August 12, 2017 6:58 PM
I received quite a few questions this week by email on a variety of subjects, from the Perseid meteor shower to an inquiry about why the earliest sunset of the year doesn’t happen on the shortest day.
As travelers on Earth, we orbit the sun at about 67,000 mph. At this point in our annual journey, our world is blasting through the stream of debris that lies along the orbit of the comet Swift-Tuttle, which circles the sun about every 133 years. This produces the Perseid meteor showers.
Because the remains from this comet are moving in an opposite direction to Earth’s orbit, its small bits and fragments slam into the Earth’s upper-atmosphere at a combined velocity of 130,000 mph. At speeds like this, even the most minuscule pieces of debris can produce beautiful meteor showers. Unfortunately, this year’s display probably won’t be as good as previous years — and here’s why.
The Earth passed through heaviest part of the comet’s debris field in the middle of the day Saturday. By the time it starts to get nice and dark after sunset Saturday, a three-quarters full moon rises at 11:06 p.m., obscuring the night sky with moonlight. I headed inland Friday night to escape the marine layer in hopes to capture a few photos, but by the time it got dark enough to get a good view, the moon was already on the rise. At least the new moon on Aug. 21 will create a total eclipse of the sun that will sprint eastward across a swath from Oregon to South Carolina.
Speaking of the sun, as we head toward the first day of winter, we are losing daylight at an ever increasing rate. It will peak on the fall equinox on Sept. 22, when we lose more than two minutes of sunlight each day.
The shortest day of the year will occur on the winter solstice on Dec. 21, when the sun will rise at 7:08 a.m. and set at 4:55 p.m. However, at our latitude, the earliest sunset for this year will happen about 4:51 p.m. Dec. 7. In other words, the earliest sunset will transpire before the winter solstice, and the latest sunrises occur after — which didn’t make a lot of sense to me, but here’s why.
The Earth’s orbit around the sun is not a perfect circle, but rather an ellipse. During the Earth’s closest point of approach to the sun — or perihelion — the Earth’s velocity increases, and combined with the Earth’s tilt, it produces earlier sunsets before the winter solstice and later sunrises afterward.
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 the United States on the Fourth of July.
Here on California’s Central Coast, our shortest day has about nine hours and 47 minutes of sunlight, while our longest day in June has about 14 hours and 31 minutes of sunshine.
If you traveled north in December, the days would become shorter and shorter until you reach the North Pole, with its seemingly perpetual darkness. If you then continued over the North Pole and headed south, the days would become longer as you approach the South Pole with its 24 hours of sunlight.
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Nearly 70 percent of the electricity PG&E delivered to its customers in 2016 came from greenhouse gas-free resources such as nuclear (Diablo Canyon Power Plant) and large hydro. An average of 32.8 percent of its electricity in 2016 came from renewable resources, including solar, wind, geothermal, biomass and small hydroelectric power plants.