Celestial Equator | Celestial Sphere | Constellations | Ecliptic | Equinox
Galactic Ages | Galactic Equator | Galactic Intersection | House
Modern Western Tropical Astrology | Precession of the Equinoxes
The imaginary line in the sky that is the projection of the Earth's equator (from the centre of the Earth) on to the celestial sphere. It is a plane perpendicular to the Earth's axis of spin, and is near enough stationary in the short term relative to the distant stars.
Inside surface of an imaginary sphere with the Earth at the centre, which is conveniently used to visualise and map the sky, and as a two dimensional co-ordinate system. Objects on the same line of sight are regarded as being in the same place on the sphere.
Groups of stars that form familiar shapes in the sky. The boundaries of the constellations (on the celestial sphere) are currently accepted as being defined by the International Astronomical Union. Note that constellations are not the same as the zodiac signs with the same names in modern Western tropical astrology. The signs were named after the constellations 2000 or so years ago, but precession of the equinoxes has shifted the signs since then.
The path of the Sun among the stars as seen from the Earth as we move round the Sun once a year. It is the plane of the Earth's orbit on the celestial sphere, and it is close to the plane of orbit around the Earth of the Moon, and the orbits around the Sun of the other planets in our solar system, so the Moon and the planets follow a path close to that of the ecliptic. This broad path is known as the zodiac.
One of two intersections between the ecliptic and the celestial equator. When we see the Sun at these points, it is above the equator, and day and night are of equal length. At other times of the year, the Sun is either north or south of the equator, and causes the seasons on Earth. The Sun is currently at Equinox on 20 or 21 March and 22 or 23 September. The equinox used as the reference point for the celestial co-ordinate system and the 12-part division of the ecliptic into the zodiac signs is the March equinox, the northern spring equinox.
Twelve Great Ages each lasting 2100 years or so as defined by the movement of the equinox relative to the galactic intersection. This movement is the precession of the equinoxes. The motion of the equinox all the 360° around the ecliptic relative to the galactic intersection is the Great Year of about 25,000 years. The Ages begin when the distance along the ecliptic from the equinox to the galactic intersection (the ecliptic longitude of the galactic intersection) is 0°, 30°, 60°, etc. The Age of Aquarius begins when the ecliptic longitude of the galactic intersection is 270°.
The projection on the celestial sphere of the plane of the Milky Way galaxy.
One of two intersections between the ecliptic and the galactic equator. The one used as the reference point for the Galactic Ages is the intersection currently in the constellation Sagittarius, near (on the celestial sphere) to the centre of the galaxy.
The zodiac is not only divided into twelve signs, starting at the equinox, but also into twelve houses, starting at the Ascendant (the part of the ecliptic currently rising on the eastern horizon). There are different systems for determining the houses, but in all systems the first house is just below the eastern horizon. Opposite, the seventh house is just above the western horizon. The houses signify different facets of our lives; the seventh is about relationships.
House
Modern Western Tropical Astrology
The system of twelve equal zodiac signs of 30° each, starting with Aries at the northern spring equinox. 2000 or so years ago the signs were named after the twelve constellations of the zodiac, but because of the precession of the equinoxes, they are now out of synch by about one sign, an indication that it is time for the beginning of a New Age. Other systems are possible. The Hindu system is sidereal, or star-based, and rotates the signs by about 50' a year to take account of precession. Even in this system the signs do not match up with the constellations because the adjustment starts from a point in the 9th Century CE, when the signs were already half moved away from their original constellations. Some people use a Western sidereal system which uses unequal signs derived from the intersections of the ecliptic with the constellation boundaries. My feeling is that the tropical system is most useful, but it is perhaps a pity that the signs are named after constellations, causing some confusion if you read that Mars is in the sign Cancer, for example, when all the time in the night sky it is clearly in the constellation Gemini.
The Earth's axis of spin is not stable relative to the distant stars, but has a slow wobble due to the gravitational pull of the Moon and Sun on the Earth's equatorial bulge. The wobble has a period of about 25,000 years, and the relative movement of the ecliptic and equatorial plane causes their intersections, the equinoxes, to move around 360° in the same period. The movement of the equinox relative to the distant stars causes the entire co-ordinate system, which depends on the equinoxes to rotate too, and not the least of the effects is that the zodiac signs rotate relative to the constellations, by about 50' a year. The signs no longer cover the constellations they are named for: in fact they are on average one constellation out. A smaller wobble is superimposed on precession caused by the relative distances and angles of the Moon and Sun. This is called nutation.
first posted September 1999