The 12 Signs of the zodiac are shown below in South Indian style [the most
logical]. The top left corner is
always Pisces/Mina. A circle has
360 degrees. Divided by 12 gives 30 degrees for each Sign.
In this south Indian style chart, the signs are always fixed. The ascendant is marked as 1 or with a slanting line.
Constellation is a group of stars forming a design as seen from earth. A constellation is not exactly 30 degrees. Nor does the group form any particular design. For example the Leo constellation can look like a triangle and a question mark. Then why is this called a Leo? Why not a cow?
Thousands of years ago the names of the zodiac was fixed in Hindu Astrology. In Sanskrit language. By Indian Sages who understood the link between humans and the planets only as a result of the power of their meditation.
Some common properties were found in humans. Such groups of humans had planets in same sign in their horoscope.
For easy rememberance, the constellation names were selected of animals that had these properties. Therefore we have a Lion as Leo Sign.
The same names have been used later by Egyptian and European astrologers for same reason. Astronomers luckily did not change the names to numbers.
The planets go around the sun in almost a level plane. Therefore, the Sun, Moon and other planets form a narrow path in the sky when seen from earth. This path is called the zodiac. The signs are called by the name of the main constellation on it.