1669 in science

The year 1669 in science and technology involved some significant events.

Astronomy

February 23 – Isaac Newton writes his first description of his new invention, the reflecting telescope.{{cite book|first=A. Rupert|last=Hall|authorlink=A. Rupert Hall|title=Isaac Newton: Adventurer in Thought|publisher=Cambridge University Press|year=1996|page=67}}
Geminiano Montanari detects the variability of the eclipsing binary Algol.
Jean Picard begins measurement of 1 degree of Earth's meridian arc in France.

Marcello Malpighi publishes Dissertatio Epistolica de Bombyce in London, a study of Bombyx mori which is the first published monograph on an invertebrate.
Robert Morison publishes Praeludia Botanica, emphasising use of the structure of a plant's fruits for its classification.
Francis Willughby and John Ray publish "Experiments concerning the motion of sap in trees, made this spring".Philosophical Transactions of the Royal Society 4: pp. 963–965.
Jan Swammerdam publishes Historia Insectorum Generalis in the Netherlands, explaining the process of metamorphosis in insects.

Phosphorus is discovered by German alchemist Hennig Brand,{{cite book|last=Emsley|first=John|title=Nature's Building Blocks: an A–Z guide to the elements|publisher=Oxford University Press|year=2001|isbn=978-0-19-850341-5}} the first chemical element to be discovered that was not known since ancient times.{{cite journal|doi=10.1021/ed009p11|title=The discovery of the elements. II. Elements known to the alchemists|journal=Journal of Chemical Education|volume=9|issue=1|page=11|year=1932|last1=Weeks|first1=Mary Elvira|bibcode=1932JChEd...9...11W}}

Nicolas Steno puts forward his theory that sedimentary strata had been deposited in former seas, and that fossils are organic in origin.De solido intra solidum naturaliter contento dissertationis prodromus.

October 29 – Isaac Newton is appointed Lucasian Professor of Mathematics at the University of Cambridge.{{cite book|last=Palmer|first=Alan|last2=Palmer |first2=Veronica|year=1992|title=The Chronology of British History|publisher=Century Ltd|location=London|pages=190–191|isbn=0-7126-5616-2}}

Rasmus Bartholin publishes his observation of the birefringence of a light ray by Iceland spar (calcite).{{cite book|first=Erasmus|last=Bartholin|title=Experimenta crystalli islandici disdiaclastici quibus mira & insolita refractio detegitur|location=Copenhagen|publisher=Daniel Paulli|year=1669}} English translation: Experiments with the double refracting Iceland crystal which led to the discovery of a marvelous and strange refraction tr. by Werner Brandt. Westtown, Pa., 1959.
Robert Boyle publishes A Continuation of New Experiments Physico-mechanical, Touching the Spring and Weight of the Air, and Their Effects.

Richard Lower publishes his Tractatus de Corde on the workings of the heart.
The Chinese traditional herbal medicine company Tong Ren Tang (同仁堂) is established in Beijing by imperial physician Yue Xianyang.

April 12 – Abdias Treu, German mathematician (born 1597)
c. April – Nicasius le Febure, French-born royal chemist, alchemist and apothecary (born 1615)