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The Zodiac Print E-mail
Written by Administrator   
Thursday, 25 August 2011 17:11


The Zodiac is a band around the sky about 18° wide, centered on the ecliptic, in which the Sun, Moon, and planets move.

The significance of the zodiac stems from the fact that the ecliptic -- the narrow path on the sky that the Sun, Moon, and planets appear to follow -- runs directly through these star groupings. Since ancient times, the Sun, Moon, and planets have been known as special astronomical objects -- they "wander" through the background stars of the zodiac, which remain fixed with respect to each other. It was reasoned that these zodiacal constellations must be special to make up this path, and the relative positions of the "wandering stars" within them bore great importance.

The ecliptic is the great circle cut in the celestial sphere by an extension of the plane of Earth's orbit; equivalently, the apparent annual path of the Sun against the background of the stars. Because of Earth's axial tilt, the ecliptic is inclined at about 23.4° to the celestial equator, an angle known as the obliquity of the ecliptic. The ecliptic intersects the celestial equator at the equinoxes. The ecliptic poles are the two points on the celestial sphere that lie 90° north and south of the plane of the ecliptic. The orbits of the planets, with the exception of Pluto, lie close to the ecliptic.

A great cirlce is the line of intersection of the surface of a sphere and any plane that passes through the center of the sphere. The shortest route between two points on a sphere, such as the Earth, is along the great circle that connects these points. A great circle is a geodesic.

A geodesic is a path on a given surface that is as straight as possible; in other words, a path that doesn't deviate either to the left or to the right, and only bends when forced to do so by the curvature (if any) of the surface. If the surface is an ordinary plane, the geodesics are straight lines; on a sphere the geodesics are great circles.

The celestial sphere is the imaginary sphere upon whose inner surface celestial objects can be considered to lie for the purpose of describing their position. The celestial sphere is centered on the origin of whatever system of celestial coordinates is being used.

In astronomy and navigation, the celestial sphere is an imaginary sphere of arbitrarily large radius, concentric with the Earth and rotating upon the same axis. All objects in the sky can be thought of as projected upon the celestial sphere. Projected upward from Earth's equator and poles are the celestial equator and the celestial poles. The celestial sphere is a very practical tool for positional astronomy.

Perspective of Earth and celestial sphere, showing the ecliptic (plane), the celestial equator (overhead the Earth's equator) and the Earth's polar axis (which also points to the celestial poles). The intersection shown between the equator and the ecliptic (the vernal equinox point) is not associated with any particular location on the Earth (despite the diagram), because the Earth rotates daily, while the celestial sphere does not.

We can again see that the ecliptic is an imaginary line that shows the path the Sun apparently "travels" against the fixed stars (celestial sphere) as the earth revolves around the Sun during the year. In more accurate terms, it is the intersection of the celestial sphere with the ecliptic plane, which is the geometric plane containing the mean orbit of the Earth around the Sun.

The ecliptic serves as the center of a region called the zodiac, which constitutes a band of 9° on either side. Traditionally, this region is divided into 12 signs of 30° longitude each. By tradition, these signs are named after 12 of the 13 constellations straddling the ecliptic.

The celestial axis is the projection of Earth’s rotation axis, north and south, onto the celestial sphere. The celestial poles are the two points where an extension of Earth’s axis intersects the celestial sphere and about which the celestial sphere appears to rotate daily. As a result of precession, the celestial poles complete a circle around the ecliptic poles every 25,800 years.

Precession is a slow, periodic conical motion of the rotation axis of a spinning body, most familiar in the wobbling of a toy top or gyroscope. Earth's axis precesses, once around every 25,800 years, as a result of it not being perpendicular to the ecliptic (the plane of Earth's orbit around the Sun). Being tipped over at about 23.5°, it is affected by gravitational perturbations from other bodies in the Solar System, including the Sun, Moon and planets. These bodies pull harder on the part of Earth's equatorial bulge nearest them than on the part farthest away, resulting in a torque(force) that is the underlying cause of precession.

