Winter Solstice

San Francisco winter solstice sunset
San Francisco winter solstice sunset, 12/20/2023, over the ParkMerced apartment towers.
Sun declination -23.43 degrees, azimuth 240.62 degrees. Right click for larger image.
San Francisco winter solstice sunset
San Francisco winter solstice sunset, 12/21/2023, over the ParkMerced apartment towers.
Sun declination -23.44 degrees, azimuth 240.61 degrees. Right click for larger image.
San Francisco winter solstice sunset
The NOAA solar calculator shows that a line for sunset on December 21, 2023,
starting at the stone bench in Brooks Park, goes above the second ParkMerced tower.
"Solstice" is derived from the Latin words for "sun stands still", and refers to the observation that there is very little change in the declination of the sun during the summer or winter solstices. At the winter solstice, the sun is located over the Tropic of Capricorn. The declination of the sun is (-)23.34 degrees on December 16, (-)23.44 degrees on December 21, and back to (-)23.34 degrees on December 26, a variation of only one tenth of a degree in ten days time NOAA. The Romans celebrated the winter solstice for a week. When was the earliest sunset in 2023? According to the Farmer's Almanac, it was on December 8, 2023, at which time sunset in San Francisco was at 4:51 P.M. PST.
On 12/21/23, sunset was at 4:55 P.M. PST, 4 minutes later. However, that is an artefact of using Pacific Standard time (clock time) cf. Equation of time

If sun time (sundial time) is used, the time from solar noon to sunset was 4 hours 49 minutes on 12/8/23, and 4 hours 47 minutes from solar noon to sunset on 12/21/23, so measured from solar noon, sunset was 2 minutes earlier on December 21.
See chart above right.

earth's orbit declination of the sun
Sun's declination, day numbers starting at the vernal equinox.
The declinations during the summer and winter solstices, at the top and bottom of the curve, vary very little per day. The declination of the sun from day to day varies the most at the equinoxes.

Stonehenge heelstone
Standing at the Stonehenge heelstone and looking through the center of the stone circle, one can see sunset on the winter solstice marking the return of the sun and the beginning of a new year.
Stonehenge winter solstice sunset
The NOAA solar calculator shows that a line for sunset on December 21, starting at the heelstone, goes through the center of the Stonehenge sarsen stone circle, showing remarkable accuracy for a structure built 4500 years ago.

Creative Commons License
Robert Karis
www.sfog.us

Alphabetical index
Extreme sundials
Geographic north
Ingleside Terraces Sundial
Sundial accuracy
Sundial Bridge accuracy
Sundial factoids
Sundial plaques
Sundials EOT tables