[[Categoria:Pagine che usano RigaIntestazione|Scientia - Vol. VIII.djvu{{padleft:40|3|0]]is really due to a motion of the sun towards Lyra, is a favourite one; it recognises the insignificance of the solar system in the universe; but it is not necessarily a truer point of view. This « solar motion », as it is called, belongs neither to the sun nor to the stars intrinsically; it is a purely relative motion attributable equally to both.

With the more complete data of recent times, the determination of the solar motion, though still attracting great interest, has come to be regarded as fairly settled. At least it is realized that whatever uncertainties and discordances remain will be best cleared up by studying the peculiarities of the distribution of the individual motions of the stars. For the precision with which the solar motion can be determined depends inter alia on the precision with which we can define the conception of the « mean of the stars », to which the motion is supposed to be referred.

What then are the principles which govern the distribution of stellar motions? In the solar system we see a group of celestial bodies, the planets, strongly under the control of a central body. The clearest evidence of this control is manifested in the shapes of their paths. But an intelligent observer might discover, simply from the instantaneous motions of the planets, that here was not a mere haphazard aggregation, but something of the nature of a system. He would notice, for instance, that each planet moves nearly at right angles to a radius drawn from the sun, and with a smaller velocity the greater the distance? a clear indication of rule and order. Is there anything even remotely analogous in the stellar universe? Anything discoverable in the statistics of the motions, which might lead us to suspect that the universe around us is not a purely haphazard aggregation of independent units? When we realize how vast are the distances which separate one star from another, and how excessively minute must be their mutual attractions, it seems almost useless to look for any such relations. But, as we shall see, relations are found to exist; not the close ties which bind the planets to the sun, but still broad traces of structure which differentiate the universe from an entirely haphazard aggregation.

Although it had been usual in researches on the solar motion to make the assumption that the motions of the stars are haphazard and directed with impartiality in all directions,