We identify four "special" days in the year based on the sun and its location in the sky. Those days mark the beginning of each of the four seasons. And tomorrow is one of those special days, the first day of winter.

For us, that means the sun's path across the sky from dawn to dusk is the lowest of any day of the year - in fact, at noontime, when the sun reaches its peak, it will be a mere 27 degrees above the horizon. With such a low track across the sky, daylight will be the shortest of any day of the year - 9 hours and 19 minutes. Compare that to the first day of summer, when the sun peaks 73 degrees above the horizon and daylight is more than 14 hours long.

There are other ways to define winter. I mentioned one a few weeks ago - meteorological winter is the three coldest calendar months of the year, December, January, and February. But officially, astronomy rules, and that makes tomorrow unique.

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