In Western astrology, astrological signs are the twelve 30-degree sectors that make up Earth's 360-degree orbit around the Sun. The signs enumerate from the first day of spring, known as the First Point of Aries, which is the vernal equinox. The astrological signs are Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius, and Pisces. The Western zodiac originated in Babylonian astrology, and was later influenced by the Hellenistic culture. Each sign was named after a constellation the sun annually moved through while crossing the sky. This observation is emphasized in the simplified and popular sun sign astrology. Over the centuries, Western astrology's zodiacal divisions have shifted out of alignment with the constellations they were named after by axial precession of the Earth while Hindu astrology measurements correct for this shifting. Astrology (i.e. a system of omina based on celestial appearances) was developed in Chinese and Tibetan cultures as well but these astrologies are not based upon the zodiac but deal with the whole sky.

Astrology is a pseudoscience. Scientific investigations of the theoretical basis and experimental verification of claims have shown it to have no scientific validity or explanatory power. More plausible explanations for the apparent correlation between personality traits and birth months exist, such as the influence of seasonal birth in humans.

According to astrology, celestial phenomena relate to human activity on the principle of "as above, so below", so that the signs are held to represent characteristic modes of expression. Scientific astronomy used the same sectors of the ecliptic as Western astrology until the 19th century.

Various approaches to measuring and dividing the sky are currently used by differing systems of astrology, although the tradition of the Zodiac's names and symbols remain mostly consistent. Western astrology measures from Equinox and Solstice points (points relating to equal, longest, and shortest days of the tropical year), while Hindu astrology measures along the equatorial plane (sidereal year).