André-Louis Danjon

French astronomer

• Born: April 6, 1890
• Birthplace: Caen, France
• Died: April 21, 1967
• Place of death: Paris, France

Danjon primarily worked to increase the precision of astronomical instruments in observing stars. The invention of the prismatic 60-degree astrolabe, or the Danjon astrolabe, is his crowning achievement. Toward the end of his career, in the 1950’s, he also investigated irregularities in Earth’s rotation.

Early Life

André-Louis Danjon (ahn-dray-lwee dahn-zhon) attended the École Normale Supérieure in Paris from 1910 until the outbreak of World War I in 1914. During the war, he served in the French army. After his release from the military in 1919, he became an astronomer at the University Observatory at Strasbourg and continued his studies. In 1928, he obtained his doctorate degree there and started a rather rapid rise through the ranks of his profession. In 1930, he accepted the position of director of the University Observatory at Strasbourg, and a year later the University of Strasbourg offered him a position on the faculty. By 1935, Danjon occupied the dean’s chair of the faculty of sciences at the university, only seven years after being granted his degree.

Life’s Work

Visual photometry interested Danjon from the beginning of his studies in astronomy. This led him to various successful attempts to improve current observational hardware, producing a greater precision in his readings. In 1921, he built a photometer for observations of the lunar eclipse. Other photometer innovations allowed Danjon to be among the first to calculate Earth’s reflectivity of the sun’s light, or albedo, and to establish the corresponding constants for the planets Mercury and Venus.

After World War II, in 1945, Danjon left the University of Strasbourg for the directorship of the Paris Observatory. There he brought his talent for the technical improvement of instrumentation and his interests in positional astronomy together. Danjon concluded that the instrument then used for the determination of the coordinates of fundamental stars, the transit instrument, had reached its maximum precision. He therefore set out to create a new system that eliminated previous shortcomings by relying on completely different principles. His innovation, the prismatic 60-degree astrolabe, did not rely on the passage of one heavenly body across another, as before. Instead, Danjon’s instrument used a prism that created two images of the same star on a graph, one descending and one ascending as Earth rotated. When these images aligned horizontally, the time at which the star attained the altitude of 60 degrees in Earth’s sky could be calculated with great accuracy. Danjon utilized a fixed altitude to minimize the error caused by variations from standard atmospheric refraction. The final version of the Danjon astrolabe , as it came to be known, proved itself so worthy that four years after its introduction at the Paris Observatory in 1956 more than thirty major observatories around the world had switched to its use.

The ability of this new device to determine more precisely star positions opened new fields of interest for Danjon, for it revealed irregularities in Earth’s rotation. The rate of Earth’s rotation can be measured using universal time, which utilizes positional astronomy (of which Danjon’s astrolabe is a part), or atomic time, which relies on atomic decay, which is constant. On occasion, scientists could determine slight differences between these two measuring techniques. In a moment of intuition, Danjon deduced that the variance of universal time from atomic time resulted from an increase in the sun’s energy. In 1958, Danjon published his results, which concluded that Earth suddenly increased its rotation in coincidence with intense solar activity on February 23, 1956.

Significance

Danjon, throughout his career, split his time between advancing the state of astronomy and supporting the efforts of other astronomers. Following his own interests, his invention of the prismatic 60-degree astrolabe allowed the compilation of the coordinates of fundamental stars and facilitated the study of Earth’s movement around its center of gravity. In this regard, his achievements should not be counted as slight merely because they seem few in number. To determine the reflectivity of Mercury and Venus, for example, required more than thirty thousand observations spread over ten years’ time, sufficient testimony to a scientist’s tenacity and productivity.

In addition to his own work, however, Danjon unselfishly spent much of his time encouraging the work of his fellows and students in various faculty and administrative positions. As an example of his directing abilities, he increased the staff of the Paris Observatory from 60 scientists to 350 in eighteen years. In addition to the time already mentioned that he spent at the observatories and universities at Strasbourg and Paris, Danjon taught at the Sorbonne and was in 1954 appointed the director of the Astrophysics Institute in Paris. His countrymen’s pride in his contributions reveals itself in his election to the French Académie des Sciences in 1948, and he was the president of the International Astronomical Union from 1955 to 1958. Danjon retired from scholarly pursuits in 1963 and died four years later.

Bibliography

Flammarion, Gabrielle Camille, and André Danjon, eds. The Flammarion Book of Astronomy. Translated by Annabel Pagel and Bernard Pagel. New York: Simon & Schuster, 1964. Though a general work on astronomy, it focuses on the solar system. Danjon worked on the chapters that cover Earth, the moon, and the planets. Contains many photographs and illustrations on all aspects of astronomy, including those from popular accounts throughout the ages.

Krisciunas, Kevin. Astronomical Centers of the World. New York: Cambridge University Press, 1988. Taking a historical look at some of the major observatories of the world, this book includes a chapter on the Paris Observatory, where Danjon worked. Its coverage of modern installations includes photographs of the telescopes used in each.

Kuiper, Gerard P., and Barbara M. Middlehurst, eds. Telescopes. Chicago: University of Chicago Press, 1960. Danjon wrote the chapter “The Impersonal Astrolabe” that details the design and effectiveness of the instrument for which he is famous, the prismatic 60-degree astrolabe. He also discusses its advantages over the transit instrument that it replaced. Includes photographs and a fairly detailed blueprint of the device, enumerating its numerous components. Bibliography.

Ronan, Colin A. Astronomy. New York: Barnes & Noble Books, 1973. A unique scrapbook of astronomical history through the ages. Extracts paragraphs and illustrations from the works of the major astronomers of the world, allowing readers to follow the thoughts of such figures as Ptolemy, Nicolaus Copernicus, Johannes Kepler, and Galileo, among others, exactly as they wrote them. Each excerpt is annotated and footnoted. Completed by superb illustrations, this chance to experience astronomy as astronomers see it should not be missed.

Sagan, Carl. Cosmos. New York: Random House, 1980. Sagan has ably attempted to paint the universe, and Earth’s and humankind’s place in it. Certainly a far-reaching book, marked by the author’s excellent text. Splendid photographs and paintings. Astronomy, its history, and its significance to life on Earth are presented here in an effective and powerful manner.

Sinnot, Roger W. “Seeking Thin Crescent Moons.” Sky and Telescope 107, no. 2 (February, 2004): 102-106. Includes information about Danjon’s astronomical discoveries.

Struve, Otto, and Velta Zebergs. Astronomy of the Twentieth Century. New York: Macmillan, 1962. Explores the advances made in astronomy since roughly 1900 from the perspective of some of Danjon’s contemporaries, astronomers who, like him, participated in these advances. Taking a historical approach, in the form of storytelling and using the scientists’ own words, it presents a unique view of how scientific inquiry proceeds by a curious combination of chance discovery, fortuitous accident, and hard work.