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Secondary Planet, against which we were before disputing:) But by the Common Center of Gravity of the Bodies, Earth and Moon, as one Aggregate.
Now supposing A B C D E to be a part of the See Fig. 2. and 3.great Orb of the Annual motion, described by the Common Center of Gravity, in so long time as from a Full-Moon at A to the next New-Moon at E; (which, though an Arch of a Circle or Ellipse, whose Center we suppose at a due distance below it; yet being but about of the whole, may well enough be here represented by a streight Line:) the Center of the Earth at T, and that of the Moon at L, must each of them (supposing their common Center of Gravity to keep the Line A E) be supposed to describe a Periphery about that Common Center, as the Moon describes her Line of Menstrual motion. (Of which I have (in the Scheme) onely drawn that of the Earth; as being sufficient to our present purpose; parallel to which, if need be, we may suppose one described by the Moon; whose distance is also to be supposed much greater from T than in the figure is expressed, or was necessary to expresse.) And in like manner E F G H I, from that New-moon at E, to the next Full-moon at I.
From A to E (from Full-moon to New-moon,) T moves (in its own Epicycle) upwards from the Sun: And from E to I, (from New-moon to Full-moon) it moves downwards, toward the Sun. Again, from C to G, (from last quarter to the following first quarter,) it moves forwards according to the Annual motion; But from G forward to C, (from the first Quarter to the ensuing last Quarter,) it moves contrary to the Annual motion.
It is manifest therefore, according to this Hypothesis, that from Last quarter to First quarter (from C to G, while T is above the Line of the Annual motion) its Menstrual motion in its Epicycle adds somewhat of Acceleration to the Annual motion; and most of all at E, the New-moon: And from the first to the last quarter (from G forward to C, while T is below the Line of the Annual motion,) it abates of the Annual motion; and most of all at I, or A the Full-moon.
So that in pursuance of Galilæo's Notion, the Menstrual add-
