Solstice 2010

On Tuesday, December 21, 2010 at 4:38 p.m. the sun will stop moving south and begin moving north. Tuesday will be the shortest day of the year at 9 hours, 21 minutes and 22 seconds. Tuesday night will be the longest night of the year at 14 hours, 38 minutes and 38 seconds.

Wednesday, December 22 will be about two seconds longer than Tuesday, and Wednesday night about three seconds shorter. And so it will go, days getting longer and nights shorter until on June 21, 2011 at 11:16 a.m., the sun will stop moving north and begin moving south. June 21 will be the longest day of the year at 14 hours, 59 minutes and 8 seconds. The night of June 21 will be the shortest of the year at only 9 hours and 52 seconds.

This is the Earth-centric view, of course. Although the day/night duration calculations are correct, and days seem to get shorter in the summer and fall and longer in the winter and spring, that’s just our perspective from the surface of the earth. In reality, so far as our solar system is concerned, the sun is fixed in the center of the system and the planets – including the Earth – orbit around it.

But Earth’s annual trip around the sun – its orbit – doesn’t account for the apparent movement of the sun – south to north in winter and spring and north to south in summer and fall. Were it as simple as that – the Earth orbiting the sun – we’d have a situation similar to that of the Moon. Luna, as the Moon is often called, orbits the Earth and rotates on its axis only once per revolution, keeping one side, the side familiar to us, always facing the Earth. The other side – the so-called dark side – never faces our planet, and only since the advent of space flight and rocket trips to the moon has man ever had a glimpse of the dark side.

But Earth, which is not tide-locked into a single rotation per orbit as the moon is, rotates much more frequently, making 365 and a quarter rotations for each orbit around the sun. Each rotation takes just under 24 hours. From these numbers, 365.25 and 24, we derive the length of our year, measured in days, and the length of our day, measured in hours.

This is all very well, but the fact that the Earth rotates once per day and orbits the sun every 365.25 days doesn’t explain the apparent north-south, south-north movement of the sun when viewed from the surface of Earth.

What causes that apparent movement is the fact that the Earth is tilted about 23.5 degrees in relation to the plane in which it orbits the sun. This plane is called “The Plane of the Ecliptic,” and is the same plane occupied by the other seven planets of our solar system as they orbit the sun. If the Earth stood vertical in the Plane of the Ecliptic, with the north pole straight up and south pole straight down, the sun would never move north or south. It would rise each morning in precisely the same location on the eastern horizon and set each evening in precisely the same location on the western horizon. Each period of day and night would be the same from day to day, and those periods would be very close to 12 hours for most of the planet.

Earth is tilted, however, and presently the axis of rotation – an imaginary line drawn through the center of the earth from the north physical pole to the south physical pole and around which the earth rotates once every 24 hours – is leaned over at 23.5 degrees.

The Earth’s axial tilt of 23.5 degrees is the secret to the seasons, and the secret to December’s Winter Solstice. (Image courtesy NASA/In the public domain)

So why the seasons? Why is there a summer solstice and a winter solstice? The secret is that the axial tilt of 23.5 degrees remains constant (at least in our very short temporal frame of reference) as the Earth orbits the sun. In the summer, the northern hemisphere is leaned over toward the sun, and the sun’s light shines more directly on the north half of the planet. In the winter, the northern hemisphere is leaned away from the sun, and the sun’s light shines less directly on the north half of the planet. If you live below the Equator, in Australia, say, the process is exactly reversed, and your summer begins in late December while your winter begins in late June.

All in all, three things have to work in concert to provide the seasons and the apparent solar movement we see. Earth has to orbit the sun. Earth has to rotate. And – perhaps most importantly – Earth has to be “leaned over” with an axial tilt of 23.5 degrees.

So enjoy next Tuesday, Dec. 21, the shortest day and longest night of the year, with the certain knowledge that as our planet continues to orbit the sun, our trusty axial tilt will mean lengthening days. Before you know it, it will once again be spring, and life will begin anew across the lovely and majestic Great Plains.