Winter solstice
The winter solstice occurs exactly when the axial tilt of a planet's polar hemisphere is farthest away from the star that it orbits. Earth's maximum axial tilt to our star, the Sun, during a solstice is 23° 26'. More evident from high latitudes, a hemisphere's winter solstice occurs on the shortest day and longest night of the year, when the sun's daily maximum elevation in the sky is the lowest.Since the winter solstice lasts only a moment in time, other terms are often used for the day on which it occurs, such as midwinter, the longest night or the first day of winter.
The seasonal significance of the winter solstice is in the reversal of the gradual lengthening of nights and shortening of days. Depending on the shift of the calendar, the winter solstice usually occurs on December 21 to 23 each year in the Northern Hemisphere, and June 20 to 23 in the Southern Hemisphere.
Worldwide, interpretation of the event has varied from culture to culture, but most cultures have held a recognition of rebirth, involving holidays, festivals, gatherings, rituals or other celebrations around that time.
Date
In 46 BCE Julius Caesar in his Julian calendar established December 25 as the date of the winter solstice of Europe (Latin: Bruma). Since then, the difference between the calendar year (365.2500 days) and the tropical year (~365.2421897 days) moved the day associated with the actual astronomical solstice forward approximately three days every four centuries, arriving to December 12 during the 16th century. In 1582, Pope Gregory XIII decided to restore the exact correspondence between seasons and civil year but, doing so, he did not make reference to the age of the Roman dictator, but to the Council of Nicea of 325, as the period of definition of major Christian feasts. So, the Pope annulled the 10-day error accumulated between the 16th and the 4th century, but not the 3-day one between the 4th AD and the 1st BC century. This change adjusted the calendar bringing the northern winter solstice to around December 22. Yearly, in the Gregorian calendar, the solstice still fluctuates a day or two but, in the long term, only about one day every 3000 years.
The figures in the charts show the differences between the Gregorian calendar and Persian Jalāli calendar in reference to the actual yearly time of the winter solstice of the northern hemisphere, the December solstice. The error shifts by slightly less than 1/4 day per year; in the Gregorian calendar it is corrected by a leap year every 4th year, omitting three such corrections in every 400 years, so that the average length of a calendar year is 365 97/400 days; while in the Persian calendar every eighth leap-cycle is extended to 5 years, making the average 365 8/33 days, shorter than the Gregorian average by one day every 13200 years.
What happens at the solstice?
The North Pole is tilted furthest away from the sun at the solstice. (Not to scale)
The image above shows an example of what happens during the June solstice. Illustration is not to scale
The December solstice occurs when the sun reaches its most southerly declination of -23.5 degrees. In other words, it is when the North Pole is tilted 23.5 degrees away from the sun. Depending on the Gregorian calendar, the December solstice occurs annually on a day between December 20 and December 23. On this date, all places above a latitude of 66.5 degrees north (Arctic Polar Circle) are now in darkness, while locations below a latitude of 66.5 degrees south (Antarctic Polar Circle) receive 24 hours of daylight.
Use the Sunrise and Sunset calculator to find the number of daylight hours during the December solstice in cities worldwide.
The sun is directly overhead on the Tropic of Capricorn in the southern hemisphere during the December solstice. It also marks the longest day of the year in terms of daylight hours for those living south of the equator. Those living or travelling south from the Antarctic Circle towards the South Pole will see the midnight sun during this time of the year.
On the contrary, for an observer in the northern hemisphere, the December solstice marks the day of the year with the least hours of daylight. Those living or traveling north of the Arctic Circle towards the North Pole will not be able to see the sun during this time of the year.
Observances
Sunrise at Stonehenge on the Winter Solstice
Direct observation of the solstice by amateurs is difficult because the sun moves too slowly at either solstice to determine its specific day, let alone its instant. Knowledge of when the event occurs has only recently been facilitated to near its instant according to precise astronomical data tracking. It is not possible to detect the actual instant of the solstice (by definition, one can not observe that an object has stopped moving until one makes a second observation in time showing that it has not moved further from the preceding spot, or that it has moved in the opposite direction). Further, to be precise to a single day one must be able to observe a change in azimuth or elevation less than or equal to about 1/60 of the angular diameter of the sun. Observing that it occurred within a two day period is easier, requiring an observation precision of only about 1/16 of the angular diameter of the sun. Thus, many observations are of the day of the solstice rather than the instant. This is often done by watching the sunrise and sunset or vice versa or using an astronomically aligned instrument that allows a ray of light to cast on a certain point around that time.
Before the scientific revolution many forms of observances, astronomical, symbolic or ritualistic, had evolved according to the beliefs of various cultures, many of which are still practiced today. The following is an alphabetical list of observances believed to be directly linked to the winter solstice.
Lawrence Hall of Science visitors observe sunset on day of the winter solstice using the Sunstones II
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