The winter solstice occurred this morning at 5:02 am. This marks the start of astronomical winter when the direct rays of the sun lie farthest to the south in the Southern Hemisphere and slowly move toward the equator again. It is also known as the shortest day of the year and many assume that daylight hours start to increase right away and that sunrises get earlier and sunsets get later. But that’s not quite the case. If one looks closely at sunrise and sunset times, they notice an anomaly to the popular belief.
In our mid-latitude region, the earliest sunset of the year comes two weeks before the winter solstice and the year’s latest sunrise comes two weeks after. The reason is a difficult one to explain. There are actually two things that help to determine the local time of sunrise and sunset: the equation of time and the sun’s declination.
Believe it or not, a day is not always exactly 24 hours long. The equation of time is a way of describing this variation. The equation of time is defined as the difference between 12:00 noon on a clock and the actual time of when the sun reaches its peak across the exact center of the time zone. The length of the solar day varies considerably over the year. It is itself determined by two factors, both dependent on the position of the Earth in its orbit. From mid-November to early February these two factors work together to make the solar day longer than 24 hours. In late December, the actual solar day is as much as 30 seconds longer than 24 hours. Since we don’t adjust our clocks for this effect, the Sun’s transit moves later and later each day throughout this period. In other words, because the solar days are longer than our days, both the sunrise and sunset become a little bit later each day. This is part one of our answer.
The other part has to do with what we call the sun’s declination. This is what most people already understand about the equinoxes and seasons. This is the measure of how far above the horizon the sun reaches. During the winter solstice, the sun is at its lowest point in the sky, so we expect the sun to be above the horizon for the least amount of the day, giving us a longer night. Most of the year, this effect causes the largest day-to-day difference in the time of sunrise and sunset. However, thanks to the fact that the orbit of the Earth is not perfectly round, the day-to-day change within a couple of weeks of either side of the solstice drops to near zero. This means if we had a perfect 24-hour day, the sunrise and sunset would be virtually the same over this period. However, if we take the first factor into account, both sunrise and sunset near the winter solstice get close to 30 seconds later each day. This is why both are later in the period that you are describing. After January 6th, the declination starts to win out again and the sunrises begin to occur earlier, giving us longer periods between sunrise and sunset.