Hipparchus was the first among the Greeks who observed some change in the constellations with regard to the equinoxes,or rather who learnt it from the Egyptians.Philosophers ascribed this motion to the stars;for in those ages people were far from imagining such a revolution in the earth,which was supposed to be immovable in every respect.They therefore created a heaven in which they fixed the several stars,and gave this heaven a particular motion by which it was carried towards the east,whilst that all the stars seemed to perform their diurnal revolution from east to west.To this error they added a second of much greater consequence,by imagining that the pretended heaven of the fixed stars advanced one degree eastward every hundred years.In this manner they were no less mistaken in their astronomical calculation than in their system of natural philosophy.As for instance,an astronomer in that age would have said that the vernal equinox was in the time of such and such an observation,in such a sign,and in such a star.It has advanced two degrees of each since the time that observation was made to the present.Now two degrees are equivalent to two hundred years;consequently the astronomer who made that observation lived just so many years before me.It is certain that an astronomer who had argued in this manner would have mistook just fifty-four years;hence it is that the ancients,who were doubly deceived,made their great year of the world,that is,the revolution of the whole heavens,to consist of thirty-six thousand years.But the moderns are sensible that this imaginary revolution of the heaven of the stars is nothing else than the revolution of the poles of the earth,which is performed in twenty-five thousand nine hundred years.It may be proper to observe transiently in this place,that Sir Isaac,by determining the figure of the earth,has very happily explained the cause of this revolution.
All this being laid down,the only thing remaining to settle chronology is to see through what star the colure of the equinoxes passes,and where it intersects at this time the ecliptic in the spring;and to discover whether some ancient writer does not tell us in what point the ecliptic was intersected in his time,by the same colure of the equinoxes.
Clemens Alexandrinus informs us,that Chiron,who went with the Argonauts,observed the constellations at the time of that famous expedition,and fixed the vernal equinox to the middle of the Ram;the autumnal equinox to the middle of Libra;our summer solstice to the middle of Cancer,and our winter solstice to the middle of Capricorn.
A long time after the expedition of the Argonauts,and a year before the Peloponnesian war,Methon observed that the point of the summer solstice passed through the eighth degree of Cancer.
Now every sign of the zodiac contains thirty degrees.In Chiron's time,the solstice was arrived at the middle of the sign,that is to say to the fifteenth degree.A year before the Peloponnesian war it was at the eighth,and therefore it had retarded seven degrees.Adegree is equivalent to seventy-two years;consequently,from the beginning of the Peloponnesian war to the expedition of the Argonauts,there is no more than an interval of seven times seventy-two years,which make five hundred and four years,and not seven hundred years as the Greeks computed.Thus in comparing the position of the heavens at this time with their position in that age,we find that the expedition of the Argonauts ought to be placed about nine hundred years before Christ,and not about fourteen hundred;and consequently that the world is not so old by five hundred years as it was generally supposed to be.By this calculation all the eras are drawn nearer,and the several events are found to have happened later than is computed.I do not know whether this ingenious system will be favourably received;and whether these notions will prevail so far with the learned,as to prompt them to reform the chronology of the world.Perhaps these gentlemen would think it too great a condescension to allow one and the same man the glory of having improved natural philosophy,geometry,and history.This would be a kind of universal monarchy,with which the principle of self-love that is in man will scarce suffer him to indulge his fellow-creature;and,indeed,at the same time that some very great philosophers attacked Sir Isaac Newton's attractive principle,others fell upon his chronological system.
Time that should discover to which of these the victory is due,may perhaps only leave the dispute still more undetermined.
