As discussed in Chap. 3, the LMC, the SMC and the Galaxy form an interacting system. At times the interactions have had severe effects. Features observed in the LMC and the SMC, including radial velocities of individual objects, are therefore not necessarily determined solely by their rotation and motions.

When the first HI 21 cm-line radial velocities were measured, it was concluded that both Clouds were rotating systems, flattened and tilted and with extensive spiral structures (Kerr and de Vaucouleurs 1955a,b). The rotational motions were at first looked for using the optical centres, 5h24m, −69°.8 for the LMC and 0h51m,−73°. 1 for the SMC. In order to obtain symmetrical rotation curves it was, however, necessary to introduce radio centres of rotation: in the LMC, at 5h20m, −68°.8, appreciably displaced from the optical centre, and in the SMC 1h10m, − 73°.25. The centre of rotation in the LMC has since been a source of much discussion (see Sect. 3.4.4). In the SMC the problems are of another nature.

The structure and kinematics of the LMC

Images of the LMC in most wavelength regions are dominated by radiation from its Extreme Population I constituent (stellar associations, supergiants, etc.) or the connected gas (HII regions, HI complexes, molecular clouds) and dust which display the regions of recent star formation as an asymmetric pattern, not completely at random but with some structure.