On the morning of Tuesday, March 20th at 1:14 AM the sun will cross the Celestial Equator from South to North. This event marks the beginning of spring.
The place where the sun crosses the equator is called the First Point of Aries, and it is the zero point for the celestial coordinate system astronomers use to locate objects in the sky.
Objects in the sky have positions described in Right Ascension and Declination. The First Point of Aries has the Right Ascension of 0 hours 0 minutes, and a declination of 0 degrees o minutes.
Right Ascension and Declination are the Longitude and Latitude of objects in the sky. The position of any star can be given by using its Right Ascension and Declination, for example the bright springtime star Arcturus has a Right Ascension of 14 hours 16 minutes, and a Declination of +19 degrees 7 minutes. The value of Right Ascension is the time difference between the moment when the First Point of Aries crosses the meridian and the moment when the star (or object) crosses the meridian. The value of Declination is the angle of the star (or object) North or South of the Celestial Equator, measured from the center of the Earth.