langley Research Center (NASA LaRC), Hampton, Virginia^[24]
• Langley Air Force Base
• Langley Hall at the University of Pittsburgh
• Langley High School in Pittsburgh
• Langley Memorial Aeronautical Laboratory
• Langley unit of solar radiation
• Mount Langley in the Sierra Nevada
• USS Langley (CV-1)
• USS Langley (DE‑131), laid down 10 July 1942 and renamed Hammann on 1 August 1942
• USS Langley (CVL-27)
• Seadrome Langley, intended as one in a chain of Atlantic aviation way-stations, cancelled due to the Depression
• SS Samuel P. Langley, U.S. Liberty Ship
• Samuel P. Langley Elementary School in Hampton, VA.

In 1963, Langley was inducted into the National Aviation Hall of Fame in Dayton, Ohio.^[25]

In the 1978 film The Winds of Kitty Hawk, he was portrayed by actor John Hoyt.

• Manly–Balzer engine

• Finding Aid to Samuel P. Langley's Papers at the Smithsonian Institution Archives
• Finding Aid to the Samuel P. Langley Collection at the National Air and Space Museum Archives Division Archived March 9, 2012, at the Wayback Machine
• Samuel Pierpont Langley, Flying Machines
• Samuel Pierpont Langley, Invention of the Airplane
• Centennial of Flight
• Allegheny Observatory – "UNDAUNTED: The Forgotten Giants of the Allegheny Observatory?"
• National Academy of Sciences Biographical Memoir