The equinox occurs when the North and South poles of the Earth point at an angle of 90 degrees to the sun (i.e. when they are on the terminator). The Tropics of Cancer and Capricorn are about 23.5 degrees from the Equator, which means that during Summer solstice the pole makes an angle of (90 - 23.5) = 66.5 degrees with the sun (with the angle being measured as from the centre of the Earth). The equinoxes occur mid-way between the solstices. At the equinox, every point on the Earth (except the poles) experiences 12 hours of daylight and 12 hours of night.
BaronCarlos's writeup highlights one of the problems that some scientists have with the Zodiac, and the way it is used and misused. Take the fact that the equinox is when the sun enters Aries or Libra, the fact that 'These points are found to be moving backward or westward, at the rate of 50" of a degree in a year', the fact that Aries and Libra are constellations, and the fact that a constellation has absolutely no connection whatsoever with the precession of Earth's equinoxes. There's a gaping hole somewhere in that chain of logic.