Re: Sun's distance

[brian whatcott <inet@INTELLISYS.NET>]

At 00:00 2/21/00 +0000, Peter Vajk wrote:
> In 1963...  I
> researched the work of the French 18/19th century astronomer de La
> Lande,  who published a three-volume Encyclopedia of Astronomy circa
> late 1790's,  with a supplement published around 1810...
> While relative sizes of orbits were known since antiquity,  the absolute
> scale (i.e., the length of the Astronomical Unit (A.U.) -- the semimajor
> axis of the Earth's orbit) was very difficult to find.

In defining the AU, we prefer the Copernican to the Keplerian construction
so we call it the *mean* distance between Sun & Earth (149 597 870 km)

> ... When combined with
> observations of both transits from more northern latitudes,  these
> observations succeeded in pinning down the A.U.  to a fraction of one
> percent....
> Peter Vajk

Triangulation of sightings of the near approach of the Eros asteroid
to Earth in 1931 provided further refinements.

I should also mention, lest we fall to the easy trap of "they then
thought x...we now know y..." that the Keplerian notion of elliptical
orbits has to be embellished with what I could call numerous epicycles
if an accurate prediction for the Lunar orbit is required.
(The solar influence and gravitational anomalies of the Earth and Moon
are complicating factors).

I want to say how much I enjoyed Peter's exposition.


brian whatcott <inet@intellisys.net>
Altus OK