December solstice

Gregorian calendar

Iranian (Persian) calendar

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 Southern 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.

The date of the solstice is not the same as the date of the latest sunrise and both are not the same as the date of earliest sunset. Because the Earth is moving along its solar orbital path, for each solar day the Earth has to do more than one full rotation. Because the Earth's orbit is elliptical, the speed at which the Earth moves along its orbit varies. Consequently, solar days are not the same length throughout the year. "Mean time" is our way of modifying this, for our convenience, making each day the same length, i.e. 24 hours. The maximum correction (see Equation of Time) is ± 15 minutes to the mean but its value changes quite rapidly around the solstices. If solar time were used rather than mean time, the latest sunrise and earliest sunset and therefore also the shortest day would all be at the December solstice in the northern hemisphere; the opposite would be true in the southern hemisphere, with the earliest sunrise, latest sunset, and longest day.

• Christmas
• Dies Natalis Solis Invicti
• Brumalia
• Dongzhi Festival