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Spring begins on March 20th with the vernal equinox.  This is when the Sun’s path, known as the ecliptic, intersects with the celestial equator making night and day equal throughout the world.  During the vernal equinox the Sun is moving in a northerly direction; thus, it appears higher in the sky each day until it reaches its peak at Tropic of Cancer.  This is marked by the summer solstice which occurs on June 21. 

The Sun’s path then travels in a southerly direction until it again intersects with the celestial equator on the autumnal equinox.  It continues to travel south, making days darker and colder in the northern hemisphere, until it reaches its lowest point at the Tropic of Capricorn.  This marks the winter solstice around December 21.  The Sun then proceeds back to the north of the celestial sphere and the whole process begins again.

Relationship between Earth’s axial tilt (ε) to the tropical and polar circles. The orthographic projection was rendered from the NASA Blue Marble Next Generation.