LET. XI.

OF THE EARTH.

3*

had performed her revolution round the earth,in an orbit like that of the moon.

In like manner, Mercury is always found tokeep in the neighbourhood of the fun, withoutever receding from him so far as Venus; butas he is continually hid in the splendor of thesun’s rays, he can seldom be seen without theassistance of a telescope. The superior bright-ness of this planet affords, likewise, a sufficientproof, that he must be much nearer to the sunthan any of the rest; and from all this it is evi-dent, that the orbit of Mercury is includedwithin the orbit of Venus, and that, like thatplanet, he regards the fun as the center of hismotion.

Mars, Jupiter, Saturn, and the GeorgiumSidus, being superior planets, must necessarilyinclude the earth in their orbits; but from theirvarious elongations, or distances from the fun.at different times, as well as from their station-ary and retrograde appearances, it is plain thatthe fun, and not the earth, must also be thecenter of their motions, or the body roundwhich they perform their respective periods andrevolutions.

And from hence it appears, that the earth it-self must likewise move round the sun. Forstnce, by the place it obtains in the system, ithas those moveable bodies Mercury and Venuson one side, nearer to the fun, and Mars, Jupi-ter.