Weather Watch

SLO County experienced a ‘cold moon’ on the winter solstice. Here’s the science behind it

A “moonset” over Bishop Peak in San Luis Obispo on the morning after the winter solstice on Dec. 21.
A “moonset” over Bishop Peak in San Luis Obispo on the morning after the winter solstice on Dec. 21. Special to The Tribune

This year’s winter solstice was not only the longest night of the year but was also accompanied by a rare nearly-full “cold moon” that created a beautiful moonset over the Pacific Ocean early the following morning as gusty Santa Lucia (northeasterly) winds generated clear skies.

This occurrence won’t happen again for another 11 years, in 2029.

This phenomenon led to some keen observations by our local weather watchers and photographers who asked why the Earth’s only permanent natural satellite that’s fully illuminated doesn’t set at the same latitude or spot over the Pacific about every 30 days.

To start, as we move toward the first day of summer, the sun will rise and set farther and farther north, and the days will grow longer until the sun is directly over the Tropic of Cancer, which will be the summer solstice on June 21, 2019. This condition is caused by the 23½-degree tilt of the Earth on its axis compared to a direction perpendicular to the plane on which it orbits the sun. This condition produces the seasons and uneven heating of the Earth’s surface, which give rise to the winds and weather.

With that, why doesn’t the moonset follow the same pattern? For the answer, I asked Dr. Ray Weymann. Ray is a retired Director and Chair of the Astronomy Department at the University of Arizona and Director and Staff Member Emeritus of the Carnegie Observatories in Pasadena.

“If we could dim the sun and trace its path among the stars during the year, it would seem to move in a path which is tilted by this same amount compared to the path it would follow if the Earth’s axis of rotation were not tilted,” he said.

But what about the moon? Does it follow the same pattern of changing its rising and setting points as it moves through the sky every month?

“The answer is yes, but not quite. You see, the moon’s orbit about the Earth doesn’t quite occur in the same plane as the Earth’s orbit around the sun. If it did, then it would duplicate the north- south change of rising and settings of the sun, but once a month. (Too bad also, because if it did, we would be treated to eclipses every single month!).

“It turns out though, that the moon’s orbit is tilted with respect to the earth’s orbit by a little over 5 degrees. But here is where it gets a bit tricky.

“If we imagine tracking both the sun and moon’s path in the sky, the point where the moon’s path crosses the sun’s path slowly changes over a cycle of about 18 ½ years. What this means is that sometimes that 5-degree tilt is added to the 23 ½ tilt of the sun’s path, and sometimes it is subtracted. When it is added, the north/south monthly swings in the moon’s rising and setting points are even more extreme than for the sun.

“This situation, called a ‘lunar major standstill,’ will next occur in April 2025. But a little over nine years ago (in October 2015) that 5-degree tilt was subtracted from the sun’s tilt (‘lunar minor standstill’) and the north/south swings in the moon’s rising and setting were less than the sun’s.

“Right now, we are about halfway between those two dates, so the monthly north/south swings of the moon’s rising and setting points are about the same as for the yearly swings of the sun.

“But note that these extremes in north/south setting points for the moon are not connected to the phase of the moon at which these extremes occur. All this is much easier to visualize if you can have a look at a celestial sphere, or better yet, have a visit to a planetarium fancy enough to show these lunar and solar motions as the months and years roll by.”

By the way, on Jan. 20 a total lunar eclipse (blood or copper moon) will take place with totality of the event between 8:41 and 9:43 p.m. It’ll coincide with the super moon, which will be at perigee (the closest point in its orbit to the earth) and will appear approximately 14 percent larger and 30 percent brighter than usual. This will also be a wolf moon (first full moon of the year). Consequently, this will be called a super wolf blood moon.

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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