From: Henry Spencer <henry@zoo.toronto.edu> Newsgroups: sci.space.science Subject: Re: Astronomical Units Date: Tue, 24 Mar 1998 05:31:13 GMT In article <6f1ud3$7rj@lace.colorado.edu>, Frank Crary <fcrary@rintintin.Colorado.EDU> wrote: >...If you mean using the distance between the Earth and the Sun as a unit >of length, then the use is ancient... It is, in fact, an obvious thing to do, given that observational astronomy works almost entirely in angles, which means it tells you proportions but not absolute distances. So if you want to work with distances, you almost inevitably end up picking one as the unit and expressing all others as multiples of that. From the 18th century well into the 20th, measuring the size of the AU was a significant concern of astronomy. Remember that the primary objective of one of Captain Cook's voyages (the first?) was not to explore the hinterlands, but to observe the transit of Venus from a known location roughly on the other side of the Earth from Europe, to determine the distance from the Earth to the Sun as a multiple of Earth's diameter. (That technique did not work very well because it's hard to determine exactly when Venus crosses the edge of the Sun's disk. Various other semi-satisfactory methods were used over the years, all of which were rendered completely and finally obsolete when the distance to Venus was measured directly by interplanetary radar.) >As far as actually calling this distance the ``astronomical unit'', I'm >not sure who first used that. The usage dates from somewhere between >the mid-nineteenth century and the early twentieth century. A bit farther back, I think. The first person to do serious astronomical mathematics in recognizably modern form was Gauss, at the beginning of the 19th century. In particular, he originated the idea of putting together a complete system of units -- for calculations, not just results -- with the AU as the unit of length, the day as the unit of time, and the Sun's mass as the unit of mass, with constants (notably the constant of gravity) adjusted to match. Almost certainly "astronomical unit" is an English translation of whatever term he used. (Just when that translation became common usage among English-speaking astronomers I'm less sure. Alas, the OED -- at least, the old edition -- does not list it.) Incidentally, the Earth's mean distance from the Sun is 1.00000003 AU, not 1 AU. It was realized quite early that the exact sizes of Gauss's basic units might change slightly as measurements improved. So it was decided that Gauss's value for the constant of gravity was exact by decree, and that *that*, and not the Earth's orbit, defined the size of the AU -- this turned out to require much less recalculation. -- Being the last man on the Moon | Henry Spencer is a very dubious honor. -- Gene Cernan | henry@zoo.toronto.edu

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