The parallactic instrument of Kapteyn is a

measuring instrument A measuring instrument is a device to measure a physical quantity. In the physical sciences, quality assurance, and engineering, measurement is the activity of obtaining and comparing physical quantities of real-world objects and events. Establ ...

created by the Dutch astronomer

Jacobus Kapteyn Prof Jacobus Cornelius Kapteyn FRS FRSE LLD (19 January 1851 – 18 June 1922) was a Dutch astronomer. He carried out extensive studies of the Milky Way and was the discoverer of evidence for galactic rotation. Kapteyn was also among the fi ...

around 1886. Using this instrument, Kapteyn analyzed over 1,700 glass plate photos of

stars A star is an astronomical object comprising a luminous spheroid of plasma held together by its gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night, but their immense distances from Earth ma ...

seen from the southern hemisphere.van der Kruit, P. C. (2015) Jacobus Cornelius Kapteyn, Born Investigator of the Heavens. Springer, Switzerland (p.176-182, j 204 (citing Wessel Krul), 217, h 496) This research contributed to the

Cape Photographic Durchmusterung In astronomy, Durchmusterung or Bonner Durchmusterung (BD) is an astrometric star catalogue of the whole sky, compiled by the Bonn Observatory in Germany from 1859 to 1903. The name comes from ('run-through examination'), a German word used for ...

, a

star catalogue A star catalogue is an astronomical catalogue that lists stars. In astronomy, many stars are referred to simply by catalogue numbers. There are a great many different star catalogues which have been produced for different purposes over the years, ...

containing 454,875 entries. Together with the measurements of stars seen from the northern hemisphere (the

Bonner Durchmusterung In astronomy, Durchmusterung or Bonner Durchmusterung (BD) is an astrometric star catalogue of the whole sky, compiled by the Bonn Observatory in Germany from 1859 to 1903. The name comes from ('run-through examination'), a German word used for ...

) the measurements of Kapteyn formed a complete star catalogue with a scope and accuracy that was impressive for its time. The instrument is currently located in the collection of the University Museum of Groningen.

Origin

Since Kapteyn lacked an observatory of his own in

Groningen Groningen (; gos, Grunn or ) is the capital city and main municipality of Groningen province in the Netherlands. The ''capital of the north'', Groningen is the largest place as well as the economic and cultural centre of the northern part of t ...

, he used a homemade instrument for the analysis of glass plate photos of stars, made by his colleague David Gill in

Cape Town Cape Town ( af, Kaapstad; , xh, iKapa) is one of South Africa's three capital cities, serving as the seat of the Parliament of South Africa. It is the legislative capital of the country, the oldest city in the country, and the second largest ...

. Kapteyn built the instrument with several parts from other (measuring) instruments. Although Kapteyn called it a ‘parallactic instrument’, the instrument is not related to the parallax effect. The name may come from the chassis of the instrument, which is originally from an instrument with a 'parallactic mount'.

Use

Parallactic Instrument of Kapteyn and Glass Plate Holder

Glass plates - Durchmunsterung Vol1 - facing page xx

Three researchers were needed to perform measurements with the instrument, each with their own task: # Aiming the lens at a star, estimating the

diameter In geometry, a diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints lie on the circle. It can also be defined as the longest chord of the circle. Both definitions are also valid for ...

of the star, and reading the

declination In astronomy, declination (abbreviated dec; symbol ''δ'') is one of the two angles that locate a point on the celestial sphere in the equatorial coordinate system, the other being hour angle. Declination's angle is measured north or south of the ...

. # Reading the

right ascension Right ascension (abbreviated RA; symbol ) is the angular distance of a particular point measured eastward along the celestial equator from the Sun at the March equinox to the (hour circle of the) point in question above the earth. When paired w ...

using a small microscope. # Writing down the results, as told to him by the other researchers. To use the instrument, the researcher must look through the

ocular Eyes are organs of the visual system. They provide living organisms with vision, the ability to receive and process visual detail, as well as enabling several photo response functions that are independent of vision. Eyes detect light and con ...

(part J), and aim the lens (H) at a glass plate photo (see drawing). The distance between the center point of the instrument and the plate to be measured must be the same distance as the

focal length The focal length of an optical system is a measure of how strongly the system converges or diverges light; it is the inverse of the system's optical power. A positive focal length indicates that a system converges light, while a negative foca ...

of the telescope that was used to take the photos (in the case of Gill's photos: . By rotating the right axis (B) the researcher can aim the lens at a star of interest. The researcher can read the position of the star on the wheel (D) below the right axis (B). Similarly, parts A and C can be used to determine the right ascension. Part L is no longer on the instrument. Using this smaller telescope the researcher could correctly position the instrument in relation to the glass plate photo. For each position on the sky, Kapteyn used two photos (each made on a different night). He placed these photos in sequence (with approximately 1 millimeter of space in between), with one being slightly displaced. This allowed him to easily distinguish stars from dust particles on the glass plate.

