C.V.L. Charlier’s book on stellar statistics is an enjoyable read. He writes clearly. The book is online, via Project Gutenberg, and is about 50 pages. The URL is below.
Charlier in 1911 proposed the word “siriometer” for interstellar distances. One siriometer is 1 million astronomical units, or approximately twice the distance from Sol to Sirius. Alternatively, 1 siriometer is 15.79 lightyears, or a parallax of 0.206265 arseconds. That is, a parsec is 0.206065 siriometer, and a siriometer is 4.848 parsecs.
Charlier notes (section 4) that Seeliger (and other German astronomers) had long used another distance measure, the Siriusweite which is defined as a parallax of 0.200 arcseconds. The idea of using (twice the) Sol-Sirius distance as a baseline, one variant or another, has a long pedigree. I think one reason the parsec (a term introduced in 1912) caught on is that it is brief — two syllables instead of five.
Another remark of Charlier, also in section 4, is upon the inadvisability of defining a length unit (the Siriusweite, or the parsec) in terms of an angle, particularly an angle which corresponds to the harmonic mean distance of a star, not to its arithmetic mean distance. That remark has merit. We are “saved” from that confusion nowadays because we have redefined the parsec in terms of a distance, a certain number of meters. Astrometric observations, such as by the Gaia space observatory, make appropriate corrections to their measurements of angular displacement. The angle vs distance confusion is not unlike the question of temperature T versus inverse temperature 1/T in thermodynamics, mentioned in my earlier post about Peter Atkins’ book, “Four Laws that Drive the Universe”.
In section 19, Charlier describes the technique used to prepare the original Bonner Durchmusterung catalogue of stars. Fascinating to read! Quite similar to Gaia’s method. A wide field of observation, 6 degrees, with all stars of 9th magnitude or brighter passing through a 1-degree-wide strip of declination noted, manually recording time, declination and magnitude as the stars passed a central hour line. One person called out the declinations and magnitude, another (in a lighted adjacent room) recorded that info with time. They could attain a data rate as high as 30 transits per minute. The original BD catalog was prepared by Argelander, over 625 observing nights in 1852-1859, and contained 324,198 stars. What an achievement!
It’s interesting, when reading an astronomy book from a century ago, to reflect upon the many changes which have happened, improvements in instruments and in concepts. That time distance may be 3 generations, but it is only 2 professional careers.
Relevant links:
The Project Gutenberg page for reading Charlier’s book, “Lectures on Stellar Statistics”, 1921:
http://www.gutenberg.org/ebooks/22157
The Wikipedia page for Siriometer:
http://en.wikipedia.org/wiki/Siriometer
The motivation to read Charlier came via the Project Gaia DPAC newsletter number 05, July-2009:
http://www.cosmos.esa.int/web/gaia/dpac/newsletter
My earlier post about Peter Atkins’s book:
Best wishes,
Ken Roberts
27-Jan-2014
Logic, Astronomy, Science, and Ideas Too 