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.

Science of the seasons: Shedding moonlight on daylight saving time

As we leave daylight saving time behind and set our clocks back an hour Sunday, I don’t want to start a discussion on its merits or lack thereof.

I’d rather talk about why we have time zones in the first place. Historically, each location on Earth had its own time, still called local time. It is defined by the sun reaching its highest point in the sky — which it does so exactly in the south. That’s called local noon. Local noon doesn’t depend on latitudes but on longitude only, hence we find time zones spread across a world map horizontally.

Local noon makes sense as it describes time conveniently at each longitude. A keen observer can be replaced by a good time piece.

The disadvantage came — big time — with the change in communication and especially transportation needs of the 19th century: train schedules didn’t just include the travel times, but they needed to accommodate the change in time depending on each destination’s longitude. The solution to that was to set up time zones, with New Zealand being the first country to do so in 1868, the United States and Canada in 1883 (by law in 1918). In 1884, 25 nations (12 American, 11 European, as well as Liberia and Japan) met for the Meridian Conference in Washington, D.C., and established Greenwich as the Prime Meridian and created 24 time zones of 15 degrees longitude each.

There are exceptions to the latter, especially concerning national and state borders. Usually these exceptions are subtle: Ogallala, Neb., is on Mountain Time while Plateau, Texas, is on Central, even though it’s two degrees farther west. Some exceptions are more extreme: all of China is on one time zone, although the country spans 60 degrees (and therefore should technically be divided into four time zones). Lublin in Poland and Santiago de Compostela in Spain are in the same time zone, even though they are 30 degrees apart. In contrast, Russia is in accordance with the meaning of the Meridian Conference, spanning 10 time zones over 160 degrees.

Alaska is on one time zone (with the exception of the Aleutians West of Umnak Island), although Ketchikan and Nome are 35 degrees apart. What that means is Southeast Alaska’s local noon occurs near 12 noon, central Alaska’s local noon is around 1 p.m. and Western Alaska’s local noon around 2 p.m. Sunrise and sunset are equally shifted, with Yakutat having both its sunrise and sunset 1.5 hours earlier than Bethel throughout the year.

Harvest moon

The harvest moon is the full moon closest to the autumnal equinox, and therefore occurs within two weeks before or after Sept. 22. At that time of the year the moon is in the constellations of Pisces and Aries, which stay close to the horizon when they rise in the east; hence the moon stays close to the horizon, as well.

That gives rise to a couple of phenomena. One is based on the physics of diffraction: close to the horizon, moonlight (which is reflected sunlight) has to travel through more of Earth’s atmosphere. Air, which is mostly nitrogen and oxygen molecules, scatters more of the blue part of the spectrum, leaving the moon more reddish.

The other is completely psychological: what is called the moon illusion makes the moon appear larger closer to the horizon than up in the sky (although either are of the same angular size). Even though there is no single hypothesis explaining the effect and different people may experience it differently, one such hypothesis holds that to us the moon looks farther away on the horizon than somewhere in the sky (where it is surrounded by sky) – because it is still of the same angular size. By appearing farther away, it also appears to be bigger. In other words, this is not what the moon really looks like, but it’s based on how our brains interpret what our eyes see. Good comparisons are optical illusions that fool all of us (like the same size dots where one appears larger than the other, the parallel lines that appear nonparallel, etc.).

Also, while the moon’s orbit around Earth is a nice ellipse, describing its motion and appearance relative to the horizon becomes quite complicated, because two different spherical coordinate systems are merged. One consequence is that while, on average, the moon rises 50 minutes later each night, during the fall it rises only 30 minutes later each night. Therefore twilight turns quickly into a brightly lit evening, assisting farmers to get their crops in.

Andy Veh is a physics and astronomy professor at Kenai Peninsula College’s Kenai River Campus. E-mail your science questions to redoubtreporter @alaska.net.