# 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?

## 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.

## 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).

Calendar | Introduced | Average Year Length | Approximate Error |
---|---|---|---|

Persian calendar | 2nd millennium BCE | 365.2421986 days | < 1 sec/year (1 day in 110,000 years) |

Revised Julian calendar | 1923 CE | 365.242222 days | 2 sec/year (1 day in 31,250 years) |

Mayan calendar | ~2000 BCE | 365.242036 days | 13 sec/year (1 day in 6500 years) |

Gregorian calendar | 1582 CE | 365.2425 days | 27 sec/year (1 day in 3236 years) |

Jewish calendar | 9th century CE | 365.246822 days | 7 min/year (1 day in 216 years) |

Julian calendar | 45 BCE | 365.25 days | 11 min/year (1 day in 128 years) |

365-day calendar (no leap years)* | - | 365 days | 6 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.

## 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.

## 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.

### In This Article

- Year Longer Than 365 Days
- Is There A Perfect Calendar?
- The Julian Calendar
- The Gregorian Calendar
- The Revised Julian Calendar

### Calendar Types

- The Gregorian Calendar
- The Julian Calendar
- The Revised Julian Calendar
- The Mayan Calendar
- The Chinese Calendar
- The Roman Calendar
- Switch from Julian to Gregorian
- Is There a Perfect Calendar?

### Create Calendar With Holidays

### Leap Day Library

- Leap Day is February 29
- Customs & Traditions
- Common Year vs. Leap Year
- Born on February 29
- February 30 Was a Real Date

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