298. As this explanation of the ebbing and flowing of the Sea is deduced from the Earth’s constantly falling toward the Moon by the power of gravity, some may find a difficulty in conceiving how this is possible when the Moon is Full, or in opposition to the Sun; since the Earth revolves about the Sun, and must continually fall towards it, and therefore cannot fall contrary ways at the same time: or if the Earth is constantly falling towards the Moon, they must come together at last. To remove this difficulty, let it be considered, that it is not the center of the Earth that describes the annual orbit round the Sun; but the [63]common center of gravity of the Earth and Moon together: and that whilst the Earth is moving round the Sun, it also describes a Circle round that centre of gravity; going as many times round it in one revolution about the Sun as there are Lunations or courses of the Moon round the Earth in a year: and therefore, the Earth is constantly falling towards the Moon from a tangent to the Circle it describes round the said common center of gravity. Let _M_ be the Moon, _TW_ part of the Moon’s Orbit, and _C_ the center of gravity of the Earth and Moon: whilst the Moon goes round her Orbit, the center of the Earth describes the Circle _ged_ round _C_, to which Circle _gak_ is a tangent: and therefore, when the Moon has gone from _M_ to a little past _W_, the Earth has moved from _g_ to _e_; and in that time has fallen towards the Moon, from the tangent at _a_ to _e_; and so round the whole Circle. [Sidenote: PLATE IX.] 299. The Sun’s influence in raising the Tides is but small in comparison of the Moon’s: For though the Earth’s diameter bears a considerable proportion to it’s distance from the Moon, it is next to nothing when compared with the distance of the Sun. And therefore, the difference of the Sun’s attraction on the sides of the Earth under and opposite to him, is much less than the difference of the Moon’s attraction on the sides of the Earth under and opposite to her: and therefore the Moon must raise the Tides much higher than they can be raised by the Sun. [Sidenote: Why the Tides are not highest when the Moon is on the Meridian. Fig. I.]