Weather Watch

The Cold Moon will mark the shortest day of the year in SLO. Here’s the science behind it

The sun will set over the Central Coast marking the shortest day of the year Dec. 21.
The sun will set over the Central Coast marking the shortest day of the year Dec. 21. Special to The Tribune

My doctor told me I need to exercise more often, in fact about every other day. Huffing and puffing I can now be found riding up Pecho Valley Road to Montaña de Oro State Park on a vintage Marin Zig Zag Trail mountain bike after work.

The challenge that I and many others have experienced as we move into winter is you need start early each day to beat the sunset. Since Nov. 28, the sun has set at 4:51 p.m. here on the Central Coast, and the sunset will remain at this time through Tuesday. But it will start to set later and later beginning Wednesday until the first day of summer in 2019.

This year, the winter solstice with the shortest day and longest night of the year arrives Dec. 21, at 2:23 p.m. as the full Cold Moon of December appears the next morning.

That leads to the question, “why doesn’t the earliest sunset of the year come on the shortest day?”

In fact, in the Northern Hemisphere, the earliest sunsets of the year occur about two weeks before the first day of winter. Conversely, the latest sunrises occur about two weeks after the winter solstice.

This year’s latest sunrises will occur Jan. 2 through Jan. 11 when the sun will appear at 7:12 a.m. If we remain on Pacific Daylight Time (PDT) throughout the year, the sunrise will not happen until 8:12 a.m.; most parents would not be happy sending their children to school in the dark.

So why does that paradoxical condition happen? For the answer, I asked astronomer Ray Weymann. Here is his answer:

“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. However, that number of atomic clicks is not 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. However, 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. 3 — 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.”

By the way, the winter solstice also marks the longest day in the Southern Hemisphere due to the 23 1⁄2-degree tilt of the Earth on its axis. Christmas in Australia has the same weather characteristics as some areas of the United States on the Fourth of July.

This 23 1⁄2-degree tilt of the Earth on its axis not only drives the seasonal variations in the weather but also produces additional uneven heating of the Earth’s surface. This causes differentials in pressure, which give rise to the winds and weather.

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PG&E has a long history of supporting local educational initiatives, providing more than $75 million over the last decade. Since 1989, more than $4 million in scholarships has been awarded by our various Employee Resource Groups (ERGs), including the company’s Veterans ERG. To apply, please visit www.pge.com.

John Lindsey’s column is special to The Tribune. He is PG&E’s Diablo Canyon marine meteorologist and a media relations representative. Email him at pgeweather@pge.com or follow him on Twitter: @PGE_John.
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