Sky Survey and the Galaxy
William Herschel (1738–1822), British but born in Germany, originally a musician,
began to observe double stars in a wide field of the sky in around 1776 at his house in
Bath, England. By chance he discovered a new planet, Uranus. This made him
famous, and he was appointed court astronomer to King George III. Thereafter, he
constructed large telescopes (48- and 122-cm reflectors) and carried out survey
observations. As a result, he found 2500 nonstellar objects (star clusters and nebulae). William Herschel’s surveying activity was taken over by his son John Herschel
(1792–1871), who, together with his own observations, collected the data in the
General Catalogue (GC), which contained 5079 objects and was published in 1864.
John Louis Dreyers (1852–1926) at Armagh Observatory, Northern Ireland,
revised the GC and published the New General Catalogue (NGC) in 1888, and its
supplements, Index Catalogues 1 and 2 by 1910. These catalogs contain 13,196
objects in total.
Using a statistical method for stellar parallax and proper motion, Jacobus Cornelius Kapteyn (1851–1922) of Groningen University proposed a model of the Galaxy
in 1921 and 1922. The Galaxy in his model takes an ellipsoidal form with an axial
ratio of 5: 1, and its radius in the Galactic plane is around 9000 parsec. The Sun is
located near the Galactic Center. A similar model was also proposed by Hugo H. von
Seeliger (1849–1924) of the University of Munich in 1920.
In 1918, slightly earlier than the works of Kapteyn and Seeliger, Harlow Shapley
(1885–1972) at the Mt. Wilson Observatory was measuring the distance and distribution of globular clusters, based on the period-luminosity relation found by
Henrietta Leavitt. For this purpose, he measured the absolute magnitudes of nearby
Cepheid variables. The results for six globular clusters indicated that their distance
from the Sun were in a range of 6.5 to 18.9 kpc, and their vertical heights from the
galactic plane were in a range of 1.8 to 18.8 kpc. These values are one order of
magnitude larger than those found by Kapteyn’s and Seeliger’s models. Moreover,
the distribution of globular clusters shows a large concentration toward the constellation Sagittarius. Based on their distribution, Shapley depicted the large-scale
structure of the Galaxy as a somewhat flattened stellar system with the diameter of
the galactic plane around 10 kpc, with the Sun located far from the center.
On the other hand, Shapley regarded spiral nebulae as small objects distributed
around or inside the Galaxy, accepting the claim made in 1916 by A. van Maanen at
the Mt. Wilson Observatory about the existence of proper motions in the spiral nebula M
101. If any proper motion were detectable, that object should be a nearby galactic object.
In 1920, Heber Doust Curtis (1872–1942) of Lick Observatory paid attention to
the large variety of spiral nebulae with respect to their size and form. He insisted that
spiral nebulae are very distant objects outside the Galaxy, though he accepted
Kapteyn’s rather smaller model of our Galaxy.
Since the views of Shapley and Curtis were so different, the National Academy of
Sciences in Washington, D.C., organized an open discussion called the Great Debate
in 1920. Both discussions, however, proceeded in parallel without significant progress. In 1934, Shapley and van Maanen accepted the error of measurement of
Sky Survey and the Galaxy
269
William Herschel (1738–1822), British but born in Germany, originally a musician,
began to observe double stars in a wide field of the sky in around 1776 at his house in
Bath, England. By chance he discovered a new planet, Uranus. This made him
famous, and he was appointed court astronomer to King George III. Thereafter, he
constructed large telescopes (48- and 122-cm reflectors) and carried out survey
observations. As a result, he found 2500 nonstellar objects (star clusters and nebulae). William Herschel’s surveying activity was taken over by his son John Herschel
(1792–1871), who, together with his own observations, collected the data in the
General Catalogue (GC), which contained 5079 objects and was published in 1864.
John Louis Dreyers (1852–1926) at Armagh Observatory, Northern Ireland,
revised the GC and published the New General Catalogue (NGC) in 1888, and its
supplements, Index Catalogues 1 and 2 by 1910. These catalogs contain 13,196
objects in total.
Using a statistical method for stellar parallax and proper motion, Jacobus Cornelius Kapteyn (1851–1922) of Groningen University proposed a model of the Galaxy
in 1921 and 1922. The Galaxy in his model takes an ellipsoidal form with an axial
ratio of 5: 1, and its radius in the Galactic plane is around 9000 parsec. The Sun is
located near the Galactic Center. A similar model was also proposed by Hugo H. von
Seeliger (1849–1924) of the University of Munich in 1920.
In 1918, slightly earlier than the works of Kapteyn and Seeliger, Harlow Shapley
(1885–1972) at the Mt. Wilson Observatory was measuring the distance and distribution of globular clusters, based on the period-luminosity relation found by
Henrietta Leavitt. For this purpose, he measured the absolute magnitudes of nearby
Cepheid variables. The results for six globular clusters indicated that their distance
from the Sun were in a range of 6.5 to 18.9 kpc, and their vertical heights from the
galactic plane were in a range of 1.8 to 18.8 kpc. These values are one order of
magnitude larger than those found by Kapteyn’s and Seeliger’s models. Moreover,
the distribution of globular clusters shows a large concentration toward the constellation Sagittarius. Based on their distribution, Shapley depicted the large-scale
structure of the Galaxy as a somewhat flattened stellar system with the diameter of
the galactic plane around 10 kpc, with the Sun located far from the center.
On the other hand, Shapley regarded spiral nebulae as small objects distributed
around or inside the Galaxy, accepting the claim made in 1916 by A. van Maanen at
the Mt. Wilson Observatory about the existence of proper motions in the spiral nebula M
101. If any proper motion were detectable, that object should be a nearby galactic object.
In 1920, Heber Doust Curtis (1872–1942) of Lick Observatory paid attention to
the large variety of spiral nebulae with respect to their size and form. He insisted that
spiral nebulae are very distant objects outside the Galaxy, though he accepted
Kapteyn’s rather smaller model of our Galaxy.
Since the views of Shapley and Curtis were so different, the National Academy of
Sciences in Washington, D.C., organized an open discussion called the Great Debate
in 1920. Both discussions, however, proceeded in parallel without significant progress. In 1934, Shapley and van Maanen accepted the error of measurement of
Sky Survey and the Galaxy
269