The component of precession caused by the Sun and Moon is called lunisolar precession; that caused by the action of the planets is called planetary precession. The sum of lunisolar and planetary precession, known as general precession, results in a westward movement of the equinoxes (the nodes of the ecliptic) known as the precession of the equinoxes. This movement amounts to an average of about 50.3" per year – the so-called precession constant. Precession means that a number of different stars in the circumpolar regions of the celestial sphere serve as pole stars on a cyclical basis.

An equinox is:

1. Either of the two points on the celestial sphere where the celestial equator intersects the ecliptic.

2. Either of the times at which the center of the Sun's disk passes through these points.

The vernal equinox (or spring equinox), when the Sun reaches its ascending node (i.e., crosses the celestial equator moving northward), falls on or around March 21 and marks the start of spring; the autumnal equinox, when the Sun reaches its descending node, occurs on or around September 22 and marks the start of autumn. These are the two days of the year on which, everywhere on Earth, day and night are of equal duration: hence the name.

The equinoxes drift very slightly across the sky because of precession. For example, in the time of Hipparchus, about 2,100 years ago, the vernal equinox lay in Aries and it is still referred to as the First Point in Aries, even though it has now moved into Pisces. Likewise, the autumnal equinox, also called the First Point in Pisces, now lies in Virgo.

As mentioned, the position of the vernal equinox is not fixed among the stars but due to the lunisolar precession slowly shifting westwards over the ecliptic with a speed of 1° per 72 years. A much smaller north/southwards shift can also be discerned (the planetary precession, along the instantaneous equator, which results in a rotation of the ecliptic plane). Said otherwise, the stars shift eastwards (increase their longitude) measured with respect to the equinoxes — in other words, as measured in ecliptic coordinates and (often) also in equatorial coordinates.

Using the current official IAU constellation boundaries — and taking into account the variable precession speed and the rotation of the ecliptic — the equinoxes shift through the constellations in the Astronomical Julian calendar years (in which the year 0 = 1 BC, -1 = 2 BC, etc.) as follows:[1]

- The March equinox passed from Taurus into Aries in year -1865, passed into Pisces in year -67, will pass into Aquarius in year 2597, will pass into Capricornus in year 4312. It passed along (but not into) a 'corner' of Cetus on 0°10' distance in year 1489.
- The June solstice passed from Leo into Cancer in year -1458, passed into Gemini in year -10, passed into Taurus in December 1989, will pass into Aries in year 4609.
- The September equinox passed from Libra into Virgo in year -729, will pass into Leo in year 2439.
- The December solstice passed from Capricornus into Sagittarius in year -130, will pass into Ophiuchus in year 2269, and will pass into Scorpius in year 3597.

The celestial meridian is the great circle on the celestial sphere that passes through the celestial poles and the zenith of the observer. The celestial equator is the great circle on the celestial sphere that divides the northern and southern hemispheres and serves as the zero-mark for declination; it is the projection into space of Earth’s equatorial plane.

Declination is the angular distance of a celestial body north (positive) or south (negative) of the celestial equator: the equivalent of latitude on Earth. The declination of the Sun is the angular position of the Sun at solar noon with respect to the plane of the equator.

The celestial latitude is the angular distance on the celestial sphere measured north or south of the ecliptic along the great circle passing through the poles of the ecliptic and the celestial object. The celestial longitude is the angular distance along the ecliptic from the vernal equinox eastward.

The Zodiac band is divided into 12 signs of the zodiac, each 30° long, that were named by the ancient Greeks after the constellations that used to occupy these positions; "zodiac" means "circle of animals," and only Libra is inanimate. Over the past 2,000 years, precession has moved the constellations eastward by over 30° so that they no longer coincide with the old signs.


Last Updated on Monday, 26 September 2011 23:35

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