Tuesday, December 23, 2008

Winter solstice: 5 hours, 41 minutes and counting

According to Wikipedia, “The winter solstice occurs at the instant when the sun’s position in the sky is at its greatest angular distance on the other side of the equatorial plane from the observer.”

That may be the most precise and necessary definition. In slightly more plain English, one can also say that the winter solstice occurs when the Earth’s axis in the northern hemisphere is tilted the farthest away from the sun, which also means that the sun appears the closest on the southern horizon at noon, which produces the shortest daytime and longest night of the year.

Since the Earth takes 365 and one-fourth days to orbit the sun, the true time of winter solstice shifts by six hours each year. And since time zones are generally spread from Hawaii and to Japan, the winter solstice can occur on the calendar between Dec. 20 and 23.

However, I checked tables, and in Alaska the winter solstice always seems to be Dec. 21, with five hours, 41 minutes, of daytime for Soldotna and Kenai.

It’s interesting to note that the earliest sunset and latest sunrise do not happen on the same date as the winter solstice.

In Alaska this winter, the earliest sunset occurs Dec. 17 at 3:53 p.m. for the central Kenai Peninsula, while the latest sunrise will be Dec. 27 at 10:12 a.m. for the central Kenai Peninsula.

This has to do with the tilt of the Earth’s axis and the Earth’s elliptical orbit. Although the rotation of the Earth is constant at 23 hours, 56 minutes and 4.09 seconds, the length of a day changes due to these two factors.

While the tilt of the Earth’s axis is constant in space – 23.5 degrees from the perpendicular of its orbit – it changes with respect to the sun during our orbit. We notice both throughout the year because stars at night always reach the same highest or lowest point in the sky above the southern and northern, respectively, horizons. That’s why the North Star Polaris is always in the same spot. In contrast, the sun appears very high in the summer and very low in the winter.

Because of that change in axial tilt with respect to the sun, the length of day is changing as well throughout the year. For those wanting a more lengthy explanation, check out http://www.larry.denenberg.com/earliest-sunset.html.
The second part is that, due to the Earth’s orbit being slightly elliptical, Earth is sometimes a little farther and sometimes a little closer to the sun. Therefore, the gravitational force between Earth and the sun changes a bit, which changes Earth’s speed in its orbit a little. We are fastest at 67,700 mph when we are closest to the sun around Jan. 3, and slowest at 65,500 mph when we are farthest from the sun around July 4.

Earth orbiting adds almost four minutes to its rotation, which makes for a day being an average of 24 hours long. But due to Earth orbiting faster or slower at times, and due to its axis tilt changing with respect to the sun, a day may be up to 29 seconds longer or shorter. That adds up throughout the year and, in turn, changes the exact time of local noon throughout the year. Since sunrise and sunset times are symmetrical before and after noon, that adds a slight variation.

Hence, the earliest sunset and the latest sunrise happen on different dates from the winter solstice.

Andy Veh is an astronomy and physics professor at Kenai Peninsula College Kenai River Campus.

No comments: