Letters on England [38]
be subtracted from their
computation.
Astronomical observations seem to have lent a still greater
assistance to our philosopher. He appears to us stronger when he
fights upon his own ground.
You know that the earth, besides its annual motion which carries it
round the sun from west to east in the space of a year, has also a
singular revolution which was quite unknown till within these late
years. Its poles have a very slow retrograde motion from east to
west, whence it happens that their position every day does not
correspond exactly with the same point of the heavens. This
difference, which is so insensible in a year, becomes pretty
considerable in time; and in threescore and twelve years the
difference is found to be of one degree, that is to say, the three
hundred and sixtieth part of the circumference of the whole heaven.
Thus after seventy-two years the colure of the vernal equinox which
passed through a fixed star, corresponds with another fixed star.
Hence it is that the sun, instead of being in that part of the
heavens in which the Ram was situated in the time of Hipparchus, is
found to correspond with that part of the heavens in which the Bull
was situated; and the Twins are placed where the Bull then stood.
All the signs have changed their situation, and yet we still retain
the same manner of speaking as the ancients did. In this age we say
that the sun is in the Ram in the spring, from the same principle of
condescension that we say that the sun turns round.
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
computation.
Astronomical observations seem to have lent a still greater
assistance to our philosopher. He appears to us stronger when he
fights upon his own ground.
You know that the earth, besides its annual motion which carries it
round the sun from west to east in the space of a year, has also a
singular revolution which was quite unknown till within these late
years. Its poles have a very slow retrograde motion from east to
west, whence it happens that their position every day does not
correspond exactly with the same point of the heavens. This
difference, which is so insensible in a year, becomes pretty
considerable in time; and in threescore and twelve years the
difference is found to be of one degree, that is to say, the three
hundred and sixtieth part of the circumference of the whole heaven.
Thus after seventy-two years the colure of the vernal equinox which
passed through a fixed star, corresponds with another fixed star.
Hence it is that the sun, instead of being in that part of the
heavens in which the Ram was situated in the time of Hipparchus, is
found to correspond with that part of the heavens in which the Bull
was situated; and the Twins are placed where the Bull then stood.
All the signs have changed their situation, and yet we still retain
the same manner of speaking as the ancients did. In this age we say
that the sun is in the Ram in the spring, from the same principle of
condescension that we say that the sun turns round.
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