The long and short of it

PERHAPS because of the rather chilly nights recently, there has been a lot of interest in the winter solstice this year.

It took place on Wednesday, at 2.24pm our time, making it the shortest day of the year, and that night the longest night.

The solstices occur when the sun appears farthest north and south as seen from Earth. The variation over the year is caused by the tilt of Earth’s axis, which is inclined at an angle of 23.4 degrees to the vertical through the plane of its orbit. That is, Earth is “tipped over” through that angle.

One of the questions we often receive at the Launceston Planetarium at this time of the year is: how much longer are the days each side of the solstice? This year, it is about two seconds in each case, and the amount of lengthening increases day by day. The difference between yesterday and today, for example, is 10 seconds.

It is around the times of the equinoxes in September and March, when the sun is directly over the Earth’s equator that the days are lengthening or shortening at the maximum rate. At the latitude of Hobart, this amounts to nearly three minutes of change per day.

Back to the winter solstice. On Wednesday, the solstice day, the length of the day in Hobart was nine hours, 0 minutes and 53 seconds. Today, that length has increased to nine hours, one minute and 10 seconds: 17 seconds longer!

The rate of change of the length of the day depends on one’s latitude. For example, in Brisbane, today is only about eight seconds longer than Wednesday.

The actual length of the day also depends on one’s latitude, and the more extreme the latitude, the greater are the extremes in day length.

In Launceston, Wednesday was nine hours, nine minutes and 32 seconds long —nearly nine minutes longer than in Hobart. In Brisbane, the shortest day was about an hour and a quarter longer!

Of course the opposite is true at the summer solstice in December, with Hobart having the longest day of any state capital.

There have long been solstice ceremonies in many parts of the world. This has been to at least recognise the change of the seasons.

However, in the case of the winter solstice, people also needed to “ensure” that the sun began its apparent motion back in the other direction, so that the days would once again become longer and warmer. The ceremonies were seen as an important part of the reversal of the sun’s apparent path.

In Imperial China, astronomy was strongly connected with the Emperor. At the winter solstice, the Emperor would go to the top of the Royal Mound, which was just south of the forbidden city in Beijing and pay homage by facing due north. (The Chinese people considered that north was the direction of the centre of the heavens.)

An interesting structure in Tonga is the 13th century trilithon called Ha’amonga ‘a Maui, which consists of two vertical stones and one horizontal one resting on the top. Some carvings, and the orientation of the lintel, seem to indicate the directions of the sun at the solstices.

Although its astronomical connection is disputed, it is one of many possible examples of human-made structures that indicate solstice directions.

So our days, little by little, are getting longer now. Enjoy those extra few seconds of daylight! Martin George is manager of the Launceston Planetarium (QVMAG).

In Launceston, Wednesday was nine hours, nine minutes and 32 seconds long — nearly nine minutes longer than in Hobart. In Brisbane, the shortest day was about an hour and a quarter longer!