them may sometimes make their Observations together, and that from divers Experiments we may be the better assured of what certainty and exactness such kind of Observations are like to prove. And because many of the Stars which may happen to come within the compass of such an Iconism, or Map, may be such as are only visible through a good Telescope, whose Positions perhaps have not been noted, nor their longitudes, or latitudes, any where remarked; therefore each Observator should indeavour to insert some fixt Star, whose longitude, and latitude, is known; or with his Telescope he shall find the Position of some notable telescopical Star, inserted in his Map, to some known fixt Star, whose place in the Zodiack is well defin’d.
Having by this means found the true distance of the Moon, and having observed well the apparent Diameter of it at that time with a good Telescope, it is easie enough, by one single Observation of the apparent Diameter of the Moon with a good Glass, to determine her distances in any other part of her Orbit, or Dragon, and consequently, some few Observations will tell us, whether she be mov’d in an Ellipsis, (which, by the way, may also be found, even now, though I think we are yet ignorant of her true distance) and next (which without such Observations, I think, we shall not be sure of) we may know exactly the bigness of that Ellipsis, or Circle, and her true velocity in each part, and thereby be much the better inabled to find out the true cause of all her Motions. And though, even now also, we may, by such Observations in one station, as here at London, observe the apparent Diameter and motion of the Moon in her Dragon, and consequently be inabled to make a better ghess at the Species or kind of Curve, in which she is mov’d, that is whether it be sphærical, or elliptical, or neither, and with what proportional velocities she is carried in that Curve; yet till her true Parallax be known, we cannot determine either.
Next, for the true distance of the Sun, the best way will be, by accurate Observations, made in both these forementioned stations, of some convenient Eclipse of the Sun, many of which may so happen, as to be seen by both; for the Penumbra of the Moon may, if she be sixty Semi-diameters distant from the Earth, and the Sun above seven thousand, extend to about seventy degrees on the Earth, and consequently be seen by Observators as far distant as London, and St. Helena, which are not full sixty nine degrees distant. And this would much more accurately, then any way that has been yet used, determine the Parallax, and distance, of the Sun; for as for the Horizontal Parallax I have already shewn it sufficiently uncertain; nor is the way of finding it by the Eclipse of the Moon any other then hypothetical; and that by the difference of the true and apparent quadrature of the Moon is not less[errata 1] uncertain, witness their Deductions from it, who have made use of it; for Vendeline puts that difference to be but 4'. 30". whence he deduces a vast distance of the Sun, as I have before shewn. Ricciolo makes it full 30'.00. but Reinoldus, and Kircher, no less then three degrees. And no wonder, for if we examine the Theory, we shall find it so complicated with uncertainties.
Errata
