Winter Solstice

By Phil Plait | December 20, 2005 11:19 pm

Today, at 18:35 Universal Time (10:35 a.m. Pacific), the Sun will reach its lowest point in its travels around the sky. If you go out every day from now until summer and observe the Sun at the same time every day, it’ll appear a little bit higher in the sky each time.*

The map above shows this. I used some planetarium software to map the Sun’s position on the sky at three dates, but the same time of day: 10:35 a.m. Pacific time for November 21, 2005 (right), December 21, 2005 (center), and January 21, 2006 (left). The curved path is the apparent path the Sun takes in the sky as the Earth orbits it once per year. This is an illusion; the Earth is doing the moving. It’s like seeing trees appear to pass you when you’re in a car, when really it’s the car that’s moving. But you can see that today the Sun is at its lowest position.

This day is called the Winter Solstice. Some people reckon it as the first day of winter (you’ll no doubt hear that on the news today), but I think the solstice marks the season’s midpoint.

You can see in that picture that a month from now, the Sun will be substantially higher in the sky. It’ll keep getting higher until June 21, then it’ll start to dip again. By next December 22, it’ll be at its lowest point once again, and the dance will start anew.

Happy Solstice!

And the cause is not due to the Earth’s distance from the Sun! It’s actually due far more to the tilt of the Earth’s axis. In fact, the Earth will be nearest the Sun on January 4th at 15:00 Universal time. Just so’s you know.

CATEGORIZED UNDER: Astronomy, Cool stuff, Science

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