While collecting material for a book, I needed to consult an 1882 paper dealing with the orbit of Mercury, innermost planet in our Solar System, that appeared in the American Ephemeris and Nautical Almanac, published by the Bureau of Navigation in the Navy Department. In my book, I plan to show how astronomers can combine historical with contemporary observations, to extend the time base over which small annual changes accumulate to a measurable size.
The positions of Mercury, against the background of distant stars, have been tracked for thousands of years. Copernicus made use of some of these observations in his 1543 De Revolutionibus Orbium Coelestium. After the invention of the telescope in 1609, observational precision increased significantly, and by the end of the 18th Century it was noted that Mercury's elliptical orbit was not fixed in space; its long axis was rotating ('precessing') very slowly.
Some precession was expected, because of the gravitational attraction of the other planets. In 1859, Urbain LeVerrier made this calculation but could account for only about 90% of the observed precession, with an unexplained residue of 38 arc seconds per century. (There are 3600 arc seconds in one degree.) This calculation was repeated by Simon Newcomb, who made use of planetary observations dating back to 1677, and derived the more precise value of 43 arc seconds per century for which no explanation could be found. This is a remarkably small angle; the anomalous precession of 43 arc seconds comes from an annual shift of angle about as wide as an inch seen from a distance of three miles.
The recognition of this minute effect is a tribute to the precision of astronomical observations and the quality of the calculations carried out by Newcomb, who was Superintendent of the Bureau of Navigation. His definitive 1882 paper appeared in the American Ephemeris and Nautical Almanac.
One suggested cause for this discrepancy was that it came from the gravitational attraction of an as-yet-undiscovered planet. This approach had been used, with great success, to explain irregularities in the orbit of Uranus. Calculations by John Couch Adams and by LeVerrier, using Newton's theory of gravitation and based on Uranus' observed path, suggested where a new planet might be found, and Neptune (as now named) was discovered in 1846 - a spectacular confirmation of Newton's theory. It was natural that the same approach be tried to explain the discrepancy found in Mercury's orbit. After a false alarm, careful searches produced nothing.
There the matter of this puzzling anomaly rested, until 1915. Einstein, in his new General Theory of Relativity, suggested a radically different view of gravity and concluded that this should show up in several ways, including an orbital precession of 43 arc seconds per century for Mercury. As he noted, his theory "explains.... quantitatively .... the rotation of the orbit of Mercury ...without the need of any special hypothesis ..... For a few days, I was beside myself with joyous excitement."
In reviews of General Relativity, the precession of Mercury's orbit is always cited, but the full historical base that Newcomb used in his calculations is almost never mentioned. I wanted to describe the historical background, and needed to consult Newcomb's 1882 paper. I was delighted to discover the old Ephemeris and Almanac in our Archives.
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