Spring officially arrives Saturday night at 11:30 PM CDT with the vernal equinox occurring!
Equinox means "equal night" in Latin.
During the equinoxes the Sun shines directly on the equator and the length of night and day are nearly equal. You have to keep in mind latitude determines the length of day.
The March equinox marks the moment when the sun rays are directly over the equator and will start to shift north of that imaginary line. The equator divides the northern and southern hemisphere.
What is an equinox you might ask?
Earth spins around its own axis approximately every 24 hours (a sidereal day is 23 hours, 56 minutes, 4 seconds). It orbits the sun in around 365 days. And, like a spinning top, it wobbles around on its axis, making a complete rotation every 26,000 years (precession of the equinoxes).
Earth is actually tilted at an angle of around 23.4 degrees toward the celestial pole, which is a a certain point in the sky. As the Earth makes its yearly orbit, one hemisphere faces the sun more than the other, the side that has summer.
On any other day of the year, the Earth's axis also tilts a little away from or towards the Sun. However, the equinoxes marks the exact moment twice a year when the Earth's axis is not tilted toward or away from the Sun at all. However, the axis tilt of around 23.4 degrees toward the celestial pole, remains the same.