Heinrich Olbers was born #OTD in 1758. His name is usually associated with the famous "paradox" of the night sky's darkness, but he is also the discoverer of two of the largest asteroids, Pallas and Vesta. en.wikipedia.org/wiki/Heinric...

Henry Cavendish was born #OTD in 1731. A very careful experimentalist, he is mostly known for the experiment to determine the Earth’s density, using a torsion balance. He obtained a value within 1% of the currently accepted figure. en.wikipedia.org/wiki/Henry_C...

Karl Schwarzschild was born #OTD in 1873. While in the German Army on the Russian front, he worked out the first exact solution to Einstein’s General Relativity, describing space-time near non-rotating spherical objects. en.wikipedia.org/wiki/Karl_Sc...

Ejnar Hertzsprung was born #OTD in 1873. With H. Russell, he devised the diagram that bears their names, where stars are plotted as a function of their colors and luminosities, a major step towards understanding stellar evolution. en.wikipedia.org/wiki/Ejnar_H...

Niels Bohr was born #OTD in 1885. One of the pioneers of quantum physics, his debates with Einstein regarding the fundamentally non-local nature of the new physics were central to our understanding of the physical world. en.wikipedia.org/wiki/Niels_B...

Meghnad Saha was born #OTD in 1893. An Indian astrophysicist whose study of thermal ionization led him to formulate the equation that bears his name, relating the populations of various ionization stages at thermal equilibrium. en.wikipedia.org/wiki/Meghnad...

Robert Hutchings Goddard was born #OTD in 1882. He built the first liquid-fuel rocket, launched in 1926, which makes him one of the pioneers of rocket physics and space travel, alongside Esnault-Pelterie, Tsiolkovsky, and Oberth. en.wikipedia.org/wiki/Robert_...

Vitaly Ginzburg was born #OTD in 1916. A Russian theoretical physicist known for a phenomenological theory of superconductivity. He also demonstrated the production of EM radiation when a charge moves in heterogeneous media. en.wikipedia.org/wiki/Vitaly_...

Otto Frisch was born #OTD in 1904. An Austrian-born British physicist, he and his aunt Lise Meitner explained the process of nuclear fission observed by Hahn and Strassmann. He outlined the process for producing an atomic bomb. en.wikipedia.org/wiki/Otto_Ro...

He was also the founder of @iap.fr read more: www.iap.fr/institut/his...

Jean Perrin was born #OTD in 1870. Famous for experiments on Brownian motion, he was also a major advocate for the advancement of science, leading to the creation of Observatoire de Haute-Provence, Palais de la Découverte, and CNRS. en.wikipedia.org/wiki/Jean_Ba...

Enrico Fermi was born #OTD in 1901. An Italian-born American physicist who received the 1938 Nobel Prize for induced radioactivity. He was a pioneer of the atomic age, demonstrating the first self-sustaining nuclear chain in 1942. en.wikipedia.org/wiki/Enrico_...

Martin David Kruskal was born #OTD in 1925. A mathematician and physicist, he made vast contributions to the study of instabilities in plasmas, to general relativity, and to solitary wave (« soliton ») phenomena. en.wikipedia.org/wiki/Martin_...

Martin Ryle was born #OTD in 1918. In the aftermath of WWII, he was a pioneer of interferometry and aperture synthesis in radio astronomy with the Cambridge group. For this he received the 1974 Nobel Prize in Physics. en.wikipedia.org/wiki/Martin_...

Ole Rømer was born #OTD in 1644. He timed Io’s eclipses in Jupiter’s shadow from different points on the Earth’s orbit, allowing Huygens and others to determine a value of approximately 212000 km/s for the speed of light. en.wikipedia.org/wiki/Ole_R%C...

Charlotte Moore Sitterly was born #OTD in 1898. She worked extensively on the chemical composition of the Sun, based on spectroscopic data. In particular, she identified technetium, providing the first naturally occurring detection of that element. en.wikipedia.org/wiki/Charlot...

Hippolyte Fizeau was born #OTD in 1819. Working together with L. Foucault, and then as his rival, he determined the speed of light in air to within five percent, improving on Rømer’s much earlier determination. en.wikipedia.org/wiki/Hippoly...

