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Astronomy In California 1850 – 1950:
Telescope Makers, Telescopes, and Artifacts
Lick Observatory Radial Velocity Program Artifacts:
- The new Mills Spectrograph
- Radial Velocity Spectrogram Measuring Machine
- Book: Catalogue of Radial Velocities
Lick Observatory's first director, Edward S. Holden, considered the measurement of "stellar velocities in the line of sight," or "radial velocities," an important program for the Observatory. But it was left to one of his successors, William Wallace Campbell (1862-1938) to implement it. Campbell changed the direction of Lick Observatory research from that of pioneering astrophysical investigation to the steady accumulation of radial velocity data when he assumed the Directorship in 1901. He was a great observational astronomer, one of the dominant figures in American astronomy for over a quarter of a century. The spectroscopic radial velocity program that he planned, organized, and directed became the backbone of Lick Observatory for almost 30 years.
In 1907, W.W. Campbell acquired the new Mills Spectrograph, made by John A. Brashear and Co., with funds from local businessman D.O. Mills. Mr. Mills was a trustee for the will of James Lick, whose bequest of $700,000 funded the observatory in 1887. The new Mills Spectrograph was extremely rigid, insulated, and temperature-controlled so that it would not expand or contract from early evening to dawn. These improvements made it possible to take long exposures of faint stars without blurring their spectrograms.
With as many as 58 workers (astronomers and assistants), Campbell began Lick Observatory's monumental and systematic radial-velocity program. They measured the shift (increase or decrease) of all wavelengths in a star's spectrum, which directly gives the radial velocity. Photography also made it possible to make a permanent record of the spectrum of a star, a "spectrogram," on which the shifts of the spectral lines were measured and then recorded. The program proved to be long and tedious, but the results were important. By measuring the velocities of many stars all over the sky, Campbell could determine the average motion of our sun with respect to the other stars. Today, the Sun's relative motion is considered to be a rate of 12 miles per second toward a point near the constellation Hercules.
In 1928, Campbell and Joseph H. Moore published the resultant catalogue of radial velocities of stars brighter than visual magnitude 5.5. The catalogue embraced 2,771 stars and was complete down to the limiting magnitude of all but 69 objects. About 60 percent of some 15,000 spectroscopic plates analyzed for the catalogue were taken at Mount Hamilton up to the year 1927 with the 36-inch Lick refractor.
The data on stellar radial velocities that came out of the "doppler" program were Campbell's main contribution to astronomy.
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