Early Methods of Measuring the
Distance to Stars
Prepared By: Saniat Sabbir
Prepared For: Professor Clarke
Due Date:
Course Code: SCI199Y
Measuring distances is one of the most important and complex tasks in astronomy. It requires time, effort and perseverance. Thanks to the early astronomer’s dedication, sincerity, today modern astronomers are able to use high tech technologies to measure distances of various planets and stars from earth. Their methods might not have been precise or accurate enough according to today’s standard but these early methods paved the way for major breakthroughs in the invention of modern techniques.
The cosmologies of Aristotle and
Ptolemy proposed that the stars were located immediately beyond the outermost
planet. When Copernicus in 1543 argued that the orbits the sun and the fact
that the stars can be observed every six months from opposite ends of a
diameter of the earths orbit, opponents wanted to know why there had been no
annual parallaxes detected. Later in the seventeenth century after Kepler
proposed a system where the sun was at the centre of the planetary system.
Annual parallax was hoped to become means of determining the distance of stars.
But if Descartes was right about the stars being physically equal to the sun,
then there distance must be enormous, and
the detection of annual parallax with instruments subject to shrinking and
warping with the passage of the seasons was daunting task. [1] However if the brightness of a star
(Sirius) could be correlated with the brightness of the sun, then since light diminishes with the square
of a distance, this ratio could be converted to a ratio of distances. [1] One major attempt was made by
Christiaan Huygens in his posthumous Cosmotheoros (1698). Huygens observed the
sun through a pinhole made in a screen, hoping that the visible fraction of the
sun would be equal to the brightness of the star Sirius. The technique was
clumsy, but his conclusion that Sirius
lay at 26664 astronomical units showed that the scale of interstellar distance
was vast. [1]
An improved
method was advanced by James Gregory in 1668. He came with the idea of
replacing Sirius by a suitable planet, at the time when the planet’s brightness
was the same as Sirius. The required
ratio was then equal to the ratio of t he light received directly from the sun,
to the right received from the sun via the planet. This calculation had the
merit of involving only quantities related to the solar system. [1]
Even with working with obsolete values, Gregory placed Sirius at 83 190 AU;
however the true distance would be much greater, in fact about 106 AU according
to
Furthermore,
a remarkable attempt in order to measure annual parallax was made by Robert
Hooke. The problem with this measurement was the possible warping o the instrument and the uncertain effect of refraction. Hooke
solved this problem by selecting the star Gamma Draconis, which passed directly
overhead his name in
The discovery of the “adoration
of light” proved the motion of the Earth around the sun. It also revealed an
error in existing star catalogs including Flamsteed’s. Since Bradley was
unsuccessful in determining annual parallax he was able to estimate from the
accuracy of his measures that the stars must be at least 400 000 AU from the
sun. His publication in the 1729 of this
minimum distance of stars was fully compatible with
The star will appear to move slightly to respect to the background stars, and this motion is called its parallax. Using simple geometry, parallax can be used 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 was very important to early astronomers.
When viewed from Viewpoint A, the object
appears to be in front of the blue square. When the viewpoint is changed to Viewpoint
B, the object appears to
have moved to in front of the red square. [4]
The further the star is away from the Earth, the smaller amount it will appear to move with respect to the background stars. Astronomers continued to create more accurate distance measurements for stars that are further away. Galileo was the first to attempt parallax observing using a 1-inch diameter telescope in 1609. He was completely unsuccessful because the telescope was not powerful enough to detect such tiny parallax motion. Galileo was followed by such scientists such as Picard, Cassini Horrebow and Halley during the next two centuries, who were also unsuccessful because their telescopes were too small. A telescope needed to have very large magnification in order to measure such a small motion. It was not until 1838 that the first parallax was measured, and was done by three scientists independently using three different instruments to measure stellar parallax: Bessel with his heliometer, Srure with his filar micro meter and Henderson with his meridian circle.
In 1835 Struve selected Vega for study. The star was bright and had a large proper motion, and in 1837 he deduced from 17 observations a parallax of one eighth of a second of arc. Encouraged by Struve’s success Bessel used parallax to determine the distance to 61 Cygni and announced his result in 1838. To succeed it was important to choose a star which was close to the sun. Bessel’s method of selecting a star was based on his own data. He chose the star which had the greatest proper motion of all the stars he studied, correctly deducing that this would mean that the star was nearby. [3] Since 61 Cygni is relatively dim it was a bold choice based on his correct understanding of the cause of the proper motion. Bessel used Fraunhofer heliometer to make the measurements and announced his value to be 0.314 which was about 10 light years, given the diameter of the earth’s orbit. The correct value of the parallax of 61 Cygni is 0.292.
Although
Bessel made the most accurate measurements with his heliometer, a heliometer
was an instrument which is of unique design and difficult to produce and most
observatories did not possess anything of the kind. Luckily, it was soon
discovered that the meridian circle that
The first attempts to determine parallaxes
using photography were done during the period 1887-1889 by Pritchard at
In 1913 Samuel Alfred Mitchel was appointed as director of Leander Mc Mormic Observatory and started photographic parallax work with a twenty-six inch telescope under the previous director, the observatory only conducted visual observations. Mitchel chose to focus on studying stellar parallax 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 were working on parallax observation. [2] Mitchel began by building a dark room in the observatory and ordered a photographic plate holder.
Each photographic plate used, could take a picture of a 1 degree by 0.5 degree piece of the sky. In the beginning, two sky images were taken on each plate and 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 Mitchel tried to use five comparison stars when ever possible for better accuracy. [2] After 1915, Mitchel decided to take image of two separate regions on the same plate in order to conserve plate and save money. Eventually, observers started to take three, four, five, and even six regions on the same plates to save plates and money. Fortunately, there were never any problems with the 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. [2]
As astronomers at McCormick
observatory spent more time with the study of stellar parallax, it is not difficult to imagine that they
produced and needed to measure many photographic plates. [2]
Measuring a plate with a microscope and ruler seemed impractical. To solve
this problem a measuring machine made by repsold was borrowed from Dr. Harold
Jacoby at
In summary, the methods of measuring the distance of stars mentioned above were a major breakthrough for the astronomers today. As time goes on, new approaches would be developed for this import research and the dedication of early astronomers towards this would always be and inspiration as we go towards a glorious future.
List of Sources
[1]Michael, Hoskin . Stellar Astronomy (to the rise of
Astrophysics in the Mid Nineteenth Century). Nov 2000.
[2]"History of Astrometric Measurements in
Astronomy." History of Astrometric Measurements in
Astronomy.
[3]"Frederich
Bessel." Frederich Bessel.
[4]"Parallax." Parallax.