The Double Drift Theory of Star Motions


PROF. J. C. CHAMBERLIN'S planetismal hypothesis has given geologists a great deal of matter for thought, and on the whole the phenomena with which they are acquainted appear to fall into line when the earth is considered as a body that has always been solid. The cosmical aspect of the question, which Prof. Chamberlin introduced in advancing his hypothesis, geologists are unable to judge, and they are waiting until astronomers give them an opinion before adopting the hypothesis on the larger scale. On the planetismal hypothesis our stellar system is a disc the edge of which is the Milky Way; beyond lies another stellar system, the so-called nebula in Andromeda for all the most distant stars in the neighbourhood of the nebula appear to be this side of the luminous disc. If our stellar system is of the same nature as that of the nebula in Andromeda, then it must be a spiral nebula with two equivalent arms originating from a central core and winding spirally round the centre in approximately the same plane. Suppose our sun had experienced a gravitational drag and was moving at a less rate than the general average of the other stars, or suppose its spiral course was steeper than the general average, and hence its angular velocity less, then an observer regarding the rest of our stellar system from our planet would see the stars near the centre of the spiral travelling in two directions, those on this side of the centre travelling from right to left, and those on the other side in the reverse direction. Is this not a possible explanation of Prof. J. C. Kapteyn's double-drift theory of star motions? It explains why the two systems travelling in opposite directions should be of equal composition and proportions, but it necessitates that in the region of the sky opposite to that in which the double drift has been observed the drift should be simple.

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