Celestial Sphere



Definition: [Ancient Astrological Concepts] An imaginary sphere of stars surrounding the Earth and rotating around it.

In astrology we generally represent the stars and constellations as lying on this single sphere. We tend to think of the planets as lying between this sphere and the Earth and - whilst mostly moving along with the celestial sphere - possessing much smaller but independant motions of their own. [This is why the ancients called the planets the Wanderers.] This, essentially, was the view of the night skies held by the Classical astrologers, such as Ptolemy, who defined much of modern astrology.


How we imagine the night sky to be: a celestial sphere rotating around the Earth.

However, this view of the night skies isn't actually true in two important respects:

(1) In the celestial sphere view of the skies, the Earth stays still and the sphere rotates around it. This is the opposite of what really happens: in reality, the stars stay still and the Earth spins on its axis giving the illusion of stellar movement. Those small idependent movements of the planets relative to the stars that we can see happen each night are because they - and we on Earth - are orbiting the Sun.

The reality: the spinning earth.


The Rotating Celestial Sphere This way of thinking about the night sky can be confusing for we astrologers as it's not reality. The movement of the stars through the sky each night is because of the Earth's spin, not the movement of the stars around us. [West on the above sphere is to the right.] ©


The Rotating Earth This is the reality: the Earth rotating around its axis once each day and producing the illusion of the movement of the stars through the night sky. The Earth spins west to east, giving the illusion of the stars moving from east to west, parallel to the celestial equator, each night. The stars seem to rise in the east and set in the west. In the daytime, so does the Sun. Again, this is simply an illusion caused by the Earth's spin. This is a view of the Earth at Equinox, so the light and dark portions of the Earth are the same size. ©


(2) The celestial sphere implies that the stars are all the same distance away from us; this is not correct. In fact, there can be huge differences in the distances from the stars to the Earth of the stars within a single constellation.

A 1551 image of a celestial globe.  These would usually been made in pairs with Earth globes, and would have adorned the libraries of the wealthy.


A Renaissance Celestial Sphere Illustration from Opera Mathematica, Johannes Schoner, Nuremberg 1551. The two dashed black and white lies mark that can be seen mark the great circles of the Ecliptic [upper line] and the Celestial Equator [lower line]. [Click on the above diagram for a complete version, 89 kB.]

The Great Circles of the Celestial Sphere: In a tradition going back at least as far as Manilius in the first century BC, Classical astrologers used Great Circles to mark out the boundaries celestial sphere. Shown below are the Ecliptic and the Celstial Equator within the celestial sphere.

Shown right is a drawing of sixteenth century celestial sphere. These were often produced as globes, for rich patrons and working astrologers, often made as pairs with globes of the Earth.

A celestial sphere.  Such spheres are used to show the tilt of the Earth's axis away from or towards the Sun at the solstices and the lack of tilt at the equinoxes.


A Static View of the Celestial Sphere The Celestial Equator is the horizontal plane in the centre. A second plane tilted with respect to the Celestial Equator marks the Ecliptic. Where these two meet are the Equinoxes and where they lie furthest from each other are the Solstices. You could also draw a Great Circle between the Vernal Equinox and through the Celestial Poles. This marks the right vertical border on our star maps, the line of zero Right Ascension.


© Dr Shepherd Simpson, Galactic Astrologer


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