This article was first published on Dec 22, 2014, and updated on Dec 22, 2017.
SINGAPORE -The Winter or December Solstice for 2017 takes place here on Friday (Dec 22). It is known as the longest night of the year, but that really depends on which part of the world you are in.
The longest night for the northern hemisphere is also the longest day - and peak of summer - for the southern hemisphere.
Here are some things to know about the Winter Solstice:
1. From space: It's all about the tilt
Earth's axis is tilted at an angle of 23.5 degrees, which means that the amount of sunlight that hits its surface varies when it rotates. The Winter Solstice falls on the day that the north pole is tipped the furthest it can be from the sun.
In the southern hemisphere, it is the day the south pole is tilted most towards the sun, and it is known as the Summer Solstice down under.
2. The Solstice happens at the same time for everyone
This means that the Solstice happens at the same instant everywhere on Earth.
This year (2017), it's on Dec 21 at 4.28pm Universal Time, which falls on Dec 22 at 12.28am Singapore time. In London, it would happen at 4.28pm while in New York, it falls at 11.28am.
In Sydney, it would be their Summer Solstice, happening at 3.28am on Dec 22.
3. Back on Earth: Less sun, longer nights
Solstice means "the sun stands still".
For those living in the northern hemisphere, this is the day the sun is at its lowest point at noon. It will gradually get higher until the Summer Solstice in June. This means the days will gradually get longer, and the nights shorter, from this day on. The reverse is true in the southern hemisphere.
During the solstices, the Sun appears to pause in its journey across the sky and change course, which is probably how these occurences came by their name.
4. But Singapore is on the Equator, so is there really a lot of difference?
Not quite. Singapore is one degree north of the Equator, so people here get a shorter day at this time of the year. But you have to be really aware of this to notice it.
The variation between the longest day and the shortest day of the year in 2012 was only about eight minutes.
5. Doesn't really matter, we still celebrate it
There are many festivals, some old and some still celebrated, associated with the Winter Solstice.
Christmas, which is celebrated by Christians, is the most popular holiday linked to the solstice. There is no record that Jesus was born on Dec 25, so experts believe that the date was chosen to coincide with solstice celebrations already prevalent hundreds of years before he was born.
These ancient holidays include Yule, celebrated by pagans, and Saturnalia, celebrated by the ancient Romans. Modern Wiccans still celebrate the solstice at Stonehenge in England.
For Chinese, Dongzhi takes place on this day and is celebrated as a day for family gatherings. Traditionally, tang yuan or glutinous rice balls are served.
Many Indian festivals also coincide with the solstice, including Uttarayana, Makara Sankranthi and Lohri.
6. The longest night ever
Many factors affect the Earth's rotation, and it has been slowing down or speeding up due to tidal friction, the melting of glacial ice, geologic activity and other variables.
Scientists estimate that the longest day and the longest night ever experienced was in 1912.
Sources: National Geographic, Tabla, Science Centre Observatory blog, earthsky.org, ucolick.org, Vox.com