In all solar calendars , the common year includes exactly 365 days.
Regular common years
The ancient Egyptian calendar knew until the calendar reform of Ptolemy III. in 237 BC BC (→ Canopus Decree ) only common years. Therefore his calendar New Year's Day wandered through all seasons ( change of year ). Quite similar to Ptolemy, Julius Caesar led 45 BC On the advice of the astronomer Sosigenes, new leap years of 366 days for his Julian calendar , which are in contrast to the common years.
Exceptionally common years
“Exceptionally common years” are the years that would be Sosigenes' leap years according to the rule, but are not leap years according to an extended rule.
Lilian common years
The exceptional common years were introduced for better astronomical alignment of the calendar year with the tropical year as part of the Gregorian calendar reform . At the suggestion of Aloisius Lilius, there are three secular, exceptionally common years in each 400-year cycle in this calendar . So on average every 133 1 ⁄ 3 years . This means that the Gregorian calendar currently deviates by about 26¾ seconds from the tropical year. Since the latter is also shortened by around half a second per century, the annual error will soon be 27 seconds. (see below) The Gregorian rule was astronomically correct about 6000 years ago.
Milankovićian Common Years
According to this rule, the calendar year comes much closer to the tropical year. However, this rule has not been astronomically accurate for several centuries.
Madlerian common years
According to the strict 128-year rule of the Mädler calendar proposed by Johann Heinrich von Mädler in 1864 , the calendar year - according to VSOP2000 - coincides astronomically exactly with the tropical year around 2048.