What is the summer solstice? Welcome to the longest day of the year
Wednesday will mark Australia's longest day of the year, the summer solstice. Derived from latin, "solstice" means a standing still of the sun. Many ancient civilisations celebrated the summer solstice as a day of great spiritual import and an opportunity to observe the power of the sun, something that Australians are well acquainted with. Here we have compiled everything you need to know about this year's summer solstice.
What is the Summer solstice and when does it happen?
The summer solstice is the day with most daylight hours of any in the year. It occurs once annually, and always falls in December in the southern hemisphere. It usually takes place on 21 December, as it will this year, but can occur between 20 to 22 December.
Conversely, the Australian winter solstice is the shortest day of the year and generally takes place on 21 June, though it can fall between 20 and 22 June. In the northern hemisphere, this phenomenon is reversed, so Wednesday will be their winter solstice instead.
Why does the summer solstice happen?
The summer solstice occurs on the day when the sun reaches its highest point in the Australian sky. The earth spins at a tilt of 23.5 degrees, meaning that the amount of sunlight experienced in different parts of the world varies throughout the year. This means that in the southern hemisphere the summer solstice takes place on the day on which the South Pole is most inclined towards the sun.
How is the summer solstice celebrated?
The summer solstice was a day of cultural significance for many ancient civilisations, who marked it with magnificent festivals and celebrations. The Vikings were known to use the summer solstice, or "midsummer", as a time to convene and discuss legal matters and resolve disputes. They also visited wells believed to offer healing powers and built huge bonfires. The bonfire tradition has carried over to modern day, with Iceland being a popular destination for "Viking" style summer solstice celebrations.
In Britain, the Druids are thought to have practiced ritual celebrations during midsummer, with some believing these took place at Stonehenge. The historical accuracy of this has become unimportant to many Britons and tourists alike, who now gather at the ancient site to watch the sun rise. In the southern hemisphere, celebrations are less common, but extended daylight hours will give Sydneysiders a chance to get out and about as Christmas approaches.
Why isn't the summer solstice on the same day every year?
For the past two years, Australia's summer solstice has fallen on 22 December, but this year will return to the most common date of 21 December. The reason for this is due to the discrepancy between our calendar year and the actual time it takes the Earth to orbit the sun. While we regard a year as 365 days, it is approximately 365 days, five hours, 48 minutes, and 45 seconds. This varies by a few seconds every year because the Earth's orbit is not just caused by the Sun's gravitational pull, but also by the pull of the planets and moons in the Solar System, whose positions change on a yearly basis.
Because of this variation the solstice occurs about six hours later every year, and we fix this difference between our calendar year and the length of the Earth's orbit, by adding an extra day to February every four years. This means that every leap year (as 2016 was) the summer solstice jumps back a date from the previous year.
Will Wednesday have the earliest sunrise and latest sunset this year?
No, despite being the longest day of the year the summer solstice doesn't feature the yearly extremes for sunrise and sunset. The earliest sunrise is usually seen in early December, and the latest sunset generally takes place in early January.
The main reason is again due to discrepancy between how we measure time and the length of a solar day (the time between when the sun is at its highest point in the sky). The difference between solar time and the time we measure on our clocks varies about 30 seconds every day, and the extra time delays the following sunrise according to our clocks.