Our planet for a year makes a complete revolution around the Sun. Every month on her way she meets a new constellation. The ancient Greeks called the circle of the constellations of the zodiac.
Although in Greek zodiac means "animal", not all of the twelve constellations are called sentient beings. To zodiac constellations are Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius, Pisces.
Under the zodiac we understand the zone, located along the Ecliptic (the circle on the sky, which is the apparent annual motion of the Sun). This circle includes not only the Sun, but the planet and the Moon. The belt is conventionally extends within 9 degrees on either side of the Ecliptic. It has been divided into 12 equal parts, representing the rectangles width of 18 degrees and a length of 30 degrees, which are called the signs of the Zodiac.
But since the vernal equinox point moves along the Ecliptic against the movement of the Sun, move and also an imaginary rectangle that correspond to the signs of the Zodiac. For 36 years the vernal equinox point is shifted by 0.5 degrees, which is roughly equal to the diameter of the moon. Therefore, the signs of the Zodiac over the last 2 thousand years has moved in a completely different constellation, and it turned out that they were currently (conditional rectangles) do not coincide with the zodiac constellations, though, and continue to wear their original names.
If anyone of you born under the sign of Libra, the Sun was in the constellation of Virgo. But the ancient Greek philosopher or a Roman Caesar, who was born under the sign of Libra, was born when the Sun really was in the constellation of Libra.
"Modern sky" has already 88 constellations. New constellations were introduced in the era of great geographical discoveries, in XVII-XVIII centuries in the course of exploring the southern sky and fill in the "blanks" in the Northern sky. The names of these constellations do not have mythological roots.
Now the constellation look differently than a hundred thousand years ago. Differently they will look in the future. View constellations changed for several reasons: they move across the sky, some stars disappear, others are born.