The Tropical Zodiac measures the 12 zodiac signs relative to Earth's seasons rather than the stars or constellations.
The sign of Aries begins at the Sun's position at the Vernal Equinox (in April – begins spring in the northern hemisphere and fall in the southern hemisphere).
Cancer begins at the Sun's position at the Solstice (June – begins summer in the northern hemisphere, and winter in the southern hemisphere).
Libra begins at the Sun's position at the Autumnal Equinox (September – begins fall in the northern hemisphere and spring in the southern hemisphere).
Capricorn begins at the Sun's position at the other Solstice (December – begins winter in the northern hemisphere and summer in the southern hemisphere).
The tropical zodiac shifts very slowly in a counter-zodiacal direction, relative to the fixed stars. This is called Precession of the Equinoxes and is caused by gradual wobble in the Earth's rotation.
