|637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661|
( 651 )
the Equinox, upon Supposition that the Alteration from this Cause is proportional to the Time, and regular through all the Parts of the Year. I have deduced the real annual Alteration of Declination of each Star from the Observations themSelves ; and I the rather chooSe to depend upon them in this Article, becauSe all which I have yet made, concur to prove, that the Stars near the Equinoctial Colure, change their Declination at this time 1" ? or z" in a Year more than they would do if the Precession was only 50", as is now generally fup-poSed. I have likewiSe met with Some Small Varieties in the Declination of other Stars in different Years* which do not seem to proceed from the fame Caufe, particularly in thoSe that are near the Solstitial Colure, which on the contrary have altered their Declination less than they ought, if the Preceflion was jo". But whether thefe Small Alterations proceed from a regular Caufe, or are occasioned by any Change in the Materials &c. of my Instrument, I am not yet able fully to determine. However, I thought it might not be a-miSs just to mention to you how I have endeavoured to. allow for them, though the ReSult would have been nearly the Same, if I had not considered them at alL What that i?, I will shew, first from the Observations of y 'Draconis, which was found to be 39" more Southerly in the Beginning of March> than in September.
From what hath been premifed, it will appear that the greatest Alteration oS the apparent Declination, of y Draconis, on Account of the Successive Propagation oS Light, would be to the Diameter of the little Circle which a Star (as was before remarked) , would seem to defcribe about the Pole of the Ecliptick, as 39" to 49", 4, The half of-this is the Angle A C B'(as represented
( 653 )
letited in the i^.)This thereSore being AC will
be to AB, that is, the Velocity of Light to the Velocity of the Eye (which in this Cafe may be supposed the Same as the Velocity of the Earth’s annual Motion in its Orbit) as ioxio to One, from whence it would follow, that Light moves, or is propagated as far as from the Sun to..the Earth in 8' ix".
It is well known,that Mr. Romer, who first attempted to account for an apparent Inequality in the Times of the Eclipfes oS Jupiter's Satellites, by the Hypothesis of the progressive Motion of Light, SuppoSed that it Spent about n' Minutes oS Time in its Passage from the Sun to us: but it hath Since been concluded by others from the like Eclipfes, that it is propagated as far in about 7 Minutes. The Velocity of Light therefore deduced from the foregoing Hypothesis, is as it were a Mean: betwixt what had at different times been determined from the Eclipfes of Jupiter's Satellites.
Thefe different Methods of finding the Velocity of. Light thus agreeing in the Refulr, we may reafonably conclude, not only that thefe Phenomena are owing; to the Causes to which they have been ascribed; but alfo, that Light is propagated (in the fame Medium), with the fame Velocity after it hath been reflected as before: for this will be the Consequence, if we allow that the Light of the Sun is propagated with the fame Velocity, before it is reflected, as the Light of the fixt Stars. And 1 imagine this will scarce be questioned, if it can be made appear that the Velocity of the Light of all the fixt Starsis equal, and that their Light moves or is propagated through equal Spaces in equal Tiroes, at all Distances from them: both which points (as I apprehend) are Sufficiently proved from the apparent Alteration