For many purposes a gravitational system of measurement is most natural; thus we speak of a force of so many pounds or so many kilogrammes.
They may thus be fairly regarded as constituting a binary system, though the gravitational attraction between some of the wider pairs must be very weak.
Starting from a widely diffused nebula, more or less uniform, we find that, in consequence of gravitational instability, it will tend to condense about a number of nuclei.
The full working out is in general difficult, the comparatively simple problem of three bodies, for instance, in gravitational astronomy being still unsolved, but some general theorems can be formulated.
When we know the mass of the earth in gravitational measure, its product by the denominator of the fraction just mentioned gives the mass of the sun in gravitational measure.
The curve has important mechanical relations, in particular it is the orbit of a particle moving under the influence of a central force which varies inversely as the square of the distance of the particle; this is the gravitational law of force, and the curve consequently represents the orbits of the planets if only an individual planet and the sun be considered; the other planets, however, disturb this orbit (see Mechanics).
Units of this kind are called absolute on account of their fundamental and invariable character as contrasted with gravitational units, which (as we shall see presently) vary somewhat with the locality at which the measurements are supposed to be made.
We learn also that on account of the variation of g with the locality a gravitational system of force-measurement is inapplicable when more than a moderate degree of accuracy is desired.
The question remains, of course, as to how far the measurement of force here implied is practically consistent with the gravitational method usually adopted in statics; this will be referred to presently.
There are two gravitational fields which sometimes reinforce and at other times diminish each other and the effect is always a resultant one.