McCormick Museum

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."

-Samuel Alfred Mitchell, Second Director of McCormick Observatory

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.

Parallax of a Nearby

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.Galileo 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. Friedrich
Bessel 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 Observatory

Samuel Alfred Mitchell When 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.

Proper Motion

While parallax observations give us information about how far away a star is from us, the study of proper motions tells us how these stars are moving in space, relative to each other, in 3 coordinates. Although we see the motion of celestial objects projected on the plane of the sky (in 2 dimensions), in reality, these objects are moving in 3 dimensions. Radial motion is the motion of an object along our line of sight, and this can be calculated using the Doppler shift of the object's spectral lines. For astronomers, this is a relatively easy task. Finding the motion of an object in the other 2 dimensions, or coordinates, is a bit tougher. The motion of an object in these other two coordinates, perpendicular to the line of sight, are what is known as the proper motion. Finding the proper motion of a celestial object takes up much time an energy in the life of an astronomer because he or she has to wait long periods of time to actually observe the physical motion across the sky of the object. Only after seeing how the star moves over many years relative to background stars can we calculate the proper motion of the star.

Barnard's Star in 1950Barnard's Star in 1997
Barnard's star was discovered in 1916 by E.E. Barnard and is has the largest known proper motion of any star, about 10 arcseconds per year. The picture on the left above is the star in 1950 and on the right is the star in 1997.

Edmund HalleyEdmund Halley (at left) was the first to discover the proper motion of stars, in 1718. He was using positions of stars recorded by Ptolemy in the second century A.D. and realized that the positions of some of th e stars were not the same as they had been 1600 years earlier. Some of the stars were slightly displaced from the rest, by a small but noticeable amount. Later studies showed that the motion of these stars was constant in speed and direction. In 1783, William Herschel discovered the solar motion, or the sun's motion relative to the stars in its galactic neighborhood. And by the mid-1800s, the study of proper motions was one of the largest fields of astronomical research being done in the world.

Proper Motion Work at McCormick Observatory

Even though the work of McCormick Observatory was initially focused mostly on parallax observations, Peter van De Kamp the arrival of Peter van de Kamp (at left) in March of 1923 from Holland started what turned out to be a very productive study of the proper motions of stars. van de Kamp and Harold Alden began the study by measuring positions of stars on parallax plates, which went back almost 10 years by now, and so they could get the proper motions of some stars by comparing their positions on the older plates with those on newer ones. Accuracy of the proper motion studies depended very much on the length of time between Peter van de Kamp and Alexander
Vyssotsky at McCormick Observatoryphotographic plates--the longer the time interval, the more accurate the measurement could be made. Nowadays, 10 years does not seem like nearly enough time to make a good measurement by these techniques. As van de Kamp himself stated in 1969, "In the old days we were happy if we had an interval of ten years; now these people have an interval of 50 years. Just imagine. Do they know how lucky they are?"

Although van de Kamp was only funded for a year's residence at the observatory, he and Alden managed to measure a total of 321 Boss star proper motions in 287 regions, relative to 1905 comparison stars. Van de Kamp joined the observatory permanently in 1925, and along with Alden and Alexander Vyssotsky, he measured proper motions until leaving in 1937 to direct the Sproul Observatory at Swarthmore College. Van de Kamp and Vyssotsky are pictured at right next to the McCormick telescope.

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