In the West, it is common for people to ask, "What's your sign?" Most people understand this to mean the zodiacal sign that the Sun was passing through at the time of the person's birth. Most people also have a rudimentary astronomical understanding that the Zodiac is a band of sky filled with constellations of stars, which falls in the background of planets as they move through the heavens. The whole thing seems rather simple. All you have to do is look up in the heavens, find the Sun on the person's day of birth, find out what constellation of stars falls in the background of the Sun and that is the Sun Sign, right?
Unfortunately, it is not that simple. Or at least in the case of Western astrology it is not that simple. First of all, when you read in the newspaper that you are a Cancer or a Leo, this does not mean that the Sun was actually in the sign of Cancer when you were born. In fact, most of the time when a Western Astrologer says that you are a Cancer, the Sun was actually in the sign of Gemini. This is because Western astrology uses what is called a Tropical Zodiac.
The Tropical Zodiac, as the name suggests, is a zodiac which is oriented to the seasons. There is a symbolic relationship between the seasons and the signs. Aries for example symbolizes new beginnings. Springtime also symbolizes new beginnings. According to Tropical Astrology, regardless of which actual sign is in the background of the Sun, the first day of springtime will always be considered to be the first day of Aries.
Vedic Astrology, however, uses a Sidereal Zodiac. The Sidereal Zodiac is aligned with the stars in the heavens. So if you ask a person what's your Vedic Sun sign, he will tell you the sign that you will actually see in the background of the Sun on his day of birth. It is usually surprising for most people to find out that the Sun was actually nowhere near the astrological sign which has become so familiar through years of reading the daily newspaper horoscope. In other words, there are two zodiacs. The one you read in the newspaper, the Tropical Zodiac that is used by westerners, does not correspond to what you see in the heavens. The one which is used by the Vedic Astrologer, the Sidereal Zodiac, does correspond to what you see in the heavens on your birthdate.