Understanding the Julian Calendar

Aligning the calendar with the Earth’s orbit around the sun is important, but not easy. The calendar needs to synchronise with the Earth’s orbit to ensure the seasons fall at the same time every year. To do this, the calendar year matches the time it takes the Earth to complete an orbit of the sun. The duration of the orbit, called a solar year, is not an equal number of whole days. This has caused a few problems in the past.

The Roman Republic Calendar, for example, was not great at keeping in sync with the solar year so, on the day we now call 1^st January in 45 BC, the Roman general Julius Caesar introduced the Julian calendar. It simplified the time keeping process and made calendar alignment with the solar year much easier.

Roman Republican Calendar

Like many calendars, the Roman Republic Calendar was based on a lunar year. It had the same 12 months as other calendars, but some were shorter than they are now. Therefore, the calendar only had 355 days which was at least 10 less than the solar calendar. Keeping the two in sync was a complicated and less scientific process.

Adding an extra month to the calendar, known as Interkalaris or Interkalarius, gave the year the extra days it needed. It involved adding 27 or 28 days after 23rd February and suspending the rest of the days in the month. Over a four-year period, the calendar may include the following months, each with the indicated number of days:

A body of priests, known as the pontifices, order intercalation when they thought it necessary. Caesar himself became the pontifex maximus in 63 BC and was responsible for intercalation. The pontifices were politicians and not scientists, so intercalation was based on their beliefs, not science. For example, they left out the intercalary month during times of war because people thought the extra days were unlucky. On resuming intercalation, the number of days required often led to an over compensation due to imprecise calculations. These irregular years highlighted the need for reform and it was Caesar who led it.

The Julian Calendar

Following the ‘last year of confusion‘, in 46 BC, Julius Caesar introduced the Julian Calendar. It was based on the solar year being 365 days and 6 hours, or 365¼ days. Because the Julian Calendar is 365 days long, every year it lost one quarter of a day (6 hours) and ‘fell behind’ the solar calendar. After fours years, it was a whole day behind, so adding an extra day to the calendar realigned it with the solar calendar. These extra days, known as leap days, make the year 366 days long, known as leap years.

In the Julian Calendar, the leap day was created by counting 24^th February twice. Officially, the extra day should have fallen on the day we know as 25^th February, but in reality it was 24^th February. Now, the leap day is 29^th February.

However, adding a leap day every four years is too much and causes the Julian Calendar to drift away from the solar year, falling behind by one day every 128 years. The Gregorian Calendar avoids this additional time by having fewer leap years, missing three of them over a 400-year period.

The idea of the Julian Calendar “falling behind” may seem counterintuitive at first, especially since it adds too many days rather than removing any. Therefore, imagine two trains travelling at the same speed on parallel railway lines, both heading towards the same destination to arrive at the same time. The first line represents the Julian Calendar and the second line represents the Gregorian Calendar. Both lines pass through the same stations, representing leap days and built four years apart.

The operators of the Julian train calculate that the train on its line station needs to stop at every station if it is to make its destination on time. In contrast, the operators of the Gregorian Line believe their train does not need to and the train on its line does not stop at three stations over a ‘distance’ of 400 years.

Over time, the Julian Train begins to fall behind as it makes more frequent stops. After 128 years, it is a full day behind, the Gregorian Train, with the gap widening.

If this drift were allowed to continue, after approximately 25,000 years, January in the UK would occur in the middle of summer. While this slow drift may seem insignificant in the short term, over centuries it could disrupt the alignment of seasons with cultural, agricultural and astronomical cycles, creating confusion and requiring further adjustments to the calendar.