The crowning trophies of gravitational astronomy in the r8th century were Laplace's explanations of the " great inequality ".
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.
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.
The problems of gravitational astronomy engaged the chief part of Hansen's attention.
There are two gravitational fields which sometimes reinforce and at other times diminish each other and the effect is always a resultant one.
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.
His inquiries afford the assurance of a nearly exact conformity among its members to strict gravitational law, only the moon and Mercury showing some slight, but so far unexplained, anomalies of movement.
But as things are the watersurface is broken by land, and the mean density of the substance of the land is 2 6 times as great as that of sea-water, so that the gravitational attraction of the land must necessarily cause a heaping up of the sea around the coasts, forming what has been called the continental wave, and leaving the sea-level lower in mid-ocean.
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.
Leverrier, in 1858, calculated a value of 8.95" for the solar parallax (equivalent to a distance of 91,000,000 m.) from the " parallactic inequality " of the moon; Professor Newcomb, using other forms of the gravitational method, derived in 1895 a parallax of 8.76".
From the observed motion of the node of Venus, as shown by the four transits of 1761, 1769, 1874 and 1882, is found Mass of (earth +moon) _Mass of sun 332600 In gravitational units of mass, based on the metre and second as units of length and time, Log.
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.
It is true that rotational instability alone is not competent to explain the separation into two components; but the existence of gravitational instability, pointed out by J.
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.
Hence there is a certain point, fixed relatively to the assemblage, through which the resultant of gravitational action always passes; this resultant is moreover equal to the sum of the forces on the several particles.
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.
Although Adams's researches on Neptune were those which attracted widest notice, the work he subsequently performed in relation to gravitational astronomy and terrestrial magnetism was not less remarkable.
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.