Page:Philosophical Transactions - Volume 001.djvu/411

 considers the principal Observations, hitherto made of the Motion of the Planets and the Fixed Stars, of their Magnitude, Figure, and other Accidents; drawing thence several Conclusions, in which he establishes his Hypothesis. The Second contains his Astronomical Tables, made according to the Hypothesis of the First Part, together with Instructions teaching the manner of using them.

Thirdly, That Astronomers will find in this Book many very remarkable things, concerning the Apparent Diameter of the Sun and the other Stars, the Motion of the Libration of the Moon, the Eclipses, Parallaxes, and Refractions: And that this Author shews, that there is a great difference between Optical and Astronomical Refraction, which Tycho and many others have confounded; undertaking to prove, that, whereas these Astronomers believed, that the remoter any Star is, the less is its Refraction, on the contrary the Refraction is the greater, the more a Star is distant. And among many other things, he ingeniously explicates the two contrary Motions of the Sun, from East to West, and vice versa, by one onely Motion upon a Spiral, turning about a Cone.

Fourthly, That he represents, How uneasie it is to establish sure Principles of this Science, by reason of the difficulties of making exact Observations, So, for example, in the Observation of the Equinox, every one is mistaken by so many Hours, as he is of Minutes, in the Elevation of the Pole, or the Diameter of the Sun, or the Refraction, or in any other circumstance. In the Observation of the Solstice, the error of one only Second causeth a mistake of an Hour and an half: mean time it is almost impossible to avoid the error of a Second; and even the sharpest sight will not be able to perceive it, except it be assisted with an Instrument of a prodigious bigness. For to mark Seconds, though Lines were drawn as subtil as the single threds of a Silk-worms Clew, (which are the smallest spaces to be discerned by the sharpest Eye) by the Calculation made by this Author there would need an Instrument of 48. feet Radius, since Experience shews, that there needs no more at most, than 3600. threds of Silk to cover the space of an inch. But, suppose one could have a Quadrant of this bigness, who can assure himself, that dividing it into Rh