The paper outlines the history of attempts to explain the Milky Way, from Antiquity to the early-twentieth century, with special reference to the eighteenth and nineteenth centuries. Also discussed is the relationship of the Galaxy to other star systems, and particularly the question of whether there are other galaxies in the visible universe.

If Jacobus Cornelius Kapteyn (1851–1922) were here today, he would undoubtedly be among those asking the new questions on the cutting edge of contemporary astronomy. It is likely that he would even find a survey of his own contributions to the study of the Milky Way irrelevant. Nevertheless, Kapteyn's life-long interest in the Milky Way shaped the work of many astronomers, including, of course, his students, Willem de Sitter, H.A. Weersman, Pieter van Rhijn, and through van Rhijn: Jan Oort, Bart Bok and many others. But Kapteyn's influence extended far beyond his native Holland. After Kapteyn became a close colleague of George Ellery Hale and a Research Associate at the Mount Wilson facilities in 1908, Hale began to employ a number of Dutch astronomers, including van Rhijn, Adriaan van Maanen, and Kapteyn's Danish future son-in-law, Ejnar Hertzsprung. Moreover, Kapteyn's astronomical colleagues world-wide found his enthusiasm and penetrating insights infectious.

“The Copernicus of the sidereal system is not to be expected for many generations”. So wrote R.A. Proctor1 in his Essays in Astronomy in 1872. Indeed things did look bleak at this time for those who hoped for a good understanding of the size and structure of the Galaxy. Why was this so, and why was there to be such an astonishing transformation of this situation between 1918 and 1930? Certainly these twelve years saw the widespread acceptance of no less than six fundamentally new ways of viewing the Galactic System. These profound shifts, occurring in such a short time, form, I would suggest, one of the most exciting chapters in the entire history of astronomy. And in this paper I shall attempt to describe and analyze what these changes were, what led up to them, as well as to examine the events surrounding them.

Attempts in the 1930s and 1940s to determine the spiral structure of the Milky Way by star counting methods, essentially the continuation of the work of the Kapteyn Astronomical Laboratory, failed to reach this goal. A new foundation for the search was laid by Walter Baade in his studies of stellar populations. With the recognition that highly luminous objects, especially H II regions, would outline the spiral structure, W.W. Morgan and his young associates Sharpless and Osterbrock carried out the observational program that first delineated, in 1951, the nearby arms of the Milky Way. The full paper was never published, so the historical details have remained somewhat vague, primarily because the 21-cm discoveries so quickly overtook the optical researches.

Mass models of the Galaxy play an important role in studies of the structure of the Galaxy. The various populations or components combine to yield a gravitational field that produces the observed rotation curve. For the spheroid and disk this requirement can be used to set limits on some of their properties. The properties of the dark corona are entirely defined this way.

The construction of models is the most effective tool for a synthesis of various observational data and for a quantitative study of physical and dynamical structure and evolution of stellar systems. Classical models of spiral galaxies were based on rotational velocities, which were identified with circular velocities. They were designed to represent the galactic attraction force in the radial direction.

Although rotation-curve studies of spiral galaxies have unambiguously established the presence of dark matter, and theoretical studies have shown that its location is likely to be in a separate spheroidal halo component (Binney, 1978; Tubbs and Sanders, 1979; Monet, Richstone and Schechter, 1981), very little is known about its spatial distribution and its nature. Recently, Faber and Lin (Faber and Lin, 1983; Lin and Faber, 1983) have shown that, if one can get a rough idea of fundamental parameters like the halo scale length and the halo-to-disk ratio, it is also possible to put strong constraints on the nature of non-luminous matter.

At the IAU General Assembly in Patras in August 1982, Commission 33 set up a Working Group on the Galactic Constants. The Working Group is charged with developing a critical review of the values of the main galactic constants, for publication before the General Assembly in 1985. It has not been specifically charged to come up with a proposal for a revised set of values, although it can do so if it wishes.

New radial velocities and optical distances for partly very distant HII regions give evidence for a rising rotation curve in the outer parts of the Galaxy.

The most recent determination of Oort's galactic rotation constant ‘A’ from open clusters was made by Taff and Littleton (1972). They obtained A=15 km/s/kpc, but unfortunately they omitted a detailed description of their cluster sample. For the present work we compiled a catalogue of O-B3 clusters for which radial velocities and distances are available. The individual cluster references given by Janes and Adler (1982) have been used to find best distances for the clusters. Radial velocities were taken from the list of Dr. Wramdemark (private communication) and from Hron et al. (1984). The catalogue contains 105 clusters distributed over the four galactic quadrants as 16:27:35:27.

The existence of variations from a smooth curve, in the form of waves, in the rotation curves of galaxies was pointed out earlier, and an interpretation was proposed based on the argument that the waves were the manifestation of the coexistence of different populations in a galaxy (see for example PişLmiş 1965, 1974). Observations in the past few years have shown that “undulations” in the rotation curve of spiral galaxies are rather common phenomena; maxima and minima occur roughly at arm and interarm regions, respectively. The velocity fields of the majority of the 23 galaxies compiled by Bosma (1978) exhibit well-defined waves. In particular the velocity field in the 21-cm HI line of M81 by Visser shows clearly the correlation of the waves with the spiral structure.

Using unpublished 21-cm data from Jackson and Kerr of the entire Galaxy at b = 0°, we have determined the terminal velocities, vm, to be (Gill & Shuter, 1983)

We review the kinematical and chemical properties of the old population of our Galaxy. Comparison is made with the properties of the old population in other disk galaxies.

The inner region of the Galaxy has been explored by means of near-infrared observations; the distribution and population of the stars are studied from the near-infrared brightness mapping and star counts in the Milky Way, while the magnetic-field configuration is probed by the near-infrared polarimetry.

We find values of the interstellar absorption from J, H and K scans of a 6.8 × 200 arcmin strip of sky extending from the Sgr I field to the Galactic Centre. The visual absorption, excluding dark clouds, increases from 3 to 30 magnitudes in this region.