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The year 1821 in science and technology involved some significant events, listed below.

Astronomy

Johann Franz Encke calculates that Comet Encke has a periodic orbit, the second comet after Comet Halley for which this has been discovered.
Alexis Bouvard detects irregularities in the orbit of Uranus.

Swedish mycologist Elias Magnus Fries begins publication of Systema Mycologicum, a key work in the modern taxonomy of mushrooms.
William Jackson Hooker publishes Flora Scotica; or, A description of Scottish plants.
Prideaux John Selby begins publication of Illustrations of British Ornithology, the first set of life-sized illustrations of British birds.

January 28 – Russian explorer Fabian Gottlieb von Bellingshausen discovers Alexander I Land in Antarctica, later determined to be the world's second-largest uninhabited island.
December 6 – The South Orkney Islands are discovered by George Powell and Nathaniel Palmer.^[2]^[3]
December 15 – The world's first geographical society, the Société de géographie, is established in Paris.
William Parry starts his second voyage to find the Northwest Passage.

Augustin-Louis Cauchy gives the first complete presentation of calculus using limits.

Charles Bell presents a paper to the Royal Society, "On the Nerves, Giving an Account of some Experiments on their Structure and Functions, which led to a New Arrangement of the System", identifying Bell's palsy; and also publishes his book Illustrations of the Great Operations of Surgery: Trepan, Hernia, Amputation, Aneurism, and Lithotomy (illustrated by himself).
Jean Marc Gaspard Itard publishes Traité des maladies d'oreille et de l'audition in Paris, a major work on otology.

William Buckland finds the remains of a hyena's den in Yorkshire, containing the bones of lions, elephants and rhinoceros.

Antide Janvier publishes his textbook on the theory and practice of watchmaking, Manuel Chronometrique, ou, precis de ce qui concerne le temps, ses divisions, ses mesures, leurs usages, etc. in Paris.

^ Kidd, John (1821). "Observations on Naphthaline, a peculiar substance resembling a concrete essential oil, which is apparently produced during the decomposition of coal tar, by exposure to a red heat". Philosophical Transactions of the Royal Society. 111: 209–221. doi:10.1098/rstl.1821.0017. S2CID 97798085.
^ Headland, Robert K. (1989). Chronological list of Antarctic expeditions and related historical events. Cambridge University Press. ISBN 978-0-521-30903-5. OCLC 185311468.
^ "South Orkney Islands | Location & Facts".
^ "Copley Medal | British scientific award". Encyclopedia Britannica. Retrieved 22 July 2020.