_A._ But because it is a thing impossible for me to know, I will proceed in my own way of inquiry. And seeing you ascribe this compounded motion to the sun and earth, I would grant you that the earth (whose annual motion is from west to east) shall give the moon her monthly motion from east to west. But then I ask you whether the moon have also that compounded motion of the earth, and with it a motion upon its own centre, as hath the earth? For seeing the moon has no other planet to carry about her, she needs it not. _B._ I see reason enough, and some necessity, that the moon should have both those motions. For you cannot think that the Creator of the stars, when he gave them their circular motion, did first take a centre, and then describe a circle with a chain or compass, as men do? No; he moved all the parts of a star together and equally in the creation: and that is the reason I give you. The necessity of it comes from this phenomenon, that the moon doth turn one and the same face towards the earth; which cannot be by being moved about the earth parallelly, unless also it turn about its own centre. Besides, we know by experience, that the motion of the moon doth add not a little to the motion of the sea: which were impossible if it did not add to the stream of the air, and by consequence to that of the water. _A._ If you could get a piece of the true and intimate substance of the earth, of the bigness of a musket-bullet, do you believe that the bullet would have the like compounded motion to that which you attribute to the sun, earth, and moon? _B._ Yes, truly; but with less strength, according to its magnitude; saving that by its gravity falling to the earth, the activity of it would be unperceived. _A._ I will trouble you no more with the nature of celestial appearances; but I pray you tell me by what art a man may find what part of a circle the diameter of the sun’s body doth subtend in the ecliptic circle? _B._ Kepler says it subtends thirty minutes, which is half a degree. His way to find it is by letting in the sun-beams into a close room through a small hole, and receiving the image of it upon a plane perpendicularly. For by this means he hath a triangle, whose sides and angles he can know by measure; and the vertical angle he seeks for, and the substance of the arc of the sun’s body. _A._ But I think it impossible to distinguish where the part illuminate toucheth the part not illuminate. _B._ Another way is this: upon the equinoctial day, with a watch that shows the minutes standing by you, observe when the lower brim of the sun’s setting first comes to the horizon, and set the index to some minute of the watch; and observe again the upper brim when it comes to the horizon: then count the minutes, and you have what you look for. Other way I know none. ========== CHAPTER V. OF THE MOTIONS OF WATER AND AIR.