Zodiac sign calculator
Western astrology uses the tropical zodiac, fixed to the equinoxes rather than to the stars. Because the Earth axis precesses over about 26,000 years, the constellations have shifted roughly one whole sign since the dates were set two millennia ago — so someone born under "Aries" now has the Sun in front of Pisces. Astrologers are aware of this and consider it irrelevant to the system; astronomers consider it decisive. Both positions are internally consistent.
Astrology and numerology are traditions, not sciences. There is no evidence that any of it predicts anything. This calculates what the tradition says, accurately.
Star signs follow the tropical zodiac: Aries from 21 March, Taurus from 21 April, and so on in roughly monthly bands. Each sign has an element (fire, earth, air or water), a quality (cardinal, fixed or mutable) and a ruling planet.
How to find your star sign
The element and quality are more informative than the sign alone for anyone reading astrological writing, because they are what the tradition actually reasons with. Elements group signs into four temperaments; qualities group them into three modes of acting. Together they give twelve unique combinations, which is exactly why there are twelve signs — each is one pairing of four elements and three qualities, with no repeats.
Questions
Roughly the 20th to the 22nd of each month. The exact boundary shifts by a day between years.