Michael Faraday was born #OTD in 1791. Arguably one of the most brilliant experimental physicists in history, his work on the relationship between magnetism, electricity, and light laid the path for Maxwell’s electromagnetic theory. en.wikipedia.org/wiki/Michael...

Heike Kamerlingh Onnes was born #OTD in 1853. He received the 1913 Nobel Prize for his investigations of matter at very low temperatures, leading to the first liquefaction of helium and the discovery of superconductivity. en.wikipedia.org/wiki/Heike_K...

James Dewar was born #OTD in 1842. A British physicist and chemist, he invented a vacuum flask with which he was able to produce large quantities of liquid oxygen, that he then showed to be attracted by magnets. en.wikipedia.org/wiki/James_D...

Masatoshi Koshiba was born #OTD in 1926. He received the 2002 Nobel prize in Physics for pioneering work in neutrino astronomy, solving the solar neutrino problem by detecting evidence of neutrino oscillation with S-Kamiokande. en.wikipedia.org/wiki/Masatos...

Léon Foucault was born #OTD in 1819. The Foucault pendulum’s oscillation plane appears to rotate due to Earth’s own rotation. Foucault’s currents occur when a conducting material is subjected to a varying magnetic field. en.wikipedia.org/wiki/L%C3%A9...

Visualisation des aberrations géométriques et chromatiques d’une lentille simple. On voit bien les nappes sagittale et tangentielle, et l’influence d’un indice optique plus élevé dans le bleu que dans le rouge.

Oskar Klein was born #OTD in 1894. He proposed that the fifth dimension introduced by Kaluza to generalize Einstein’s GR theory may be « curled up » and microscopic, an idea central to the development of string theory. en.wikipedia.org/wiki/Oskar_K...

Irène Joliot-Curie was born #OTD in 1897. She received the 1935 Nobel Prize in Chemistry, together with her husband Frédéric, for their discovery of artificial radioactivity, opening the path to harnessing atomic power. en.wikipedia.org/wiki/Ir%C3%A...

Félicitations à Stéphane ! Les outils qu’il a développés pour comprendre l’efficacité des algorithmes d’apprentissage automatique sont utiles dans de nombreux domaines… dont l’astrophysique, ce qui a valu la médaille de bronze à Erwan Allys dans notre équipe cette année.

⚡ ALERTE INFO. La médaille d'or 2025 du CNRS est décernée au mathématicien Stéphane Mallat, notamment pour ses travaux sur le stockage de données ➡️ https://l.franceinfo.fr/lyO

James Jeans was born #OTD in 1877. Jeans’ criterion gives the maximum mass for a gas cloud to be stable under the effects of self-gravity and thermal pressure. It is a fundamental tool to approach problems in star formation. en.wikipedia.org/wiki/James_J...

Arthur Compton was born #OTD in 1892. The Compton effect describes the scattering of a photon by a charged particle, usually an electron. Depending on who gains energy in the process, the effect is « direct » or « inverse ». en.wikipedia.org/wiki/Arthur_...

Hans Georg Dehmelt was born #OTD in 1922. He received the 1989 Nobel Prize in Physics for developing the Penning trap, able to store charged particles using an axial magnetic field and a quadrupolar electric field. en.wikipedia.org/wiki/Hans_Ge...

James Van Allen was born #OTD in 1914. A pioneer of the scientific exploration of space, he devised sounding rockets launched from balloons (« rockoons »), which led to the discovery of the radiation belts surrounding the Earth. en.wikipedia.org/wiki/James_V...

John Dalton was born #OTD in 1766. He introduced atomic theory into chemistry and published the first scientific paper on colour blindness. Dalton’s law states that the total pressure in a gas mixture is the sum of partial pressures. en.wikipedia.org/wiki/John_Da...

Carl David Anderson was born #OTD in 1905. His discovery of the positron from cloud chamber photographs earned him the 1936 Nobel Prize and confirmed Dirac’s prediction. That same year, he also discovered the muon. en.wikipedia.org/wiki/Carl_Da...

