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General News · 16th December 2020
Mike Moore
It has been about a year since Steve Musial left us. The last time I saw him was on the field playing ultimate. I always enjoyed when he came out to play, we had such a good time joking and talking. One of our favourite topics was the mechanics behind the equinoxes and solstices. Of particular interest to Steve was why during the winter solstice, even though the overall daylight in a day decreases until solstice, the time of sunset starts to get later in the day much earlier than the solstice. For this year, 2020 in Campbell River, the earliest sunset for the year occurs on December 10th yet solstice and the shortest day is not until December 21st. Even though the sun is setting a little later each day, the sun is rising later too and the time difference between sunrises is more than that of the sunsets so the total length of daylight decreases. But why do sunsets start getting later before the solstice?

At winter solstice, the sun is directly above the earth at its furthest south latitude for the year. On December 21st the sun will shine down on the Tropic of Capricorn at 23.4 degrees south. Because the sun is over the earth’s southern hemisphere, we in the northern hemisphere will experience shorter days and longer nights. During the spring equinox in March, the sun will be directly above the earth’s equator and on that day everywhere on earth will have a 12 hour day and a 12 hour night. Then of course the sun will appear to rise higher in the sky in the northern hemisphere and the days will get longer until the summer solstice on June 21st when the sun lies over the Tropic of Cancer at 23.4 degrees north. This explains why the days get longer or shorter but it does not explain why sunsets start getting later before the winter solstice.

Steve loved the concept of the Equation of Time. Our cell phones and digital watches keep very accurate time and measure the length of a day as being exactly 24 hours. But often that isn’t really how long a day is. We can measure a day as being the time between noon to noon from one day to the next and we can define solar noon as being when the sun is over observer's line of longitude. If the earth’s orbit around the sun was a perfect circle, noon to noon would indeed be 24 hours. But the orbit is elliptical instead. Imagine the earth travelling at a constant speed through space but following an egg shaped path. The angular distance the earth travels relative to the sun will be more on the flatter sides of the egg than the highly curved ends. As the earth does its daily spin on its axis and simultaneously travels forward on its orbit around the sun, the time of solar noon will be different whether it is on the curved ends or flat sides of its orbit. The solstices occur on the orbit’s curved ends and the equinoxes happen on the flat sides. The length of the solar day is longer than 24 hours around the solstices and shorter than 24 hours at the equinoxes.

In 2020, the length of day between solar noon on December 16th and December 17th is 24 hours and 30 seconds. So by our watches sunrise and sunset will occur 30 seconds later than the day before. And although this effect happens independently of the sun’s change in latitude over the earth, the two effects need to be combined to accurately predict the times of sunrise and sunset. On December 17th of this year, the sun will still be shifting south to its maximum at solstice, creating a shorter day and longer night but also the earth will also be travelling the curved end of its elliptical orbit so that the solar noons from one day to the next will be more than 24 hours making sunrise, noon and sunset occur later in the day as measured by our clocks.

Steve and I would circle and whirl about each other, mimicking the motion of the earth around the sun to try to understand the Equation of Time. Happy Solstice Steve! I imagine you whirling about in timeless eternal light.