Christmas Day

Christmas Day Sunrise

Christmas to me has been, and still is, about light. I love the lights on the Christmas trees and displays in yards and on houses. So festive and welcome during these long nights.

The Christmas season is also the Winter Solstice season. The Winter Solstice marks the day when the amount of day light slowly, gradually, starts to increase. The daily addition of a few seconds and minutes of sunlight results in the days of June being hours longer than the days of December. And here in the Northern Hemisphere, not only do we get a change in quantity of sunlight after the Winter Solstice, we start to get a change in quality too as the Sun rays become more intense.

We are not sensible of the Earth's perpetual tilt (perpetual in terms of human scale that is) towards the North Star but it's this perpetual tilt to another star beyond the Sun that means the Northern Hemisphere lists towards the Sun roughly April through September and away from it October through March.

Despite this confusing description which makes it sounds as if the Earth is bobbling back and forth, the Earth stays steady with this tilt to the North Star. There is no bobbling. Rather, it is the tilted Earth's position on the orbital path around the Sun that determines whether the Northern Hemisphere is leaning towards or away. If you were in deep outer space looking at the Earth, Sun and North Star you would see a change in how they were lined up. At certain times you might see the Earth-Sun-North Star with the Northern Hemisphere leaning towards the North Star with the Sun between them, as if it were in the way.  Then Earth trundles along its orbit so six months later the alignment changes to Sun-Earth-North Star and Earth, still tilted toward the North Star, is on the other side of the Sun which is no longer between Earth and North Star. Same tilt. Different line up.

This change in Earth's location on its orbital path means that Southern Hemisphere sticks out towards the Sun October through March. Somewhat unintuitively, it's not the proximity of the Hemisphere to the Sun that causes the intensification of the Sunlight but rather the directness of the rays. If you hover a flashlight over a dark piece of construction paper, you can see how the intensity of the light changes as you tilt the construction paper (not the flashlight!) this way and that.

The effects of this change in increased light quantity and quality are still months out. We actually don't hit peak response (dog days of summer) until after the Summer Solstice which begins the reduction of light quantity and intensity. Consider, then, the seasons are a very delayed dose response to the Sun.

But back to Christmas. I'm that person in the neighborhood that leaves their lights on till Jan 6. This is the day that at my latitude we have a full 9 hours of daylight, an increase of 10 minutes from December 21 just two short weeks ago.

I'll turn off the lights then, even though we have months of cold left. The Sun returns.

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