```From: Henry Spencer <henry@zoo.toronto.edu>
Newsgroups: sci.space.science
Subject: Re: Astronomical Units
Date: Tue, 24 Mar 1998 05:31:13 GMT

>...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

>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

```