During the course of a year the Sun, as seen from the earth, appears to move in a circle against the background of the stars, this circle is known as the ecliptic. The axis of the earth's rotation is inclined at an angle to the plane of the ecliptic, so, if the earth's equator is imagined to be extended into space, it cuts the plane of the ecliptic at two points known as the equinoxes.

One of these points, the vernal or spring equinox, is taken to be the beginning of the circle and is known, both in Astronomy and Astrology, as the first point of Aries. Astrologers then divide the ecliptic into twelve sections of 30 degrees each thus generating the twelve signs.

The autumn equinox is directly opposite the first point of Aries and is the beginning of the sign of Libra. When the Sun is at these points, days and nights are of equal length.

During the summer months the Sun appears to rise higher and higher in the sky, reaching its highest point at midsummer or the summer solstice, the beginning of the sign of Cancer.

This is the time of the longest day and shortest night.

Similarly, it reaches its lowest point in the sky in midwinter or the winter solstice, the beginning of the sign of Capricorn.

This is the time of the shortest day and longest night.

The signs of zodiac therefore, has nothing to do with actual star constellations, but is based on the relationship between Sun and Earth.