twice as great as that from the Sun to us) in the star of Saturn is almost totally imperceptible, and in Jupiter not very observable, what shall we think of the fixed stars, which I believe you will not scruple to place twice as far off as Saturn? In Mars, which for that it is nearer to us———
Simpl.Pray Sir, put your self to no farther trouble in this particular, for I already conceive that what hath been spoken touching the unaltered apparent magnitude of the fixed stars may very well come to passe, but what shall we say of the other difficulty that proceeds from not perceiving any variation in the mutation of aspect?
Salv.We will say that which peradventure may satisfie you also in this particular. And to make short, would you not be satisfied if there should be discovered in the stars face mutations that you think ought to be discovered, in case the annual motion belonged to the Earth?
Simpl.I should so doubtlesse, as to what concerns this particular.
Salv.I could wish you would say that in case such a difference were discovered,If in the fixed stars one should discover any annual mutation, the motion of the Earth would be undeniable. nothing more would remain behind, that might render the mobility of the Earth questionable. But although yet that should not sensibly appear▪ yet is not its mobility removed, nor its immobility necessarily proved, it being possible, (as Copernicus affirmeth) that the immense distance of the Starry Sphere rendereth such very small Phænomena unobservable; the which as already hath been said, may possibly not have been hitherto so much as sought for, or if sought for, yet not sought for in such a way as they ought,It is proved what small credit is to be given to Astronomical Instruments in minute observations. to wit, with that exactnesse which to so minute a punctuality would be necessary; which exactnesse is very difficult to obtain, as well by reason of the deficiency of Astronomical Instruments, subject to many alterations, as also through the fault of those that manage them with less diligence then is requisite. A necessary argument how little credit is to be given to those observations may be deduced from the differences which we find amongst Astronomers in assigning the places, I will not say, of the new Stars or Comets, but of the fixed stars themselves, even to the altitudes of the very Poles, in which, most an end, they are found to differ from one another many minutes. And to speak the truth, who can in a Quadrant, or Sextant, that at most shall have its side ** Braccia Italian. 3. or 4. yards long, ascertain himself in the incidence of the perpendicular, or in the direction of the sights, not to erre two or three minutes, which in its circumference shall not amount to the breadth of a grain of ** Or Mill. Mylet? Besides that, it is almost impossible, that the Instrument should be made, and kept with absolute exactnesse. Ptolomeysheweth
