Image 1 Caption: The 1922 Kapteyn universe (a Milky Way model) of Jacobus Kapteyn (1851--1922) displayed in a contour map of a cross section of the Milky Way disk.

Features:

1. The Kapteyn universe is symmetrical about the horizontal axis and only the upper half is displayed in Image 1.

2. The Kapteyn universe is at least approximately an ellipsoid of stars with diameter ≅ 17 kpc, maximum thickness ≅ 3 kpc, and the Solar System offset from the center by ≅ 0.65 kpc (Imamura: Herschel, Kapteyn, and the Milky Way, No-490).

3. The Kapteyn universe was determined by a statistical method that was similar to, but much more advanced, than the method of star gages used by William Herschel (1738--1822) in his pioneering work to map the Milky Way.

   Both Herschel and Kapteyn failed to get the correct Milky Way structure and the Solar System location in the Milky Way.

   The 2 essential reasons for failure were (1) they could NOT measure distances to stars and (2) interstellar dust prevents visible band observations of the Milky Way disk beyond ∼ 3 kpc (FK-563). The Milky Way disk is ∼ 30 kpc in diameter, but the value depends on how exactly one defines the Milky Way diameter (see Wikipedia: Milky Way: Size).

   Image 2 Caption: Jan Oort's (1900--1992) illustration of the discrepancy between the Kapteyn universe of Jacobus Kapteyn (1851--1922) and Harlow Shapley's (1885--1972) system of Milky Way globular clusters. It originally appeared in Willem de Sitter's (1872--1934) book Kosmos (1932).

   Shapley's system gave a much better idea of the size of the Milky Way and the Solar System's location relative to the Milky Way center (AKA Galactic center of mass): distance = 74--87 kpc = 24--28.4 kly.

1. Christopher Palma, before 2024, The Shape of the Milky Way from Starcounts.
2. J.C. Kapteyn, 1922, 55, 302, First Attempt at a Theory of the Arrangement and Motion of the Sidereal System, NASA/ADS.
3. James Imamura, before 2024, Astronomy 123: Cosmology and Our Place in the Universe: Herschel, Kapteyn, and the Milky Way.
4. No-490.
5. P.C. van der Kruit, 2014, p. 3.
6. Wikipedia: Jacobus Kapteyn: Biography.

Images:

1. Credit/Permission: Jacobus Kapteyn (1851--1922), 1922 / Believed to be Public domain.
   Download site: The Shape of the Milky Way from Starcounts by Christopher Palma.
   Image link: Itself.
   
2. Credit/Permission: © Jan Oort's (1900--1992), 1932 (appearing in P.C. van der Kruit, 2014, p. 3) / No permission.
   Image link direct: Itself.
   Image link: Placeholder image: alien_click_to_see_image.html.