Astronomy Picture of the Day: Julius Ceasar and Leap Days

February 29 is a Leap Year Day

This year February has an extra day, according to a leap year rule that has caused a lot of confusion. This is the first time a leap year day has been added to a century year in the United States. This is the story.

When Julius Caesar imposed his Julian calendar in 46 B.C., the year was considered to be exactly 365.25 days long. That extra quarter day was saved for three years and added all at once as a whole day. (Caesar had it added to February, not because it was the shortest month, but because it was then the last month of the year. The names of the last four months of the year come from their positions then. For example, September is Latin "sept" for seven.) For the next 16 centuries, people faithfully added a leap year day to every fourth February.

The year, however, is actually 365.2422 days long. The difference between 0.2422 and 0.25 day is 11 minutes, and that error added up. People added too many leap year days, and the calendar slowly fell out of step with the seasons. The error had accumulated to ten days by the late sixteenth century. The spring equinox, which fell on March 21 by the calendar, was in reality occurring on March 11.

This caused a problem for church authorities responsible for fixing holy days. The date of Easter is determined by the date of the spring equinox and the phase of the moon, and because the equinox was wrong, people were celebrating Easter on the wrong day. In 1582, Pope Gregory XIII decreed that a one-time adjustment be made and immediately dropped 11 days. October 4 that year was abruptly followed by October 15, and that brought the calendar back into synchronization with the sky. Pope Gregory's second decree modified the rule for adding leap year days to keep the calendar synchronized with the sky. The new rule calls for there to be three fewer leap year days every 400 years. Prior to 1582, all years divisible by four were leap years. Henceforth, beginning with the year 1600, century years are not leap years unless they are divisible by 400. Thus, 1700, 1800, and 1900 were not leap years, but 2000 is. 2100 will not be. This is the second century leap year since Gregory's reform of 1582.

The pope's calendar reform was adopted immediately in Catholic countries, but Protestant countries resisted. The Gregorian calendar was adopted in Britain and her American colonies in 1752, in Japan in 1873, and in Russia not until 1918.

The term "leap year" comes from the way the dates cycle through the days of the week during the years. A common year of 365 days contains exactly 52 weeks of seven days plus one day. The same date falls one day later in the week each successive year. March 1, for example, fell on Saturday in 1997, on Sunday in 1998, and on Monday in 1999. This year it "leaps over" Tuesday and falls on a Wednesday.

Pope Gregory's Gregorian Calendar will not need another adjustment for 3000 years - and we'll worry about it then.

For additional leap year day information go to these web sites:
Royal Observatory of Greenwich: Leap years
U.S. Naval Observatory: Leap Years

Griffith Observatory                   Griffith phone: (323) 664-1181
2800 East Observatory Road               Griffith fax: (323) 663-4323
Los Angeles, California 90027 USA

Astronomische Persberichtendienst

27 februari 2000


Over twee dagen is het 29 februari 2000: een schrikkeldag in een eeuwjaar. Zoiets maken we slechts eens in de 400 jaar mee. Veel mensen dachten dan ook even dat hun kalenders en agenda's voor 2000 fout waren.

Lees op meer over de historie van de schrikkeldag. Het betreft een artikel uit het laatste nummer van het tijdschrift Zenit.

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