Jean Sylvain Bailly and the Orbit of Halley’s Comet

Jean Sylvain Bailly

Jean Sylvain Bailly

On September 15, 1736French astronomer, mathematician, freemason, and political leader of the early part of the French Revolution Jean Sylvain Bailly was born. Bailly computed an orbit for Halley’s Comet (1759) and studied the four satellites of Jupiter then known. He was the first Mayor of Paris and presided over the Tennis Court Oath.

Jean Sylvain Bailly came from a family of artists. His father was an artist and supervisor of the Louvre and his grandfather was Nicholas Bailly, an artist and court painter. While it is believed that during his early life, Bailly intended to follow his family’s footsteps, however he became highly enthusiastic for science and especially astronomy. It is further assumed that he was influenced by Nicolas de Lacaille.

In 1759, he calculated an orbit for the next appearance of Halley’s Comet. Halley’s Comet was the first comet to be recognized as periodic. Edmond Halley used Newton’s new laws in his 1705 Synopsis of the Astronomy of Comets, used  to calculate the gravitational effects of Jupiter and Saturn on cometary orbits. This calculation enabled him to determine that the orbital elements of a second comet that had appeared in 1682 were nearly the same as those of two comets that had appeared in 1531, observed by Petrus Apianus, and 1607, observed by Johannes Kepler. Halley correctly concluded that all three comets were the same object returning about every 76 years, a period that has since been found to vary between 74–79 years. The comet was first named in Halley’s honour by French astronomer Nicolas Louis de Lacaille in 1759.

Next to his work on Halley’s Comet, Jean Bailly further reduced Lacaille’s observations of 515 stars. Bailly participated in the construction of an observatory at the Louvre. Due to these accomplishments, Bailly was elected to the French Academy of Sciences in 1763. He published numerous works that established him a great reputation as an astronomer throughout Europe. In 1766 he published his Essay on The Theory of the Satellites. In 1771 he released his dissertation On the Inequalities of Light of the Satellites of Jupiter and a few years later he was elected a foreign member of the Royal Swedish Academy of Sciences.

During the later 1770s, Jean Bailly devoted his career to the history of science. He published several works on the topic including A History of Ancient Astronomy published in 1775 and A History of Modern Astronomy published in 1782.

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