We have two birthdays today.
Johann Oskar Backlund was born on April 28th 1846 in Länghem, Sweden, but after university spent his career in firstly Tartu, Estonia (part of Imperial Russia at the time) and then Pulkovi (also Russia). He was a dab hand at celestial mechanics, and became so well known in Russian astronomical circles for his work on comet 2P/Encke that Russian sources sometimes refer to it as Encke-Backlund.
2P/Encke is believed to be the source of the Taurid meteor shower, but Backlund was more interested in the effect it might be having on Mercury, and used the perturbations produced by the motion of the comet to predict the mass of the planet. If I had a copy of the 1961 journal Soviet Physics I might be able to share his results with you (unless they were in Russian, which seems likely).
Jan Oort was also born on this day, in 1900, in Franaker, Friesland (the Netherlands). He was mostly drawn to radio astronomy, and his day job was as a professor at the University of Lieden, under Director Ejnar Hertzsprung. Among Oort’s many career highlights were the discovery of a group of stars outside the Milky Way (the galactic halo), the calculation of how far away and in what direction lies the centre of the galaxy, and of course the idea that comets originate in what is now commonly known as the Oort Cloud, a roughly spherical region of icy planetesimals surrounding the Sun at distances of up to an almost unbelievable 50,000 AU (defining the limits of our home star’s gravitational supremacy).
1903 – Discovery of asteroid 509 Iolanda (a.k.a. 190LR) by Max Wolf. Iolanda is an S-type main belt asteroid, and the NASA JPL Small-Body Database Browser gives it a diameter of just under 53km, an absolute magnitude of 8.40, and a rotation period (day) of 12.306 hours.
1928 – Birth of Eugene Shoemaker, a leading light in the development of astrogeology, but mostly remembered these days as co-discoverer of comet Shoemaker-Levy 9, which famously collided with Jupiter in 1994.
1999 – Launch of the ABRIXAS X-Ray Telescope by the German Deutsches Zentrum für Luft und Raumfahrt from the Kapustin Yar facility in Russia. The mission lasted approximately three days, thanks to an accident involving an overcharged battery.
2003 – Galaxy Evolution Explorer (GALEX) launched.