Editing United States Naval Observatory (section)

=== Measuring the astronomical unit ===
An early scientific duty assigned to the Observatory was the U.S. contribution to the definition of the [[Astronomical unit|Astronomical Unit]], or the {{sc|AU}}, which defines a standard [[arithmetic mean|mean distance]] between the Sun and the Earth. This was conducted under the auspices of the congressionally-funded U.S. Transit of Venus Commission. The astronomical measurements taken of the [[transit of Venus]] by a number of countries since 1639 resulted in a progressively more accurate definition of the {{sc|AU}}.

Relying strongly on photographic methods, the naval observers returned 350&nbsp;[[photographic plate]]s in 1874, and 1,380&nbsp;measurable plates in 1882. The results of the surveys conducted simultaneously from several locations around the world (for each of the two transits) produced a final value of the [[parallax|solar parallax]], after adjustments, of 8.809″, with a probable error of 0.0059″, yielding a U.S.-determined Earth-Sun distance of {{cvt|92,797,000|mi|km}}, with a probable error of {{cvt|59,700|mi|km}}. The calculated distance was a significant improvement over several previous estimates.<!-- The preceding paragraph contains text written by a U.S. Government employee, which makes it copyright-free and thus in the public domain. --><ref name="NASA-Dick">{{cite journal |last=Dick |first=Steven J. |orig-year=2004 |title=The American Transit of Venus Expeditions of 1874 and 1882 |journal=[[Proceedings of the International Astronomical Union]] |pages=100–110 |doi=10.1017/S1743921305001304 |date=23 May 2005 <!-- Published online 23 May 2005 --> |volume=2004 |bibcode=2005tvnv.conf..100D |type=abstract |doi-access=free}}</ref>