Current Moon Phase

Waxing Crescent
7% of full

Farmers Almanac
The 2017 Farmers Almanac
Farmers' Almanac

Did the Calendar Manufacturers Make a Mistake?

Share on Facebook Share on Twitter Add to Google+ Share on Pinterest Subscribe by Email Print This Post
Did the Calendar Manufacturers Make a Mistake?

“Thirty days have September, April, June and November; all the rest have thirty-one, except for February alone, that has 28,” goes the old rhyme. Except for this year, that is. This is a leap year, which means this month has 29 days. But have you ever wondered why some years we have an extra day?

The reason has everything to do with historical attempts to line the calendar year up with the astronomical year. Unlike the calendar, which organizes each year into a neat 365 days, the seasons do not repeat every 365 days, but rather every 365.2422.

Leap years, or intercalary years, as they are also called, date back to the reign of Roman emperor Julius Caesar, in 46 BC. At that time, Caesar in consultation with the astronomer Sosigenes of Alexandria, decreed that a calendar year would be 365 days in length, and contain 12 months. Prior to that, the Romans followed an evolving series of calendars that were roughly based on the Greek lunar calendar, with a total of 354 days, and a “leap month” thrown in every few years to even things out.

Days were added to various months to bring the total number up to 365. Because the seasons didn’t exactly fit the 365-day year, the calendar ended about one-quarter day early, resulting in the calendar becoming a full day off every fourth year. To make up for the error, the Julian calendar, as Caesar’s calendar came to be called, added an extra day to the month of February every fourth year. Any year evenly divisible by four would be a leap year, which made the average length of the calendar 365.25 days.

However, the Julian calendar was still slightly off the mark. Caesar’s correction made the year 11 minutes and 14 seconds too long, which meant that, after 128 years, the calendar would end a full day later than the astronomical year. In 1582, Pope Gregory XII stepped in and ordered yet another correction to the calendar, resulting in the Gregorian calendar, which we use today. According to this reform, century years are not leap years, unless they are evenly divisible by 400. Thus, 1900 was not a leap year, but 2000 was. This made the average length of the calendar 365.244 days and reduced the calendar error to only one day in 3322 years. During the 19th Century, astronomer John Herschel suggested dropping a leap year every 4000 years, to obtain even greater accuracy, however his suggestion never received official support, in part because contemporary astronomers believe the point of the vernal equinox will change by the year 8000, making Herschel’s correction irrelevant.

So why do we call it a “leap” year, anyway?
Common (non-leap) years are composed of exactly 52 weeks, plus one day. This extra day means that if your birthday falls on a Tuesday in one common year, it will fall on a Wednesday the next common year, and so on. However, a leap year changes this scenario. A leap year is comprised of 52 weeks plus two days. So, if your birthday fell on a Wednesday last year, in a leap year it “leaps” over Thursday and lands on Friday. Thus, the name “leap year.”

Articles you might also like...


1 am fonda { 02.29.12 at 12:27 pm }

“leaping lizards” a brief refresher reminder of leap year traditional adaptation, way to go almaknacking farmers…..

2 Terry { 02.29.12 at 11:17 am }

The leap year discussion was always a ‘fun’ one in my fifth grade classes. Children loved learning the ‘whys’ of that ‘extra day.’ Thanks for the article and fond reminders.

Leave a Comment

Note: Comments that further the discussion of the above content are likely to be approved. Those comments that are vague or are simply submitted in order to promote a product, service or web site, although not necessarily considered "spam," are generally not approved.

If you notice a hole in the upper left-hand corner of your Farmers' Almanac, don't return it to the store! That hole isn't a defect; it's a part of history. Starting with the first edition of the Farmers' Almanac in 1818, readers used to nail holes into the corners to hang it up in their homes, barns, and outhouses (to provide both reading material and toilet paper). In 1919, the Almanac's publishers began pre-drilling holes in the corners to make it even easier for readers to keep all of that invaluable information (and paper) handy.

Spring Is Almost Here – Sign Up Today!

The Farmers' Almanac is a gardener's best friend. Get 365 days of access to our online weather and gardening calendars + a copy of the 2017 Almanac
for only $13.99 $11.99!

Subscribe Today »