Woldemar Voigt was born #OTD in 1850. Astronomers dealing with spectroscopic data know him for the Voigt profile, which is a convolution of a Gaussian and a Lorentzian, therefore encompassing all line broadening mechanisms. en.wikipedia.org/wiki/Woldema...

Montée vers l’observatoire de Meudon pour accueillir la nouvelle promotion du Master 2e année « Astronomie, Astrophysique et Dynamique Céleste ».

Francis William Aston was born #OTD in 1877. After studying electrical discharges in gas-filled tubes, he devised mass spectrometers sensitive enough to identify 212 naturally occurring isotopes of chemical elements. en.wikipedia.org/wiki/Francis...

Hermann von Helmholtz was born #OTD in 1821. A German physicist, physician, and philosopher, he made vast contributions to thermodynamics, fluid mechanics, and electrodynamics. Many results in these fields bear his name. en.wikipedia.org/wiki/Hermann...

Ernest Rutherford was born #OTD in 1871. He is considered the father of nuclear physics, through his discovery of alpha and beta decays and his interpretation of the Geiger-Marsden experiments that show the lacunar nature of atoms. en.wikipedia.org/wiki/Ernest_...

Godfrey Hounsfield was born #OTD in 1919. Independently from A. McLeod Cormack, he came up with a design for an X-ray computer-assisted tomographic device, which led to the development of the CT scanners that are now commonplace. en.wikipedia.org/wiki/Godfrey...

Norman Foster Ramsey, Jr. was born #OTD in 1915. A student of Rabi, he improved upon his method to determine transition frequencies, a fundamental element of atomic clocks that earned him the 1989 Nobel Prize in Physics. en.wikipedia.org/wiki/Norman_....

Beautiful new Nature Astronomy cover, featuring a very high-resolution simulation of compressible, magnetized turbulence in the interstellar medium. www.nature.com/natastron/vo... @natastron.nature.com 🧪🔭

Johann Heinrich Lambert was born #OTD in 1728. A Swiss mathematician and physicist who made important contributions to optics, such as the Beer-Lambert law of attenuation, he also was a pioneer of map projections. en.wikipedia.org/wiki/Johann_...

Arthur B. C. Walker, Jr. was born #OTD in 1936. A professor at Stanford, he developed normal-incidence multilayer XUV telescopes (e.g., MSSTA) to observe the solar corona. Similar technology is now employed onboard SOHO/EIT. en.wikipedia.org/wiki/Arthur_....

Sarah Frances Whiting was born #OTD in 1847. An American physicist and astronomer, she set up one of the first undergraduate physics labs in the USA, at Wellesley College, and was the first director of Whitin Observatory. en.wikipedia.org/wiki/Sarah_F...

Samuel Langley was born #OTD in 1834. An American astronomer and physicist, he invented the bolometer, which allowed for the measurement of solar power received at Earth. Bolometers are now widespread in astronomical facilities. en.wikipedia.org/wiki/Samuel_...

Nikolay Bogolyubov was born #OTD in 1909. A Russian mathematician and theoretical physicist, he made vast contributions to the fields of non-linear mechanics, statistical mechanics, dynamical systems, and quantum field theory. en.wikipedia.org/wiki/Nikolay...

John Flamsteed was born #OTD in 1646. He was the first Astronomer Royal and laid the foundations of the RGO. The « Atlas Cœlestis », published posthumously, was the first stellar atlas based on telescopic observations. en.wikipedia.org/wiki/John_Fl...

Pierre Méchain was born #OTD in 1744. A surveyor and astronomer, he measured the Dunkirk-Barcelona meridian arc leading to the definition of the metre, and discovered several nebulæ that ended up in Messier's catalogue. en.wikipedia.org/wiki/Pierre_...

Louis de Broglie was born #OTD in 1892. His theory extended the particle-wave duality of light to material particles, with the de Broglie wavelength of the wave associated to a particle being related to the momentum of the particle. en.wikipedia.org/wiki/Louis_d...

Une partie du Cygne à droite et αCep à gauche, pas mal pour un simple appareil photo (Sony α 7C II, 204800 iso, au 1/8e).