As 2025 rolled in, amid the cheers and the popping of prosecco, you may have been wondering why January is the first month of the year, or why years have twelve months. Admittedly, as the clock struck midnight, I was not exactly in a state to be pondering deep questions about the world. However, after waking up bleary-eyed and stepping outside on a bitingly cold New Year’s Day, I did ask myself, why does the year have to begin in the depths of winter?

A little bit of history

Today, the calendar most used around the globe is the Gregorian calendar. Many cultures retain their own calendars, but these are mostly used only for a few purposes, such as marking religious festivals. The Gregorian calendar is a modification of the earlier Julian calendar, which as the name suggests was proposed by Roman dictator Julius Caesar. The problem with Caesar’s calendar was that it assumed an average year length of 365.25 days, when in fact the actual length of the solar year is 365.242 days. This difference may seem tiny, but it corresponds to a difference of about 11 minutes per day. At the time of Pope Gregory XIII’s reform in 1582, the dates of the equinoxes were off by 10 days.

“This difference may seem tiny, but it corresponds to a difference of about 11 minutes per day”

The Gregorian calendar changed the rules for when leap years occur to correct the difference. You may be familiar with the rule that every year divisible by four is a leap year. However, there is a caveat to this – years which are divisible by 100 but not 400 are not leap years! This means that 1700, 1800 and 1900 were not leap years, whereas 2000 was. Interestingly, the idea of the 29th of February as a leap day is quite modern, since it used to be that the 24th February was counted twice. Overall, this leads to an average year length of 365.2425 days, reducing the drift to 11 seconds per year. Great work Gregory!

“The winter was completely ignored initially, since farmers couldn’t plant during this season so it had no relevance to the agricultural cycle”

As for why January is the first month of the year, we must look earlier in history to the pre-Caesar Roman calendar. The year used to begin in spring with the month of March and continued until December, so there were only ten months. The months followed the cycles of the Moon, beginning and ending with the new moon. Due to double counting of new moon days, this led to six months of 31 days, and four months of 30 days. The winter was completely ignored initially since farmers couldn’t plant during this season so it had no relevance to the agricultural cycle. However, this calendar was later reformed by King Numa Pompilius and January and February were added to the beginning of the year, covering the winter. The calendar was still out of whack with the seasons since the number of days was too small, so sometimes random ‘intercalary months’ were added to correct it. This ended when the Julian calendar was adopted in 45 BC, giving the months and the year their familiar numbers of days.

Not all calendars follow the solar year like the Gregorian calendar does – some are lunar calendars, such as the Islamic calendar, which has 354 or 355 days depending on the year. It has 12 months, each lasting one lunar cycle or 29 days. It is one of very few examples of a pure lunar calendar, since it doesn’t have any modifications like intercalary months to correct the drift of the seasons. Calendars which make this correction are called lunisolar, such as the Chinese traditional calendar.

Getting things astronomically wrong

A day is commonly defined as the time taken for the Earth to make a complete 360-degree rotation, in the same way the hour hand would make its way around a clock face every twelve hours. However, this is not as straightforward as you might think. Things are complicated by the fact that the Earth orbits the Sun. If we pretended the Sun wasn’t there and compared its rotation to the very distant so-called ‘fixed stars’ in the sky, one day would be 23 hours and 56 minutes long. This is known as a sidereal day. In fact, the Earth has to rotate about 361 degrees in order to ‘catch up’ from its orbital movement and face the Sun straight-on again the next day, giving us our synodic day of 24 hours.

Similarly, a sidereal year can be defined using the ‘fixed stars’ as a reference, giving a result that is slightly longer than our solar year. This is because the solar year needs the Sun to return to the same position in the sky at the beginning and end of the year. Earth’s axis of rotation itself is moving around; think of a spinning top once it starts to slow down. This gives the planet a little helping hand and makes it face the Sun about 20 minutes earlier than it otherwise would, making the solar year 20 minutes shorter than the sidereal year.

So next time you look out onto yet another cold and dreary New Year’s Day, you know who to blame for this predicament – the second king of Ancient Rome.

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