1845 in science

Astronomy

• April – Lord Rosse discovers that the nebula M51 has a spiral structure.
• September–October – Cornish mathematician John Couch Adams communicates to James Challis and George Biddell Airy his calculations demonstrating that a body is perturbing the orbit of Uranus.
• November 10 – Urbain Le Verrier presents to the Académie des sciences in Paris a memoir showing that existing theories fail to account for the motion of Uranus.
• Construction begins in Ireland of the "Leviathan of Parsonstown", a telescope built by Lord Rosse.

  Biology

• August–September – Previously unknown Potato blight strikes the potato crop in Ireland: start of the Irish Potato Famine.

  Chemistry

• March 17 – Stephen Perry patents the rubber band in England.
• Edmond Frémy discovers the oxidizing agent Frémy's salt.

  Exploration

• August – John Franklin's expedition with HMS Erebus and HMS Terror to find the Northwest Passage is last seen entering Baffin Bay prior to its mysterious disappearance.

  Medicine

• December 27 – Anesthesia is used in childbirth for the first time, by Dr Crawford Long in Jefferson, Georgia.

  Physics

• September 13 – Michael Faraday discovers that an intense magnetic field can rotate the plane of polarized light, the Faraday effect.
• C. H. D. Buys Ballot confirms the Doppler effect for sound waves.
• Kirchhoff's circuit laws are first described by German physicist Gustav Kirchhoff.

  Technology

• July 26–August 10 – Isambard Kingdom Brunel’s iron steamship Great Britain makes the Transatlantic Crossing from Liverpool to New York, the first screw propelled vessel to make the passage.
• The saxhorn family of valved brass instruments is patented by Adolphe Sax in France.

  Publications

• January 14 – Physikalische Gesellschaft zu Berlin established and begins publishing Fortschritte der Physik and Verhandlungen.
• August 28 – The journal Scientific American begins publication.
• Alexander von Humboldt's Kosmos: Entwurf einer physischen Weltbeschreibung begins publication.

  Awards

• Copley Medal: Theodor Schwann
• Wollaston Medal for Geology: John Phillips

  Births

• March 3 – Georg Cantor, Russian-born German mathematician.
• March 27 – Wilhelm Röntgen, German physicist, discoverer of X-rays, Nobel laureate.
• April 21 – William Healey Dall, American malacologist and explorer.
• May 4 – William Kingdon Clifford, English geometer.
• May 16 – Élie Metchnikoff, Russian-born microbiologist, Nobel laureate.
• June 16 – Heinrich Dressel, German archaeologist.
• July 4 – Thomas Barnardo, Irish-born physician and philanthropist.
• September 11 – Émile Baudot, telegraph engineer.
• November 14 – Ulisse Dini, Italian mathematician.

  Deaths

• January 11 - Etheldred Benett, British geologist.
• March 13 – John Frederic Daniell, English chemist and physicist.
• March 18 – 'Johnny Appleseed' , American nurseryman.
• April 10 – Thomas Sewall, American anatomist.
• October 18 – Dominique, comte de Cassini, French astronomer.
• Jean Henri Jaume Saint-Hilaire, French botanist.