As San Luis Obispo County heads toward winter, many of us are commuting back to our homes with a sky painted in hues of red and orange, or most likely with stars.
As the sun sets earlier with each consecutive day, after-school soccer practices now occur under athletic field lights. Many other high school sports have retreated to gyms.
In a recent email, Marty Kaliski of San Luis Obispo asked, “Why doesn’t the year’s earliest sunset occur on the year’s shortest day?”
It may seem contradictory, but in the Northern Hemisphere, the earliest sunsets of the year occur about two weeks before the first day of winter, the date with the shortest day and longest night of the year. Conversely, the latest sunrise occurs about two weeks after the Winter Solstice.
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So why does this paradoxical condition happen? For the answer, I asked astronomer Dr. Ray Weymann, who provided this explanation:
“Sunrises and sunsets have to do with the real sun rather than the ‘fictitious sun.’ This means that what we call solar time and clock time get out of whack, and this is measured by that which is called the equation of time. The two effects described below cause the equation of time to change rapidly in December, and so the day of earliest sunset measured by clock time occurs well before the shortest day of the year, and the day of latest sunrise well after the shortest day of the year.
“To understand this, we need to understand the difference between a ‘clock day’ and a ‘sun day.’ We also need to understand the two main motions of the Earth and how they relate to keeping time. Time is now kept with the super-regular ticks of atomic clocks. A ‘sun day’ is the number of ticks of that clock from one solar noon to the next. Solar noon is also called simply ‘midday.’
“It refers to that instant when the sun reaches its highest point for the day. But that number of atomic clicks is not exactly the same for every day of the year. Why is this?
“The Earth rotates on its axis, and this causes the stars and sun to rise and set every day. Now imagine a stick pointing to the sun and also to some distant star right next to the sun when they are together at their highest in the sky. The next day, the stick will be pointing to the same star. But because the Earth has moved a little way in its orbit around the sun, the sun will no longer be lined up with that star but will have appeared to have shifted a bit to the east of that star. So it will take a little extra time — about four minutes longer — before the Earth rotates a bit more until the sun reaches its highest point. “Because Earth’s orbit around the sun is not a perfect circle but rather an ellipse, the speed of the Earth in its orbits varies.
“During the Earth’s closest point of approach to the sun — called perihelion (which occurs around Jan. 4 during our Northern Hemisphere winter) — the Earth’s orbital speed increases, so the sun takes slightly longer than average to reach its highest point and conversely a little less when it is farthest from the sun.
“But there is another second effect: the Earth’s axis of rotation compared to the plane of the Earth’s orbit is tilted compared to the Earth’s equator, which also causes a comparable amount of variation in the number of ticks in a solar day. The ‘fictitious sun’ is an imaginary one which has the same length of a ‘fictitious solar day’ every day of the year, and that is how our 24-hour clock time is defined.
“On Dec. 7, the sun reaches its noontime position at 11:52 a.m. clock time — what we call local standard time. Two weeks later — on the winter solstice — the sun will reach its noontime position around 11:59 a.m. That's seven minutes later than on Dec. 7.”
This tilt of the Earth on its axis not only drives the seasonal variations but also produces additional uneven heating of the Earth's surface. This causes differentials in pressure, which give rise to the winds, but that's a story for another column. • • • PG&E safety tip: Driving in fog can be every bit as treacherous as rain and snow. Please slow down, as distances are especially hard to judge in fog. Turn on your low-beam headlights. Be on the lookout for pedestrians, animals or stopped cars. Remember that the road is wet, so it will take you longer to stop.