Sect. i. Of the SOLAR SYSTEM. i Z

the Numbers 1*2,3, &c. its equable Motionround the Sun; the correspondent Numbers1, 2, 3, &c. in the Circles, b } c, d } the Mo-tion of Mars as it would be seen from theEarth. It is plain from this Figure, that ifthe Earth stood still, the Motion of Marswould be always progressive, (tho’ some-times very unequal;) but since Observationsprove the contrary, it necessarily follows,that the Earth turns round the Sun.

The Annual Periods of the Planets roundthe Sun are determined, by carefully observe- U rnd mo-ing the length of Time since their Departurefrom a certain Point in the Heavens (or from how com-a Fixed Star) until they arrive to the same f '" f 'again. By these sorts of Observations theAncients deetermined the periodical Revo-lutions of the Planets round the Sun; andwere so exact in their Computations, as tobe capable of predicting Eclipses of the Sunand Moon. But since the Invention ofTelescopes, Astronomical Observations aremade with greater Accuracy, and of conse-quence our Tables are far more perfect thanthose of the Ancients. And in order tobe as exact as possible, Astronomers com-pare Observations made at a great Distanceof Time from one another, including seve-ral Periods; by which means the Error thatmight be in the whole, is in each Periodsubdivided into such little Parts, as to bevery inconsiderable. Thus the mean Lengthof a solar Year is known even to Seconds.

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