How Accurate Are Calendars?
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.
Solar calendar systems, such as the modern-day Gregorian calendar, are designed to reflect the duration of a tropical year—also called a solar year, astronomical year, or equinoctial year—as accurately as possible. This is the duration of a full seasonal cycle, for example, from one equinox to the next. A tropical year is approximately 365.242189 days long on average, though 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.
The table shows how accurately the different systems reflect the length of a tropical year, sorted from most to least accurate. Calendars that are designed to reflect time spans other than the tropical year are not listed. This includes the Islamic, Buddhist, and Hindu calendar systems.
Calendar Accuracy Comparison
|Average Year Length
|2nd millennium BCE
|Less than 1 sec/year (1 day in 110,000 years)
|Revised Julian calendar
|2 sec/year (1 day in 31,250 years)
|13 sec/year (1 day in 6500 years)
|27 sec/year (1 day in 3236 years)
|9th century CE
|7 min/year (1 day in 216 years)
|11 min/year (1 day in 128 years)
|11 min/year (1 day in 128 years)
(no leap years)*
|6 hours/year (1 day in 4 years)
* There is no 365-day calendar system currently in use for civil purposes. Past examples include the ancient civil Egyptian calendar, the Maya Haab' calendar, and the Aztec Xiuhpohualli calendar.
The Modern-Day Calendar and Its Variants
The Gregorian calendar is the internationally used calendar system. It is derived from the Julian calendar developed in ancient Rome.
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, an enhanced variant of the ancient Julian calendar, 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.