Today is the longest day of the year. It also marks the start of summer.
Hang on, what about December 1?
Australia is one of just a few countries that considers December 1 the beginning of summer (or winter in the northern hemisphere), says astronomer Fred Watson.
“My understanding is that that comes from colonial times because it was the 1st of December when the troops got their summer uniforms and got out of their winter uniforms,” Professor Watson says.
Most of the rest of the world marks the start of summer (or winter) on the solstice.
Normally this happens on December 22, but this year was a leap year so it falls on December 21. Jupiter and Saturn come together tonight in rare eventSkywatchers are in for a treat as Jupiter and Saturn come so close to each other tonight they will almost look like a single shining planet in the sky in a “once in a lifetime” event.Read more
The solstice — and the seasons — are created by the tilt of the Earth.
In December, the southern hemisphere is tilted towards the Sun.
On the solstice, the Sun is directly over the Tropic of Capricorn (hello Rockhampton) and the highest in the sky from our perspective, says Andrew Jacob, a curator at Sydney Observatory.
“So we have the longest amount of hours when the Sun is above the horizon, and the shortest number of hours when the Sun is below the horizon.”
In other words, it’s the longest day and the shortest night.
But while we think of the solstice as an all-day event, like all things astronomical, it happens at precisely the same time wherever you are in the world.
This year that is 10:02am UTC (9.02pm AEDT).
At that time, Dr Jacobs estimates it would be midday above a point along the Tropic of Capricorn somewhere near Madagascar.
“The Sun would be directly overhead … so a stick or person would not cast a shadow in that area,” he says.
And just by sheer coincidence, skywatchers are in for an extra treat when Saturn and Jupiter appear their closest in nearly 400 years in a great conjunction about an hour after sunset onDecember 21.
Speaking of sunset, this is a good time to clear up a few misunderstandings that come up at this time of year.
The latest sunset (and the earliest sunrise) do not happen on the solstice
There are more hours of daylight during the solstice, but it is not the date of the latest sunset.
The latest sunset doesn’t happen until January (and the earliest sunrise happened about two weeks ago).
This mind-bending phenomenon is due to a combination of the tilt of the Earth and the fact our path around the Sun is more of an oval shape than a circle.
At this time of year, we are moving slightly faster through our orbit as we come closest to the Sun.
That has the effect of throwing out the rate at which the Sun moves across the sky and the rate at which our clocks tick.This difference between solar time and clock time is known as the equation of time.
“Each day we can add or subtract a certain number of minutes for how far ahead or behind the Sun appears to be compared to a clock,” Dr Jacob says.
Around the time of the solstice, both sunrise and sunset are getting slightly later.
So, for example, on December 21, the Sun sets in Sydney at 8:05pm, but two weeks later it sets at 8:09pm.
Likewise, sunrise in Sydney is at 5:41am, but it was four minutes earlier two weeks ago.
But when the Sun rises or sets has nothing to do with the solstice: it is the total number of daylight hours that matter.
“If you take the number of minutes between that rise and set, you find that the longest number of sunlight hours is the solstice,” Dr Jacob says.
Earth is closer to the Sun in summer, but that’s not why it’s hot
This a common misunderstanding, Professor Watson says.
“A lot of people think that is why it’s hot in summer in the southern hemisphere,” Professor Watson says.
Earth will reach its closest point to the Sun (known as the perihelion) two weeks after the solstice next year on January 3.
But the difference in distance is marginal.
“It’s only a few per cent closer than its average distance,” Professor Watson says.
The tilt of the Earth has a much greater impact than the distance from the Sun.
On top of that, local geography, weather and climate patterns dominate what’s happening to your sense of temperature.
There are different ways to define our seasons
The December solstice marks the beginning of the southern summer in an astronomical sense, but the start of the season in Australia is more about how we mark temperature and time.
Australia defines the year using meteorological seasons. These seasons are based on the Gregorian calendar, which carves the year up into 12 months roughly aligned with changes in the weather.
“The meteorological seasons are very different from what the astronomical seasons might be,” Professor Watson says.
Australia covers a large area and has many different climatic patterns.
Thanks to the tilt of our Earth, the Tropics get the most sun so it is hot all year round. Seasons are marked by changes in rainfall.
The four defined three-month seasons concept really only applies to locations south of the Tropic of Capricorn to Tasmania.
And even then, these European seasons don’t accurately reflect what happens in most of Australia, Dr Jacob says.
“You get many different patterns of weather throughout the year, so we should be using Indigenous seasons in many ways,” he says.
Indigenous seasonal calendars follow cues in the environment, says Emma Woodward of the CSIRO.
“The timing of them every year may be different by days, or weeks, or months,” says Dr Woodward, who has worked with several Indigenous communities who are recording their language and knowledge of seasonal changes for future generations.
“[The knowledge] depends on whether [the language group] has been able to maintain that connection to country to keep their knowledge strong.”
Each calendar is specific to each community and reflects the interaction of specific species of plants and animals to weather events.
The Ngan’gi knowledge holders in the Nauiyu Nambiyu community from the Daly River and its wetlands in the Top End have 13 seasons, many of which describe the lifecycle of wurr panangalan (speargrass).
The first rains of the wet season, known as kudede, is when the mundupan (green ants) are ready for eating and turtles start moving. When the wurr panangalan shoots it is wurr bengin tyerrfal season and time to collect misyawuni (bush potato).
Further south in the semi-arid region around the Fitzroy River in the Kimberley, the Walmajarri people have three main seasons divided into several sub-seasons
This time of year is known as parranga — hot weather time. It is the best time to catch parlka (barramundi) and jaminparu (black bream), and the partiri (flowers) are blooming so it’s a good time to collect honey.
First published at The ABC, Monday December 21 2020. See; https://www.abc.net.au/news/science/2020-12-21/summer-solstice-2020-sunset-seasons-saturn-and-jupiter/12956544