This coming Saturday (Dec. 21) marks one of the four major way stations on the Earth’s annual journey around the sun.
Because of the tilt in the Earth’s axis of rotation, the sun appears to rise and fall in our sky over the course of a year. It’s not the sun itself moving, but the Earth moving relative to the sun.
The Earth’s axis currently points in a northerly direction close to the second-magnitude star Polaris, also known as the Pole Star. Everything in the sky, including the sun, appears to revolve around this almost fixed point in the sky. [Season to Season: Earth's Equinoxes & Solstices (Infographic)]
Because the Earth’s axis points to Polaris no matter where Earth happens to be in its orbit, the sun appears to move over the year from 23.5 degrees north of the celestial equator on June 21 to 23.5 degrees south of the celestial equator on Dec. 21.
The sun crosses the equator travelling northward around March 21 and going southward on Sept. 21, in celestial events known as "equinoxes" (from the Latin for "equal night," as day and night are of roughly equivalent length on these dates.) The exact dates vary a little bit from year to year because of leap years.
On Dec. 21, the sun stops moving southward, pauses, and then starts moving northward. This pause is called the "solstice," from the Latin words "sol" for "sun" and "sisto" for "stop." Similarly, on June 21 the sun stops moving northward and starts moving southward.
These four dates have been extremely important to humanity since we first started to grow crops 10,000 years ago. Our ancestors have built amazing structures over the millennia to track these important landmarks. For example, Stonehenge in England was built as an astronomical observatory, its stones precisely oriented to detect the extremes of the sun’s movement.