The System of Concentric Spheres

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:

1. The Sun
2. The Moon
3. 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 in the concave side of the solid and transparent firmament, carried around by it.

• It was solid 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 attached to the same sphere with the stars.

So they assigned to each of them their own sphere.

• Each of them were attached to the concave side which carried them around the earth.

The sphere of the Sun was 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
• be affected by the motion of the Fixed Stars.

Thus, the Sun was carried around from east to west by the communicated movement of this outer sphere, which produced his diurnal revolutions.

The Sun also had a motion of his own, contrary to this, from west to east. This led to:

• its annual revolution, and
• the continual shifting of his place relative to the Fixed Stars.

This motion was easier when carried on edgeways, and not in direct opposition to the motion of the outer sphere.

• This led to the inclination of the axis of the sphere of the Sun, to that of the sphere of the Fixed Stars.

This produced:

• the obliquity of the ecliptic, and
• the consequent changes of the seasons.

The moon was below the sphere of the Sun, so it:

• had a shorter course to finish, and
• was less obstructed by the contrary movement of the sphere of the Fixed Stars.

The moon finished her period, therefore, in a shorter time, and required just 1 month, instead of 1 year, to complete it.

The Planets

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 became the 5 Planets, or wandering Stars:

• Saturn
• Jupiter
• Mars
• Venus
• Mercury.

Like the Sun and Moon, they accompany the motion of the Fixed Stars from east to west. But at the same time they have a motion of their own, which is generally from west to east.

Each of them, like the sun and moon, were attached to the inside of a solid concave and transparent sphere which had its own revolution that was almost directly contrary to the revolution of the outer heaven. But at the same time, it was hurried along by the superior violence and rapidity of the outer sphere.

This is the system of Concentric Celestial 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 the grandest and 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,

• the vicissitudes and length of day and night

This system would have stood the test of time if there were only the Sun, Moon, and Stars in the heavens.