The December solstice — the Sun’s lowest point in the sky for Northern Hemisphere observers — occurs Tuesday, December 21, at 7:42 A.M. EST. This astronomical event marks the beginning of winter for those living above the equator. Conversely, summer is beginning in the Southern Hemisphere.
Earth experiences seasons because its axis (the imaginary line joining the North and South poles) is tilted with respect to its orbital path around the Sun. Earth’s tilt measures 23.5°. In mid-June, Earth’s North Pole points toward the Sun. At this time of year, the North Pole points away from the Sun.
The result is that now, Northern Hemisphere days are shorter and the Sun appears lower in the sky. In addition, radiation from the Sun strikes Earth’s surface at a steeper angle, spreading it over a larger surface area. One square foot (meter) of sunlight in June is spread over an area many times larger in December.
At the moment of the winter solstice, the Sun is directly above an imaginary line on Earth at latitude 23.5° south called the tropic of Capricorn. Six months before or after this date, the start of northern summer finds the Sun 23.5° above the equator above a line called the tropic of Cancer.
Interestingly, the Sun is not in front of the stars of the zodiacal constellation Capricornus the Sea Goat on the first day of winter (or in front of Cancer the Crab on the first day of summer). Rather, the Sun lies in front of the stars of Sagittarius the Archer December 21, and it’s in Gemini the Twins in June. The names “tropic of Capricorn” and “tropic of Cancer” refer to the Sun’s position in front of those constellations 2,000 years ago, when those names were given.
The word “solstice” has Latin roots: sol (Sun) and stitium (a suffix that means stop). This combination refers to the fact that the Sun has been traveling southward in the sky since being at its most northerly point in mid-June. It reaches its lowest (most southerly) point in the sky December 21 and then stops. This represents both good and bad news.
The good news is the Sun will begin to climb higher in the sky afterward. The bad news is that, because of a lag in solar heating, the coldest days of the year are still ahead.