Winter solstice 2012 is on Thursday 21 June
Sunset on Port Hacking, Sydney, NSW on 17 May 2012. Photo Nick Lomb
The winter solstice is the day when the Sun reaches its furthest north position in the sky and starts moving back towards the south. It marks one of the main turning points in the year with the others being the equinoxes and the summer solstice in December. This year winter solstice is on Thursday 21 June. From that day on the days start becoming longer and night time shorter.
In ancient Europe the winter solstice (in December in the northern hemisphere) was a time of celebration. The Romans had a week-long celebration called Saturnalia during which all wars had to stop and courts did not try criminals. Later this festival became Dies Natalis of Sol Invicti or the Birthday of the Unconquerable Sun celebrated on 25 December each year.
Changes in the length of daylight in Sydney around the time of the winter solstice. Chart Nick Lomb
As indicated above the length of daylight, that is the time between sunrise and set, starts increasing from the time of the solstice. From then on we can start to look forward to longer days, but only slowly. As can be seen from the chart, there is no noticeable change from day to day on the days immediately before and after the solstice. The name solstice comes from this fact as it means the Sun stands still. Soon though the length of daylight starts to increase.
In many countries the seasons are fixed in reference to the four astronomical turning points so that we could take the start of winter from the day of the solstice. In Australia, however, the seasons traditionally start on the first of the appropriate month so that winter has already begun on 1 June. This fits in well with our weather for it takes a while for the ground and the oceans to come into equilibrium with the minimum of heat received from the Sun at this time. The coldest days hence tend to be in the middle of July and so in the middle of the three-month winter period.
On Thursday 21 June we have reached the shortest day of the year. From now on things are bound to improve. Let’s celebrate!
June 20, 2012 at 5:19 pm, David said:
Day length is a Sine function so if we assume T= 0 as September 22 (equinox) we are currently at Sine 270 which is also approx 270 days later.
A sine function is characterised by a minimum rate of change at its max and min values and a maximum rate of change (gradient) at the zero crossings (Equinox). This is why the day length varies dramatically during the transitions between winter and summer (Spring and Autumn)
By viewing the day length as a sine function it’s clear that we realy spend 4 months in winter (April 22 – Aug 22), 4 months in summer (Oct 22 – Feb 22), and 4 months (combined) in Autumn and Spring.
There are 365 days per year, so one day is approx = one degree (1/360) .
June 19, 2012 at 3:06 pm, Michael said:
Keen on hearing about the little spikes we can see in the graph!
June 20, 2012 at 12:24 pm, Nick_Lomb said:
Well noticed Michael. The spikes were artefacts from using the wrong type of chart. There were also one minute errors in the times. These have all been corrected and the chart replaced by a correct one.