Today is the day of the December solstice! This is the day on which the sun reaches its southernmost point in its yearly journey through the stars (as seen from Earth). Thus, in the northern hemisphere, the maximum daily altitude that sun reaches (when it is at the point halfway between rising and setting) is the lowest of the whole year. The length of the sun’s day-long arc above the horizon is therefore shortest. Consequently, the length of time that the sun is above the horizon is shortest, and the duration of daylight is shortest. A sun that is low in the sky and not up for very long will not warm things much. Thus in the northern hemisphere the December solstice is the “winter” solstice.
Of course the situation in the southern hemisphere is just the opposite: the sun’s maximum daily altitude, the length of the sun’s arc above the horizon, and the duration of daylight are all greatest. A sun that is high in the sky and up for a long time warms things quite well. Thus in the southern hemisphere this is the “summer” solstice.
Of course, as mentioned in the previous post on the solstice, just because today is the shortest/longest day does not mean the times of sunrise and sunset are behaving as expected. More on that to come in a future post!
*Plots are based on data from the US Naval Observatory.