Part 1

The system of concentric Spheres

by Adam Smith

Of all natural phenomena, mankind has been most curious of the celestial ones because of their:

greatness and
beauty

Those who surveyed the heavens found 3 sorts of objects:

The Sun
The Moon
The Stars

The stars appear always in the same place and at the same distance with regard to one another. They seemed to revolve every day around the earth in parallel circles which widened gradually from the poles to the equator.

They were naturally thought of as:

being fixed, like so many gems, in the concave side of the firmament, and
being carried around by the diurnal revolutions of that solid body

It was believed that the azure sky, in which the stars seem to float, was a solid body, the roof or outer wall of the universe. This was because of the uniformity of their apparent motions.

The Sun and Moon often change their distance and location relative to the other heavenly bodies. They could not be apprehended to be attached to the same sphere with them.

They assigned, therefore, to each of them, a sphere of its own.

Each of them were attached to the concave side of a solid and transparent body, by whose revolutions they were carried round the earth.

There was not indeed, in this case, the same ground for the supposition of such a sphere as in that of the Fixed Stars; for neither the Sun nor the Moon appear to keep always at the same distance with regard to any one of the other heavenly bodies.

But as the motion of the Stars had been accounted for by an hypothesis of this kind, it rendered the theory of the heavens more uniform, to account for that of the Sun and Moon in the same manner.

The sphere of the Sun was placed above that of the Moon since the Moon was seen in eclipses to pass between the Sun and the Earth.

Each of them was supposed to revolve by a motion of its own, and at the same time to be affected by the motion of the Fixed Stars.

Thus, the Sun was carried round from east to west by the communicated movement of this outer sphere, which produced his diurnal revolutions, and the vicissitudes of day and night; but at the same time he had a motion of his own, contrary to this, from west to east, which occasioned his annual revolution, and the continual shifting of his place with regard to the Fixed Stars.

This motion was more easy, they thought, when carried on edgeways, and not in direct opposition to the motion of the outer sphere, which occasioned the inclination of the axis of the sphere of the Sun, to that of the sphere of the Fixed Stars.

This again produced the obliquity of the ecliptic, and the consequent changes of the seasons. The moon, being placed below the sphere of the Sun, had both a shorter course to finish, and was less obstructed by the contrary movement of the sphere of the Fixed Stars, from which she was farther removed.

She finished her period, therefore, in a shorter time, and required but a month, instead of a year, to complete it.

Some of the Stars, when more attentively surveyed:

were less constant and uniform in their motions than the rest
changed their situations with regard to the other heavenly bodies
moved generally eastwards, yet appearing sometimes to stand still, and sometimes even to move westwards.

These, to the number of five, were distinguished by the name of Planets, or wandering Stars, and marked with the particular appellations of Saturn, Jupiter, Mars, Venus, and Mercury.

As, like the Sun and Moon, they seem to accompany the motion of the Fixed Stars from east to west, but at the same time to have a motion of their own, which is generally from west to east; they were each of them, as well as those two great lamps of heaven, apprehended to be attached to the inside of a solid concave and transparent sphere, which had a revolution of its own, that was almost directly contrary to the revolution of the outer heaven, but which, at the same time, was hurried along by the superior violence and rapidity of this last.

This is the system of concentric Spheres, the first regular system of Astronomy in the world.

It was taught in the Italian school
Afterwards, Aristotle and his philosophers, Eudoxus and Callippus, perfected it.

Though rude and inartificial, it could connect together, in the imagination, the grandest and the most seemingly disjointed appearances in the heavens.

The eclipses of the sun and moon are not so easily calculated. But they are as easily explained just like the modern system.

This system:

was taught in secret because they were afraid to make the people angry when they removed from the gods the control of those events.
explained:
- the consequent changes of the seasons,