Celestial change that causes the winter solstice

On Dec. 21, we will reach the winter solstice.

The Sun will rise latest and set earliest, be lowest in the sky at noon and we will have the shortest period of daylight.

For most of us, this does not impact us much. Our houses are warm and the stores are filled with food from all over the world. However, in the past, when people depended on how much food the community saved to cover the winter, an assurance that the days were about to become longer and spring is coming was a great reassurance and something to celebrate.

For example, in the years around 2500 BCE, on the late afternoon on the shortest day of the year, a long, solemn procession approached Stonehenge, in England, from the north-east.

The route was planned so those approaching the monument, located on the Salisbury Plain in south-west England, would see it suddenly as they climbed over a hump in the landscape, against the blaze of the winter sunset.

Archaeological studies showed the event was attended by people from all over Britain, some of them travelling for more than a month. Before the Stonehenge ceremony, the attendees spent some days celebrating nearby at another monument, now known as "Woodhenge".

Similar ceremonies were taking place in many lands. When Christianity spread across Europe, the celebration of the birth of the new year was reassigned to celebrate a different birth.

After centuries of calendar adjustments, this celebration is now three or four days after the solstice.

The annual sequence of spring equinox, summer solstice, autumn equinox and winter solstice are all due to one simple thing, the inclination of the Earth's axis.

As the planets orbit the Sun, they spin on their axes, like tops. Earth's axis is not perpendicular to the orbit plane, it leans by some 23 degrees and happens to be pointed at a star, which we therefore call Polaris, the North, or Pole Star.

That means at one point in the year, when the Sun and Polaris are on the same side of the Earth, the Northern Hemisphere is leaning toward the Sun. So we get the longest days with the Sun highest in the sky at noon—the summer solstice.

Six months later, when the Earth is on the other side of the Sun, the Northern Hemisphere is leaning away from the Sun and we get the shortest days with the Sun lowest in the sky at noon—the winter solstice.

Therefore, there have to be two occasions, between the solstices, where the Earth is leaning to the side, neither towards nor away from the Sun, which at that point is overhead at the equator. Days and nights are of equal length and are therefore called equinoxes. The one we pass through heading towards summer is the spring equinox, and the other one the autumn equinox.

The direction of the earth's axis moves slowly in a circle, taking about 26,000 years so, but the actual tilt only changes by a degree or so, which is probably crucial for life to continue being possible on our planet. With continents moving around and large areas of the earth's crust lifting and sinking, it is really quite remarkable that the tilt is not changing a lot. The reason is that Earth is being stabilized by the Moon's gravitational tugging.

The situation on the planet Mars is very different. Its two moons, Phobos and Deimos, are far too small to have any stabilizing effect on the planet's axis of rotation, so the lean of its axis varies widely over hundreds of thousands to millions of years.

That might sound like a long time, but the appearance and evolution of life takes place over millions to billions of years. The resulting instability in the Martian seasons would add another challenge to success of life on the Red Planet.

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• After sunset, Saturn lies low in the south-west, Jupiter higher in the south and Mars rising in the east.

• The Moon will reach its first quarter on Dec. 29.

Merry Christmas to all.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.