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Celestial Sphere-Orrery

3-foot celestial sphere

The author's 3-foot rotating model showing the relationship between astronomy and astrology.

To see the origin of the 12 house cusps more clearly, look at the other model below.




         On the celestial sphere above, the ecliptic is marked by the tilted colored band with beads on it. The beads represent the 10 astrological planets. Seen from the Earth's surface, the ecliptic circle appears to be a straight line. All the planets including the rising and setting Sun and Moon appear to travel in the same straight line across the sky.

         An Orrery is a mechanical apparatus illustrating the relative motions—rather than their sizes and distances from each other—of the sun, planets, and their moons. Devised by two clockmakers in 1710-1712, the Orrery was later copied by John Rowley for his patron, Charles Boyle, the fourth Earl of Cork and Orrery, from which it got its name. Orreries were originally made of wood and metal and employed various sized balls to represent planets and moons. The one above, however, is made of two, 3-foot plastic hemispheres held together by copper tubing with brass fittings. And it's not exactly an Orrery, but more precisely a "celestial sphere."

         The celestial sphere is in fact the sky as seen from Earth, hence its name. It is actually a sphere of infinite diameter, and objects (drawn) on it represent their directions as seen from the Earth, not their distances from the Earth. The Earth's polar axis extended into space intersects the celestial sphere at the latter's north and south poles. Since the Earth's pole always points in the same direction in space (except for a slight 26,000-year wobble known as the "precession of the equinoxes," pages 43-44 in You and the Universe ), the direction of its equator extended into space also remains constant. This plane is called the "celestial equator," and in this celestial sphere is the intersection circle of the two, 3-foot hemispheres

         The circles parallel to the Earth's equator between it and the poles are called lines of latitude. Being parallel to to the equator, they maintain constant direction when extended into space. Because of this, stars and other distant celestial objects maintain a constant distance north or south of the celestial equator, their "celestial latitude," year after year. The vertical circles perpendicular to the Earth's equator and passing through the poles, the longitude lines, are a little more difficult to fix in space. As the Earth rotates on its axis they rotate directionally in space as well. Celestial longitude lines can be directionally fixed in space, however, by choosing a fixed direction in space as a fiducial, or starting direction. This is comparable to choosing the Greenwich Meridian as our starting line of 0º latitude on Earth. To discuss this, however, we will need a little more background.

         First, because the planets (except probably for Pluto) originally condensed from a protodisc (see pages 70-71 in You and the Universe) or single rotating cloud of diffuse matter, they all travel in the same plane around the Sun as does the Earth. This is called the "ecliptic plane" on which eclipses occur. Secondly, because the Earth's axis is tilted at 23.5º to the ecliptic plane, its equator, being perpendicular to its polar axis, is also tilted at a constant angle to the ecliptic. These two planes, the ecliptic and the equatorial, thus intersect in space in a fixed direction, or line, called the "line of the equinoxes," similarly to the way two walls intersect in a line at the corner of a room.

         Since the direction of the celestial poles is also fixed in space, an imaginary plane passing through the celestial poles and the line of the equinoxes is perpendicular to the celestial equator and is fixed in space as well. This is our celestial prime meridian as it were, and represents the line (or plane) of 0º celestial latitude. All the other celestial latitude planes are set off from it in the same way that latitude lines are set off from the prime meridian on Earth. Celestial objects can be assigned coordinates based on this fixed celestial coordinate system, just as the latitude and longitude lines on Earth provide us with a fixed coordinate system for locating terrestrial objects. These celestial coordinates can be seen on the celestial sphere above, marked just like the corresponding lines on the surface of the Earth.

         This particular celestial sphere can be oriented with any local Earthly zenith at its zenith, and in fact this is always so when it is suspended from above as it is here. This sphere is thus a three-dimensional astrological chart, as will now be discussed. The horizontal circle of copper tubing represents the horizon plane of that place on Earth whose zenith is at the top of the sphere: the locality for which the chart is being cast.  It intersects the zodiac at the first and the seventh House cusps.

         The vertical copper-tube-circle represents a "vertical circle;" so named because it passes through the local zenith of the nativity or chart in question and the center of the Earth. That particular vertical circle which also passes through the Earth's polar axis extended into space (or through the north point on the horizon) is called the local meridian. The vertical copper circle in the above model is indeed the meridian, because the Earth's pole is pinned into it near the top of the photograph.

         The ecliptic is also known as the zodiac, although technically the two differ due to the precession of the equinoxes. The colored zodiacal belt in the model above is in turn divided into 12 differently colored segments known as the signs of the zodiac. This sphere is held within its copper tubing framework by large coupling washers visible on the polar axis at its intersection points with the copper tubing frame.

         The north star, Polaris, is located very near the celestial sphere's north pole.


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© Carl Woebcke: Orrery-Celestial Sphere, 1991-2014. All rights reserved.