Use by Kapteyn

Kapteyn and his staff members analyzed the first photo (aimed at the

south pole The South Pole, also known as the Geographic South Pole, Terrestrial South Pole or 90th Parallel South, is one of the two points where Earth's axis of rotation intersects its surface. It is the southernmost point on Earth and lies antipod ...

) on October 28, 1886 and the final photo (aimed at 85° declination) on June 9, 1887. They used in the instrument in a laboratory of Dirk Huizinga, a professor in

physiology Physiology (; ) is the scientific study of functions and mechanisms in a living system. As a sub-discipline of biology, physiology focuses on how organisms, organ systems, individual organs, cells, and biomolecules carry out the chemical ...

who made two of his rooms available to them. Kapteyn and his staff members analyzed the glass plate photos in duplicate and darkened the room to get a better view of details in the photos. Kapteyn and his staff performed some repeat measurements in 1892, 1896, 1897 and 1892. Kapteyn and Gill published their Durchmusterung in three volumes that together formed the Cape Photographic Durchmusterung: declination zones -18° to -37° (1896), -38° to -52° (1897) and -53° to -89° (1900).

Influence on the private life of Kapteyn

Working with the instrument had a significant impact on the health and private life of Kapteyn. Kapteyn often felt pain in his eyes and stomach and became easily agitated due to the intense labor. After completing one of the last measurements, Kapteyn wrote to Gill: "...- and the truth is that I find my patience nearly exhausted", with which he referred to the analysis for the Cape Photographic Durchmusterung. Additionally, Kapteyn wrote about working on the Durchmusterung: "There is a sort of fate that which makes me do my life long just what I want to do least of all."

Prisoners

The British astronomer

Arthur Stanley Eddington Sir Arthur Stanley Eddington (28 December 1882 – 22 November 1944) was an English astronomer, physicist, and mathematician. He was also a philosopher of science and a populariser of science. The Eddington limit, the natural limit to the lumin ...

claimed that prisoners were part of the staff of Kapteyn that worked with his instrument. However, this fact is deemed implausible, since prisoners only performed relatively simple tasks in this time period and because this fact was never brought up in any correspondence with Kapteyn.

Impact

The publication of the measurements performed with the instrument of Kapteyn marked a major breakthrough for Kapteyn in the field of astronomy. In 1901 Kapteyn was the first Dutchman to receive a golden medal from the British

Royal Astronomical Society (Whatever shines should be observed) , predecessor = , successor = , formation = , founder = , extinction = , merger = , merged = , type = NGO ...

. Kapteyn had been a member of this organisation since 1892. Furthermore, working with the instrument may have inspired the theories of Kapteyn about the shape of the

Milky Way The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye ...

. Kapteyn first discussed these theories in 1891 during a rectorial speech. The American astronomer

Simon Newcomb Simon Newcomb (March 12, 1835 – July 11, 1909) was a Canadian–American astronomer, applied mathematician, and autodidactic polymath. He served as Professor of Mathematics in the United States Navy and at Johns Hopkins University. Born in Nov ...

praised Kapteyn and his work: "This work he Cape Photographic Durchmusterungof Kapteyn offers a remarkable example of the spirit which animates the born investigator of the heavens." Jacob Halm remarked that the results of the Cape Photographic Durchmusterung had an accuracy comparable to that of the results of the northern hemisphere. The astronomer Henry Sawerthal, who visited the laboratory of Kapteyn in 1889, described the results as "...sufficient in the present instance to give results more accurate than those of the Northern Durchmusterung, a remark which not only applies to positions, but to magnitude (also)." The German astronomer

Max Wolf Maximilian Franz Joseph Cornelius Wolf (21 June 1863 – 3 October 1932) was a German astronomer and a pioneer in the field of astrophotography. He was the chairman of astronomy at the University of Heidelberg and director of the Heidelberg-K� ...

had such admiration for the instrument of Kapteyn that he built his own 'improved' version of the instrument.Wolf, M. (1902) Der parallactische Messaparat, Publikationen des Astrophysikalischen Instituts Koenigstuhl-Heidelberg, Vol. 1, p. 5-10
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References

{{reflist Astronomical instruments