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ference between the true and visible Place) vanishes. But if Light be propagated in Time (which I presume will readily be allowed by most of the Philosuphers of this Age) then it is evident from the foregoing Considerations, that there will be always a Difference between the real and visible Place of an Object, unless the Eye is moving either directly towards or from the Object. And in all Cases, the Sine: of the Difference between the real and visible Place of the Object, will be to the Sine of the visible Inclination of the Object to the Line in which the Eye is moving, as the Velocity of the Eye to the Velocity of Light.
If Light moved but i ooo times faster than the Eye, and an Object (fuppoSed to be at an infinite Distance) was really placed perpendicularly over the Plain in which the Eye is moving, it follows from what hath been already said, that the apparent Place of such an Object will be always inclined to that Plain, in an Angle of 89° ^6't ; 10 that it will constantly appear 3' > from its true Place, and seem fo much less inclined to the Plain, that way towards which the Eye tends. That is, if A C is to AB (or AD) as 1000 to one, the Angle ABC will be 89° 56' *, and A C B = 3' *, and B C D = i A C B = 7'. So that according to this Supposition, the visible or apparent Place of the Object will be altered 7', if the Direction of the Eye’s Motion is at one time contrary to what it is at another.
If the Earth revolve round the Sun annually, and the Velocity of Light were to the .Velocity of the Earth’s Motion in its Orbit (which I will at present suppose to be a Circle) as 1000 to one; then tis easy
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co conceive, that a Star really placed in the very Pole of the Ecliptick, would, to an Eye carried along with the Earth, seem to change its Place continually, and (neglecting the finall Difference on the Account of the Earth’s diurnal Revolution on its Axis) would seem to describe a Circle round that Pole, every Way distant therefrom 3So that its Longitude would be varied through all the Points of the Ecliptick every Year; but its Latitude would always remain the fame. Its right Ascension would allb change, and its Declination, according to the different Situation of the Sun in refpedt to the equinoctial Points ; and its apparent Distance from the North Pole of the Equator would be 7' less at the Autumnal, than at the vernal Equinox.
The greatest Alteration of the Place of a Star in the Pole of the Ecliptick (or which in Effedt amounts to the same, the Proportion between the Velocity of Light and the Earth’s Motion in itsOrbit) being known; it will not be difficult to find what would be the Difference upon this Account, between the true and apparent Place of any other Star at any time ; and on the contrary, the Difference between the true and apparent Place being given ; the Proportion between the Velocity of Light and the Earth’s Motion in its Orbit may be found.
As I only observed the apparent Difference of Declination of the Stars, I shall not now take any farther Notice in what manner such a Cause as I have here suppofed would occasion an Alteration in their apparent Places in other Refpedts; but, supposing the Earth to move equally in a Circle, it may be gathered from what hath been already said, that a Star which