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During the course of one year, the earth orbits around the sun. From the earth's point of view the sun appears to shift through a background of many different constellations. This apparent path of the sun is called ecliptic and the constellations along this path define the circle of the zodiac. They are (see labels): Aries the ram, Taurus the bull, Gemini the twins, Cancer the crab, Leo the lion, Virgo the virgin, Libra the scales, Scorpius the scorpion, Sagittarius the archer, Capricornus the goat, Aquarius the water bearer, and Pisces the fishes. The division in twelve signs of the zodiac was probably already introduced 3000 years ago in Babylonia. Following the moons phases the year was divided into twelve months. For each month an equal zone along the ecliptic was defined as a sign of the zodiac to create the first known celestial coordinate system and the predecessor of our modern calendar.
Astronomy is the natural science of celestial objects whereas Astrology wants to use celestial placements in order to explain the fate of individual persons. No scientific evidence has ever been found to support Astrologies claims. The signs of the zodiac were chosen arbitrarily and in most cases there isn't even a similarity between the sign and the star pattern in heaven. Interestingly todays astrology still uses the signs in a way they were 3000 years ago. Because of a shift of earths axis (precession) the historical signs of the zodiac do not correspond anymore to the orginal star patterns (constellations). The sign of Sagittarius (the archer) for example is mainly in todays constellation of Scorpius or partly even in the constellation of Ophiuchus which never was a sign of the zodiac.
The above constellation photos resemble more or less the impression of the starry sky we have by naked eye. They are intended to be as natural as possible, just the way the well adapted naked eye might see the stars in a clear night at full darkness or at twilight.
There is no standard for the constellation outlines, only the borders are defined by the International Astronomical Union. So the outlines are quite arbitrary but similar to most sky charts. Their purpose is just to help identifying the fixed star pattern of each celestial region.
Nevertheless, it is always impressive to identify such a fixed star pattern in the real night sky. Go, grab your sky chart and have a try! You even might find some planets along the ecliptic (image).