Both the sidereal and tropical zodiacs measure the 12 signs in 30-degree segments, even though the corresponding constellations of each sign are not usually 30 degrees wide.
The sidereal zodiac is out of sync with Earth's seasons and with the tropical zodiac. For example, spring in the northern hemisphere (or fall in the southern hemisphere) begins at the Vernal Equinox, when the Sun appears to cross the equator (reaches 0° declination). This typically happens around March 20 each year.
However, whereas the Sun in the tropical zodiac is at 0° Aries at the vernal equinox, the Sun in the sidereal zodiac is at 5° Pisces. The sidereal Sun doesn't reach 0° Aries until around April 14 each year.
The gap between the tropical and sidereal zodiacs increases by about 1° every 72 years. This is also known as the Precession of the Equinoxes.
Pronounced: Sid-DEER-ee-al or Sye-DEER-ee-al.