William Rowan Hamilton was born #OTD in 1805. His reformulation of classical mechanics in terms of a variational principle played an essential role in the development of field theories and quantum mechanics. en.wikipedia.org/wiki/William...

John Tyndall was born #OTD in 1820. He devised astute methods to study the interaction of light (both visible and infrared) with various gases. In doing so, he provided experimental evidence for the greenhouse effect. en.wikipedia.org/wiki/John_Ty...

Helen Sawyer Hogg was born #OTD in 1905. She worked on globular clusters and variable stars, finding 132 new such stars in M2 while at the Dominion Astrophysical Observatory, before pursuing this task at the University of Toronto. en.wikipedia.org/wiki/Helen_S...

Isidor Isaac Rabi was born #OTD in 1898. His discovery of nuclear magnetic resonance earned him the 1944 Nobel Prize in Physics and opened up new ways to study matter, with notable applications to chemistry and medicine (MRI). en.wikipedia.org/wiki/Isidor_...

Loránd Eötvös was born #OTD in 1848. Following Galileo’s studies of free-falling bodies, the experiment that bears his name establishes accurately the equivalence between the inertial and gravitational masses. en.wikipedia.org/wiki/Lor%C3%...

Rosalind Franklin was born #OTD in 1920. An X-ray crystallographer, her work was essential in uncovering the structures of coal, several viruses, as well as DNA and RNA. Her untimely death, at 37, prevented her from earning a Nobel Prize. en.wikipedia.org/wiki/Rosalin...

Vera C. Rubin was born #OTD in 1928. Her study of rotation curves in galaxies provided strong evidence for dark matter haloes. Her observations of peculiar motions of galaxies also supported the existence of superclusters. en.wikipedia.org/wiki/Vera_Ru...

Friedrich Bessel was born #OTD in 1784. He was the first to determine a stellar distance via parallax measurements, falling within 10% of the actual value. He noticed anomalies in the motion of Sirius, later attributed to Sirius B. en.wikipedia.org/wiki/Friedri...

Victor Regnault was born #OTD in 1810. A chemist and physicist, he designed sensitive apparatus to study the thermal properties of gases, in his laboratory of the « Manufacture Nationale de Sèvres » porcelain works. en.wikipedia.org/wiki/Henri_V...

Edward Charles Pickering was born #OTD in 1846. Long-time director of Harvard College Observatory, he discovered a series of lines that he mistakenly attributed to hydrogen. He recruited the famous « Harvard computers ». en.wikipedia.org/wiki/Edward_...

Hendrik Lorentz was born #OTD in 1853. He made vast contributions to electromagnetic theory, earning the 1902 Nobel Prize in Physics with Zeeman, and laid essential groundwork for Einstein’s special theory of relativity. en.wikipedia.org/wiki/Hendrik...

Georges Lemaître was born #OTD in 1894. He was the first to suggest the Universe was expanding and to estimate what is now called the Hubble constant. He also championed the idea of a « beginning of the world », a Big Bang. en.wikipedia.org/wiki/Georges...

Frits Zernike was born #OTD in 1888. He is remembered by radio-astronomers for the Van Cittert-Zernike theorem that relates the brightness distribution of a source to the complex visibilities observed by an interferometer. en.wikipedia.org/wiki/Frits_Z...

Leon Lederman was born #OTD in 1922. With Garwin and Heinrich, he verified parity violation independently from Wu. Later on, he discovered the muon neutrino, for which he received the 1988 Nobel prize, and the bottom quark. en.wikipedia.org/wiki/Leon_M....

Paul Drude was born #OTD in 1863. The Drude model provides an interpretation of electrical conduction properties in metals through the scattering of charge carriers (electrons) by the mostly immobile ions in the lattice. en.wikipedia.org/wiki/Paul_Dr...

Isabel Martin Lewis was born #OTD in 1881. The first woman to be hired by the USNO as an assistant astronomer, she worked on eclipses. She held regular columns in several magazines to popularize science and astronomy. en.wikipedia.org/wiki/Isabel_...

Nikola Tesla was born #OTD in 1856. A Serbian-American engineer known for his flamboyance, he is remembered for his contributions to the design of the multiphase alternating-current (AC) electricity supply system. en.wikipedia.org/wiki/Nikola_...

John Archibald Wheeler was born #OTD in 1911. He made many contributions to theoretical physics, from nuclear fission to general relativity and grand unification theories of gravity and quantum physics. en.wikipedia.org/wiki/John_Ar...

Pyotr Kapitsa was born #OTD in 1894. After working on high magnetic fields while at Cambridge, he moved to low-temperature physics upon returning to the USSR, discovering Helium superfluidity in 1937. en.wikipedia.org/wiki/Pyotr_K...

William John Macquorn Rankine was born #OTD in 1820. The Rankine cycle of steam engines consists of two isobaric and two adiabatic phases, while the Rankine-Hugoniot relations connect quantities upstream and downstream from a shock. en.wikipedia.org/wiki/Macquor...

Henrietta Swan Leavitt was born #OTD in 1868. One of the famous « Harvard computers », she discovered the period-luminosity relation of Cepheid variable stars, providing a vital ladder on the cosmological distance scale. en.wikipedia.org/wiki/Henriet...

Hans Bethe was born #OTD in 1906. Over his seventy-year career, he made vast contributions to theoretical physics and astrophysics, in particular regarding stellar nucleosynthesis through the p-p chain and CNO cycle processes. en.wikipedia.org/wiki/Hans_Be...

Félix Savart was born #OTD in 1791. He is mostly known to physicists for the Biot-Savart law that gives the magnetic field from a distribution of currents. He also studied acoustics, designing a trapezoidal violin. en.wikipedia.org/wiki/F%C3%A9...

George Ellery Hale was born #OTD in 1868. In 1908, using Mount Wilson Observatory, which he had founded 4 years earlier, he proved sunspots to be magnetic, providing the first detection of « extraterrestrial » magnetic fields. en.wikipedia.org/wiki/George_...

Maria Göppert Mayer was born #OTD in 1906. She was the second woman to win the Nobel Prize in Physics, in 1963, for the development of the « shell model » of atomic nuclei, similar to that used for the electrons in an atom. en.wikipedia.org/wiki/Maria_G...

Alexis Bouvard was born #OTD in 1767. Director of the Paris Observatory from 1822 to 1843, he noticed irregularities in the orbit of Uranus, which Le Verrier and Adams used to discover Neptune. en.wikipedia.org/wiki/Alexis_...

Charles Messier was born #OTD in 1730. Mostly interested in comets (he discovered 13 of them), he began recording these pesky objects that were not… and ended up with what is now the Messier catalogue of Nebulæ and star clusters. en.wikipedia.org/wiki/Charles...

Walther Nernst was born #OTD in 1864. He made several contributions in thermodynamics and physical chemistry. The Nernst heat theorem enabled chemists to determine equilibrium points of chemical reactions from heat measurements. en.wikipedia.org/wiki/Walther...

Victor Hess was born #OTD in 1883. He discovered cosmic rays by measuring the levels of ionizing radiation in the atmosphere and noticing that they increased with altitude, contrary to what was expected at the time. en.wikipedia.org/wiki/Victor_...

Siméon Denis Poisson was born #OTD in 1781. Poisson’s equation is fundamental in electromagnetism, Poisson brackets appear in analytical mechanics, and Poisson distributions arise, e.g., in radioactive decay processes. en.wikipedia.org/wiki/Sim%C3%...