4 The INTRODUCTION:

Length of the Thread, we alter the Positionof the Pegs, we shall then have an Ellipsisof a different Kind from the former; andthe nearer the Focus’s are together, the nearerwill the Curve described be to a Circle 3until at last the two Focus’s coincide, andthen the Pin in the doubling of the Threadwill describe a perfect Circle. The Orbitsof all the Planets have the Sun in one oftheir Focus’s, and half the Distance betweenthe two Focus’s is called the Excentricity ofF.xcentri- the Orbits. This Excentricity is differentcity. in all the Planets, but in most of them itis so small, that in little Schemes or Instru-ments, made to represent the Planetary Orbits,it need not be considered.

The six Planets above-mentioned are. ^ called Primaries, or Primary Planets-, butpiZTtl besides these, there are ten other lesser Pla-s rondo,- netS > tvhich are called Secondaries, Moons,'flanets ' 3 or Satellites. These Moons always accom-pany their respective Primaries, and performtheir Revolutions round them, whilst bothtogether are also carried round the Sun.Of the Six Primary Planets, there are but* three, as far as Observation can assure us,that have these Attendants, viz. the Earth,Jupiter, and Saturn.

The Earth is attended by the Moon, whoperforms her Revolution in about 27 IDays, at the Distance of about 3 o Diametersof the Earth from it; and once a Year is

carried