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How Accurate Are Calendars?

The Julian calendar was abolished because it did not reflect the length of a year on Earth accurately. Today's Gregorian calendar does a better job, but is there such a thing as a perfect calendar?

Illustration image

Earth's orbit around the Sun defines year length.

The length of a year on Earth is defined by the time it takes our planet to complete a full orbit around the Sun.


A Year is Not 365 Days Long

The length of a year on Earth is defined by the time it takes our planet to complete a full orbit around the Sun. Called a tropical year, solar year, astronomical year or equinoctial year, it is approximately 365.242189 days long on average. Its length changes slightly over time.

Because a common year has 365 days in today's Gregorian calendar, a leap day is regularly added to bring it in sync with the tropical year.

Without leap days, our calendar would be off by 1 day approximately every 4 years, causing the astronomical seasons to occur at an increasingly later date as time goes by. In less than 50 years, the March equinox would be in April and the June solstice would occur in July.

Which Years Are Leap Years?

Is There A Perfect Calendar?

The simple answer is no. None of the calendar systems currently in use around the world perfectly reflect the length of a tropical year. However, there are calendar systems that are more accurate than the Gregorian calendar we use today. One of them is the Revised Julian calendar.

The table shows how accurate the different systems are (sorted from most to least accurate).

CalendarIntroducedAverage Year LengthApproximate Error
Persian calendar2nd millennium BCE365.2421986 days< 1 sec/year (1 day in 110,000 years)
Revised Julian calendar1923 CE365.242222 days2 sec/year (1 day in 31,250 years)
Mayan calendar~2000 BCE365.242036 days13 sec/year (1 day in 6500 years)
Gregorian calendar1582 CE365.2425 days27 sec/year (1 day in 3236 years)
Jewish calendar9th century CE365.246822 days7 min/year (1 day in 216 years)
Julian calendar45 BCE365.25 days11 min/year (1 day in 128 years)
365-day calendar
(no leap years)*
-365 days6 hours/year (1 day in 4 years)

* The 365-day calendar has never been in use as a civil calendar.

The Julian Calendar

In the Julian calendar, a leap day is added every four years without exception, so an average Julian year is 365.25 days long. The difference between the tropical and the Julian year is about 11 minutes per year, amounting to an error of 1 day every 128 years.

Because of this inaccuracy, the Julian calendar was eventually replaced by the Gregorian calendar.

About the Julian Calendar

The Gregorian Calendar

Today's Gregorian calendar uses more elaborate leap year rules, making it far more accurate. However, it is not perfect either. Compared to the tropical year, it is 27 seconds too long, so it is off by 1 day every 3236 years.

About the Gregorian Calendar

The Revised Julian Calendar

This calendar system uses even more complex rules to determine when a leap day is to be added. With an error of only about 2 seconds per year or 1 day in 31,250, it is roughly 10 times more accurate than today's Gregorian calendar and one of the most accurate calendar systems ever devised.

About the Revised Julian Calendar

Topics: Calendar, Timekeeping, Months, Weekdays

In This Article


Calendar Types

  1. Gregorian Calendar
  2. Julian Calendar
  3. Revised Julian Calendar
  4. Persian Calendar
  5. Mayan Calendar
  6. Chinese Calendar
  7. Roman Calendar
  8. Switch from Julian to Gregorian
  9. Is There a Perfect Calendar?

Calendars Library

Create Calendar With Holidays

Leap Day Library

  1. Leap Day is February 29
  2. Customs & Traditions
  3. Common Year vs. Leap Year
  4. Born on February 29
  5. February 30 Was a Real Date

What Is a Leap Year?

Which Months Are The Same


When is the next occurrence of

Which years use the same calendar

Which months are the same

Alternative Leap Years

  1. Bahá'í Calendar Leap Year
  2. Chinese Calendar Leap Year
  3. Ethiopian Calendar Leap Year
  4. Hindu Calendar Leap Year
  5. Persian Calendar Leap Year
  6. Islamic Calendar Leap Year
  7. Jewish Calendar Leap Year

Leap Years in Other Calendars

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