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is neither in the Pole nor Plain of the Ecliptick, will seem to describe about its true Place a Figure, insensibly different from an Ellipse, whose Tranfverse Axis is at Right-angle to the Circle of Longitude pasting through the Stars true Place, and equal to the Diameter of the little Circle described by a Star (as was before supposed) in the Pole of the Ecliptick ; and whose Conjugate Axis is to its Tranfverse Axis, as the Sine of the Stars Latitude to the Radius. And allowing that a Star by its apparent Motion does ex-a&ly defcribe such an Ellipse, it will be found, that if A be the Angle of Position (or the Angle at the Star made by two great Circles drawn from it, thro’ the Poles of the Ecliptick and Equator) and B be another Angle, whose Tangent is to the Tangent of A as Radius to the Sine of the Latitude of the Star ; then B will be equal to the Difference of Longitude between the Sun and the Star, when the true and apparent Declination of the Star are the same. And if the Sun’s Longitude in the Ecliptick be reckoned from that Point, wherein it is when this happens; then the Difference between the true and apparent Declination of the Star (on Account of the Cause I am now considering) will be always, as the Sine of the Sun’s Longitude from thence. It will likewise be found, that the greatest Difference of Declination that can be between the true and apparent Place of the Star, will be ,to the Semi-Tranfverse Axis of the Ellipse (or to the Semi-diameter of the little Circle described by a Star in the Pole of the Ecliptick) as the Sine of A to the Sine of B.
If the Star hath North Latitude, the Time, when its true and apparent Declination are the same, is before
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fore the Sun comes in Conjunction with or Opposition' to it, if its Longitude be in the first or last Quadrant (viz. in the ascending Semi-circle) of the Ecliptick ; and after them, if in the descending Semi-circle ; and it will appear nearest to the North Pole of the Equator, at the Time of that Maximum (or when the greatest Difference between the true and apparent Declination happens) which precedes the Sun’s Conjunction with the Star.
Thefe Particulars being Sufficient for my preSent PurpoSe, I shall not detain you with the Recital of any more, or with any farther Explication of thefe. It may be time enough to enlarge more upon this Head, when I give a Description of the Instruments &c. if that be judged neceffary to be done; and when I (hall find, what I now advance, to be allowed of (as I flatter my self it will) as fomething more than a bare Hy-potheus. 1 have purpoSely omitted Some matters of no great Moment, and constdered the Earth as moving in a Circle, and not an Ellipfe, to avoid too perplexed a Calculus, which after all the Trouble of it would not Sensibly differ from that which I make ufe of, especially in thoSe ConSequences which I shall at preSent draw from the foregoing Hypothesis.
This being premised, I shall now proceed to determine from the Observations, what the real Proportion is between the Velocity of Light and the Velocity of the Earth’s annual Motion in its Orbit; upon Supposition that the Phenomena before mentioned do depend upon the Caufes I have here assigned. But I must first let you know, that in all the Observations hereafter men-tioned, I have made an Allowance for the Change of the Star’s Declination on Account of the PreceSTion of
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