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329. Velocity Determined from Eclipses of Jupiter's Moons.—roemer, a Danish astronomer, was led to assume a progressive motion for light in order to explain some apparent irregularities in the motions of Jupiter's satellites. A few observations of one of Jupiter's moons are sufficient to determine the time of its eclipses for months in advance. If these observations be made when the earth and Jupiter are on the same side of the sun, and the time of an eclipse occurring about six months later predicted from them be compared with the observed time of that eclipse, it is found that the observed time is about 16$2⁄3$ minutes later than the predicted time. This discrepancy is explained if it be assumed that light has a progressive motion and requires 16$2⁄3$ minutes to cross the earth's orbit, for the distance of the earth from Jupiter in the second case is about the diameter of its orbit greater than in the first.

330. Aberration of the Fixed Stars.—The apparent direction of the light coming from a star to the earth, that is, the apparent direction of the star from the earth, is the resultant of the motion of the light and the motion of the earth. As the motion of the earth changes direction the apparent direction of the star will change also, and the amount of that change will depend on the relation between the velocity of light and the change in the velocity of the earth in its orbit, understanding by change of velocity change in direction as well as in amount. This apparent change in the position of the stars is called aberration. Knowing its amount corresponding to a known change in the earth's motion, we may compute the velocity of light. This method was first employed by Bradley.

Though the agreement of the velocity of light thus determined with that measured by other methods seems to confirm the validity of the reasoning here given, yeb there are serious and unexplained difficulties in the theory of aberration, arising from the discrepant results of experiments instituted to determine the relations of the ether to bodies moving through it.

331. Fizeau's Method.—Several methods have been employed for measuring the velocity of light by determining the time required