Vernal Equinox at 4:44 AM Today

The Vernal Equinox marks the date when the center of the sun passes directly over the equator marking the beginning of Astronomical Spring. 

The earth rotates on a 23.5 degree tilt.  This tilt and the earth's path as it orbits the sun allows the sun to migrate northward until it reaches 23.5 degrees north latitude, the Tropic of Cancer.  The Tropic of Cancer marks the northern most point where the center of the sun passes directly overhead.  Havana, Cuba is one major city near 23.5 degrees north latitude.