History of Astrometric Measurements in Astronomy
"Parallax work costs a great many hours loss of sleep, and it is therefore a difficult research for one to
carry on who in addition to this work must engage in university teaching."
Parallax is the apparent change in position of a star due to the actual
change in our (the earth's) position in our orbit around the sun. A
photograph is taken of a star at one time during the year, and the
position of the star with respect to the background stars is measured.
Then a photograph is taken six months later, when the earth is on the
opposite side of the sun, and the position of the star with respect to
the background stars is measured again.
The star will appear to move slightly with respect to the background stars, and this motion is called its parallax. Using some simple geometry, we can use this parallax to calculate the star's distance away from the earth. Distances are highly prized quantities to obtain in astronomy, so the study of stellar parallaxes is of utmost importance to astronomers.
Unfortunately, the further a star is away from the earth, the smaller amount it will appear to move with respect to the background stars, so our knowledge of distances to stars which are further away than about 100 parsecs is not very good. To put that into perspective, the Milky Way Galaxy, the galaxy which contains the sun, is about 30,000 parsecs in diameter. So we really can't determine distances to very many stars even in our own galaxy in this way.
But astronomers are achieving more accurate distances to stars which are further and further away. Galileo (at left) was the first to attempt parallax observing, using a 1-inch diameter telescope in 1609. He was completely unsuccessful because the telescope just didn't have the power to detect the tiny parallax motions. Galileo was followed by scientists such as Hooke, Flamsteed, Picard, Cassini, Horrebow, and Halley during the next two centuries, who were similarly unsuccessful because their telescopes were simply too small. A telescope needs a very large magnification in order to measure such a small motion. It was not until 1838 that the first parallax was measured, and coincidentally it was done by three scientists independently using three different instruments to measure the parallax motion: Bessel with his heliometer, Struve with his filar micrometer, and Henderson with his meridian circle. Although Bessel (at right) made the most accurate measurements with his heliometer, a heliometer is an instrument which is of unique design and rather difficult to produce, and thus most observatories did not possess anything of the kind. Luckily, it was soon shown that the meridian circle that Henderson used could produce parallaxes to a satisfactory degree of accuracy, and almost every observatory at the time already owned a meridian circle. Thus, every observatory could do these kind of measurements, and every observatory could contribute to this important field of study.
The first attempts to determine parallaxes using photography were done during the period 1887-1889 by Pritchard at Oxford. Although there was considerable debate over the merits and even possibility of doing astrometry using photographs, photography turned out to be an excellent way to measure parallaxes, as the accuracy was much greater than using visual methods and the labor was much less intensive. Furthermore, by taking a photograph, a permanent record was made of the measurement, so that the image could be examined at once or later, and it could be remeasured again and again for new information. In 1900 Kapteyn designed a systematic method to take these photographs, in allowing each photographic plate to be exposed three times during a single night, and four nights spaced throughout the year, such that in the end there are twelve exposures for every star on the plate.
Parallax at McCormick ObservatoryWhen Samuel Alfred Mitchell (at right) was appointed as Director of the Leander McCormick Observatory in 1913, he deemed it appropriate to start photographic parallax work with the 26-inch telescope. Under the previous and founding director of the observatory, Ormond Stone, only visual observations had taken place, but in Mitchell's words, "It seemed wise that photographic work with the refractor should be started." He chose to begin with parallax studies mostly because so much work was already being done in the study of proper motions, how the stars moved with respect to each other, while only four telescopes at that time (Yerkes, Mt. Wilson, Grenwich, and Swarthmore) were working on parallax observations. Mitchell began by building a dark room in the observatory and by ordering a photographic plate holder from the John A. Brashear Company, a duplicate of one constructed for Sproul Observatory a few years earlier. The plate holder was delivered in May, 1914, and photographic work began in the fall of that same year.
Each photographic plate used at McCormick Observatory could take a picture of a 1 degree by 0.5 degree piece of the sky. In the beginning, two images of the sky were taken on each plate, and then once the plate was developed, it was placed under a microscope and the positions of the stars were measured with respect to each other. Three comparison stars were usually used to measure how far the parallax star had moved, although Mitchell tried to use 5 comparison stars whenever possible, for better accuracy. After 1915, Mitchell decided to take images of two separate regions on the same photographic plate, in order to conserve plates and money. This allowed the number of plates to be used and developed to be cut in half, and so was a great savings. Eventually, observers took 3, 4, 5, and even 6 regions on the same plate to save plates and money. Luckily, there was never trouble with stars from the two separate regions overlapping each other and the observers were able to relatively easily distinguish which stars were in each region by spacing out the observations in different patterns.
As astronomers at McCormick Observatory spent more and more time with the study of stellar parallax, it is not difficult to imagine that they produced and needed to measure many, many photographic plates. And although it was possible to sit and measure a plate with a microscope and ruler, it was not very practical. For this reason, as soon as the first plates were taken at the observatory, a measuring machine made by Repsold was borrowed from Dr. Harold Jacoby at Columbia University. This measuring machine had a built in microscope atop a moving stage where one could mount a photographic plate. The stage moved by way of long screws which could be twisted and then the amount of twist determined the distances between stars on the photographic plate. In 1916, a similar machine made by Wm. Gaertner and Son was purchased for the observatory's permanent use.
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