  The Venus Transit 2004  ... Extended InfoSheet G3  Printable
Page: html      PDF  The definition of the Astronomical Unit and
the other units of distance in astronomy  The unit of length in
the international system (SI) is the metre, defined as follows:
the metre is the distance traversed by light in the vacuum in
the time interval of 1/299 792 458 of a second.  In astronmy,
the basic unit of distance is the "Astronomical Unit (AU)",
defined as follows: the radius of the circular orbit of a planet
of negligible mass and free of perturbations, whose mean motion
isl "k" radians per day around the Sun, where "k" is Gauss'
gravitational constant, the unit of time being the solar day and
the unit of mass, the mass of the Sun.  The basic unit in
astronomy is thus linked to the dimensions of the solar system
and to the motion of the Earth. Indeed, all distances in the
solar system may be deduced from only one of them, thanks to
Kepler's laws. The perturbations generated by the Moon and the
other planets on the Ear