THE SUN.
15
doubtless increase as the theory of the motions of Jupiter 'ssatellites is more carefully studied and investigated.
The experimental determination of the Velocity of Light hasalso been used in connection with a certain small apparentdisplacement of the positions of the fixed stars, which is calledtheir “ Aberration'' This etfect is analogous to that by whicha shower of rain, falling vertically on a calm day, appears to^slope towards a person moving rapidly through it. It dependsupon the comparative value of the velocity of the light whichcomes from the stars to an observer, and of the velocity with*which he is at the same time carried through space as hereceives it, by the motion of the Earth in its orbit. The formervelocity being known, the latter can be calculated from theamount of any star’s aberration or apparent disidacement..And the velocity of the Earth in its orbit being thus deduced,we can tell at once how far it will travel in a year; i.e., we knowthe length of the circumference of its orbit. It is, then, avery simple matter to evaluate the radius of that orbit, or, inother words, the Earth ’s distance from the Sun. About93,000,000 miles appears to be the most probable value whichthis method gives.
Observations of the planet Mars have also been made whenit has been especially near to the Earth (as, for instance, in'1862 or 1877), with a view to the solution of this same greatproblem. The method at first adopted was to notice from twodifferent observatories how much nearer Mars appeared to be tocertain stars as seen from the one, than as seen from the other.This effect is due to the difference of the directions in whichthe two observatories look at the })lanet, the stars being so-much farther away that no appreciable .change is produced inf their apparent positions. From such observations we mayproceed to calculate the distance of Mars from the Earth by amodification of the surveying method already described (seeFigs. I. and II.); and it is found that we are able to do so withvery considerable accuracy.
The nearest distance of the planet Mars from the Earth being not much less than H times the nearest distance ofVenus , this last statement may appear to be inconsistent