The **main difference** between Julian and Gregorian calendars is **that** **an average year in Julian calendar is 365.25 days while an average year in Gregorian calendar is 365.2425 days**.

Gregorian calendar is the normal calendar we currently use to determine the date. Julian calendar was used from 46 B.C to 1582. It was replaced by the Gregorian calendar.

### Key Areas Covered

**1. What is Julian Calendar*** – Definition, Features***2. What is Gregorian Calendar*** – Definition, Features***3. Relationship Between Julian and Gregorian Calendars*** – Outline of Common Features***4. Difference Between Julian and Gregorian Calendars*** – Comparison of Key Differences*

### Key Terms

*Julian Calendar, Gregorian Calendars*

## What is Julian Calendar

Julian calendar is the 365-day calendar Julius Caesar made official in 46 B.C. It replaced the Roman calendar, which was a lunar calendar based on the phases of the moon. Julian calendar, which was designed with the aid of Greek mathematicians and astronomers like Sosigenes, is based entirely on Earth’s revolutions around the Sun. On average, it takes Earth approximately 365.242189 days (365 days, 5 hours, 48 minutes and 45 seconds) to make one orbit around the Sun.

There are two types of years in the Julian calendar: a normal year and a leap year. A normal has 365 days, divided into 12 months. Every four years, there is a leap year with 366 days. They added this leap day to February, which was then the last month of the year. This makes an average year 365.25 days long. However, this is more than the actual value of a solar year – 365.24219 days. Therefore, the Julian calendar gains a day every 128 years. After centuries of use, the Gregorian calendar replaced the Julian calendar. However, parts of the Eastern Orthodox Church still use this calendar to determine the dates of moveable feasts.

Moreover, for years from 1901 to 2099, date according to the Julian calendar is 13 days behind its corresponding Gregorian date. For example, 16 November in the Gregorian calendar is equal to 3 November in the Julian calendar. There is a 13-day difference.

## What is Gregorian Calendar

The Gregorian calendar is the calendar we all use today. It was introduced in October 1582 by Pope Gregory XIII. It’s a modification of the Julian calendar, reducing the average year from 365.25 days to 365.2425 days. It shortened the calendar year by 0.0075 days in order to stop the drift of the calendar with respect to the equinoxes. To correct this drift caused by the Julian calendar, the date was advanced ten days in October 1582. Therefore, Friday 15 October 1582 followed Thursday 4 October 1582.

Moreover, the Gregorian calendar has a new way of determining a leap year. Every year that is divisible by four is a leap year; however, years that are evenly divisible by 100 are not leap years. Moreover, centurial years are leap years if they are evenly divisible by 400.

## Relationship Between Julian and Gregorian Calendars

- Both calendars have two types of years: a normal year with 365 days and a leap year.
- Gregorian calendar is only a slight modification away from Julian calendars.

## Difference Between Julian and Gregorian Calendars

### Definition

Julian calendar is the 365-day calendar Julius Caesar made official in 46 B.C while the Gregorian calendar is the calendar currently used in most parts of the world.

### Average Year

An average year in Julian calendar is 365.25 days while an average year in the Gregorian calendar is 365.2425 days.

### Leap Years

In the Julian calendar, there was a leap year every four years. According to the Gregorian calendar, a year that is exactly divisible by 4 is a leap year; however, a year that is evenly divisible by 4 and 100 is a leap year only if it is also exactly divisible by 400.

### Determining the Date

For the time period from 1901 to 2099, the date according to the Julian calendar is 13 days behind its corresponding Gregorian date

### Conclusion

In brief, the Gregorian calendar is only a slight modification away from Julian calendars. An average year in the Julian calendar is 365.25 days while an average year in the Gregorian calendar is 365.2425 days. Hence, this is the main difference between Julian and Gregorian calendar.

##### Image Courtesy:

1. “Julian to Gregorian Date Change” By Asmdemon – Own work (CC BY-SA 4.0) via Commons Wikimedia

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