Page:Philosophical Transactions - Volume 001.djvu/389



I did apply my self the last Summer to the taking of the Diameters of the Sun, Moon, and the other Planets, by a Method, which one M. Picard and my self have, esteem'd by Us the best of all those, that have been practis'd hitherto; since we can take the Diameters to Second Minutes, being able to divide one foot into 24000. or 30000. parts, scarce failing as much as in one only part, so as we can in a manner be assur'd, not to deceive our selves in 3. or 4. seconds, I shall not now tell you my Observations, but I may very well assure you, that the Diameter of the Sun has not been much less in his Apogee, than 31.m. 37. or 40. sec. and certainly not lesse than 31. m. 35. sec, and that at present in his Perigee it passes not 32. m. 45. sec. and may be lesse by a second or two. That, which is at the present troublesome, is, that the Vertical Diameter, which is the most easie to take, is diminisht, even at Noon, by 8. or 9. sec. because of the Refractions, which are much greater in Winter than Summer at the same height; and that the Horizontal Diameter is difficult, because of the swift motion of the Heavens.

As for the Moon, I never yet found her Diameter less than 29. m. 44. or 45. sec. and I have not seen it pass 33. m. or if it hath, it was only by a few seconds. But I have not yet taken her in all the kinds of situations of the Apogees and Perigees which happen, with the Conjunctions and Quadratures, I do not mention all, what can be deduced from thence, but if you have Persons at London, that observe these Diameters, we may entertain our selves more about this Subject, another time. I shall only tell you, that I have found a Way to know the Parallax of the Moon, by the means of her Diameter: Vid. If on a day, when she is to be in her Apogee or Perigee, and in the most Boreal Signes, you take her Diameter towards the Horizon, and then towards the South, with her Altitudes Rh