WINTER SOLSTICE: Everybody knows that December 21 is the shortest day of the year but not many people know why. The Sun reaches its maximum southern declination of the year. For those of us in the northern hemisphere, that means the noon Sun is lowest in the sky of any date of the year. From latitude 40 degrees north (New York to San Francisco), the noon Sun grazes the treetops and the noon shadows are twice as long as the objects casting them. The winter solstice is also the first “official” day of winter for the northern hemisphere. After this date, the Sun is higher in the noon sky each day, which causes the days to grow longer.
In the southern hemisphere, the sky appears upside down and everything is reversed. The Sun is highest in the noon sky and the days are the longest of the year. The southern hemisphere is in summertime when the northern hemisphere experiences winter.
Celestially speaking, the Sun is lined up with the constellation Sagittarius on December 21, as seen from the Earth. In the coordinate system of the sky, the Sun is at a declination of -23.5 degrees, or 23-1/2 south. The Sun is at 18 hours right ascension, halfway around the celestial sphere from the place of the summer solstice in Gemini, the northern extreme of the zodiac. Since the Sun is high in the sky in Sagittarius at noon, Gemini (and its neighbor, Orion) is high in the sky at midnight.