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The structure of the cloud of comets surrounding the Solar System and a hypothesis concerning its origin.
[Leiden: Astronomical Institutes of the Netherlands], 1950. First edition, extremely rare offprint, of the proposal of the existence of the 'Oort cloud'. "Oort is generally regarded as one of the leading astronomers of the twentieth century. He performed very important researches in a number of areas, most notably on the structure and dynamics of the galactic system ... In comparison with his studies in galactic and extragalactic astronomy, Oort spent little time on solar system astronomy. However he did perform very important researches on comets. This line of study grew out of the researches of a student at Leiden, A. J. J. van Woerkom, who worked on comets' orbits. His interest piqued by van Woerkom's dissertation, Oort in short order developed his own theory of a cloud of protocomets that surround the solar system. He argued that when the solar system was formed, very many small bodies orbited between Mars and Jupiter. Over time, Jupiter's gravitational force acted to expel many of these objects from the solar system, but some were carried into highly elongated orbits that took them out from the Sun to distances between 50,000 astronomical units and 200,000 astronomical units. Oort further calculated that passing stars could redirect some of these bodies into the inner regions of the solar system, thereby producing a "new" comet. While Oort termed his scheme "speculative" when he advanced it in 1950, it had become widely accepted by the late twentieth century and his proposed cloud of comets has become known as the Oort cloud" (DSB). OCLC lists US Naval Observatory only. No copies in auction records. "The Oort cloud is a theoretical cloud of predominantly icy planetesimals believed to surround the Sun to as far as somewhere between 50,000 and 200,000 AU. It is divided into two regions: a disc-shaped inner Oort cloud (or Hills cloud) and a spherical outer Oort cloud. Both regions lie beyond the heliosphere and in interstellar space. The Kuiper belt and the scattered disc, the other two reservoirs of trans-Neptunian objects, are less than one thousandth as far from the Sun as the Oort cloud. The outer limit of the Oort cloud defines the cosmographical boundary of the Solar System and the extent of the Sun's Hill sphere. The outer Oort cloud is only loosely bound to the Solar System, and thus is easily affected by the gravitational pull both of passing stars and of the Milky Way itself. These forces occasionally dislodge comets from their orbits within the cloud and send them toward the inner Solar System. Based on their orbits, most of the short-period comets may come from the scattered disc, but some may still have originated from the Oort cloud. Astronomers conjecture that the matter composing the Oort cloud formed closer to the Sun and was scattered far into space by the gravitational effects of the giant planets early in the Solar System's evolution. Although no confirmed direct observations of the Oort cloud have been made, it may be the source of all long-period and Halley-type comets entering the inner Solar System, and many of the centaurs and Jupiter-family comets as well" (Wikipedia). Oort proposed the idea "as a means to resolve a paradox. Over the course of the Solar System's existence the orbits of comets are unstable and eventually dynamics dictate that a comet must either collide with the Sun or a planet or else be ejected from the Solar System by planetary perturbations. Moreover, their volatile composition means that as they repeatedly approach the Sun, radiation gradually boils the volatiles off until the comet splits or develops an insulating crust that prevents further outgassing. Thus, Oort reasoned, a comet could not have formed while in its current orbit and must have been held in an outer reservoir for almost all of its existence" (ibid.). "After studies at the University of Groningen, Oort (1990-92) was appointed astronomer to the Leiden Observatory in 1924 and became director in 1945, a position he held until 1970. In 1925 Bertil Lindblad of Sweden had advanced the theory that the Milky Way rotates in its own plane around the centre of the galaxy. Oort was able to confirm this theory in 1927 through his own direct observations of star velocities in the galaxy, and he modified the theory substantially into the form used thereafter. "Oort's subsequent work, as well as that of the school of astronomy he developed in the Netherlands, was directed toward strengthening and testing the Lindblad-Oort theory. Soon after having become a professor at the University of Leiden (1935), he determined by radio astronomy that the Sun is 30,000 light-years from the centre of the galaxy and takes 225 million years to complete an orbit around it. The discovery in 1951 of the 21-cm radio waves generated by hydrogen in interstellar space provided him with a new method for mapping the spiral structure of the galaxy. "In 1950 Oort proposed that comets originate from a vast cloud of small bodies that orbit the Sun at a distance of about one light-year, and the approach of other stars toward this cloud alters some comets' orbits so that they pass close to the Sun. The existence of this region, which was named the Oort Cloud, eventually came to be accepted by most astronomers. "From 1958 to 1961 Oort was president of the International Astronomical Union, of which he had been general secretary from 1935 to 1948" (Britannica). Offprint from Bulletin of the Astronomical Institutes of the Netherlands, Vol. 11, 1950. 4to (296 x 225 mm), pp 91-110. Stapled in self-wrappers, as issued. Very fine.
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