 History of Astrometric Measurements in Astronomy
Parallax
"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.
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 Observatory  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 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 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,  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
 photographic 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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