In principle, the combination of element and quality that is unique to each sign, decides what its character is.
Each Zodiac sign is also said to have a ruling and an exalting planet. This simply means that some pla­nets are particularly enforced by the character of that sign. A planet rules over the Zodiac sign where it fits better than the other planets do, and it exalts in a sign where it fits better than any other planet than the ruler of that sign.
Here are the ruling and exalting planets (note that also the sun and the moon are regarded as planets in this context) of the Zodiac:

SIGN RULER EXALTED
Aries Mars Sun
Taurus Venus Moon
Gemini Mercury Jupiter
Cancer Moon Venus
Leo Sun Pluto
Virgo Mercury Venus
Libra Venus Mercury
Scorpio Pluto Neptune
Sagittarius Jupiter Uranus
Capricorn Saturn Mars
Aquarius Uranus Saturn
Pisces Neptune Mercury

Since there are twelve Zodiac signs and only ten planets to divide between them, Venus and Mercury both rule and exalt in two signs each.
Regarding what planets rule and exalt in what signs, astrologers don't agree completely, but most of the above is generally accepted. In any case, the rulership and exaltation should not be overly emphasized in horo­scope interpretations. A planet's character in a sign speaks well for itself, without these generalizations.
When people are said to be born in a certain Zodiac sign, it only means that the sun was in that sign at their birth. It's their sun sign. The sun is only one component in the horoscope - although an important one - so pre­dicitons based only on that are not astrologically ade­quate. The whole horoscope has to be considered.
The sun moves steadily through the Zodiac signs, spending about 30 days in each of them. The calendar system of leap years makes the dates of change from one sign to another jump around a little, but in average, these are the dates that the sun spends in each sign:

Aries March 21 - April 19
Taurus April 20 - May 20
Gemini May 21 - June 20
Cancer June 21 - July 22
Leo July 23 - August 22
Virgo August 23 - September 22
Libra September 23 - October 22
Scorpio October 23 - November 21
Sagittarius November 22 - December 21
Capricorn December 22 - January 19
Aquarius January 20 - February 19
Pisces February 20 - March 20

If you are born on a date of change, or the day before or after it, you should have your sun position calculated by a horoscope program, to ascertain what sun sign you are.
The traditional order of the Zodiac signs starts with Aries, because it is the moment of the spring equinox. That date was also the date of the new year, in ancient Europe. At the fall equinox, the sun moves into Libra. At the summer solstice it enters Cancer, and at the winter solstice Capricorn.

References:

The following information is taken from http://www.horoscoper.net/astrology/zodi ac.htm

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