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The tenth and twelfth month in our calendar are October and December respectively. Yet, the prefixes of these months suggest otherwise. Has this ever puzzled you? The prefixes ‘oct-‘ and ‘dec-‘ come derive from Greek and they mean eight and ten respectively.

With the prefix ‘oct-‘, you have words such as ‘octagon’ (an eight-sided polygon), ‘octane’ (an alkane with eight carbon atoms), ‘octopus’ (an eight-limbed mollusc). For those of you who are musically inclined, consider the word ‘octave’. Its symbol is ‘8va’ and this is no coincidence as it is a shortened form of the word ‘octava’ (why an octave is divided into 12 semitones should be given a separate blogs post!).

With the prefix ‘dec-‘, you have words such as ‘decimal’, ‘decade’ (period of ten years), ‘decibel’, ‘decagon’ (ten-sided polygon), and ‘Decalogue’ (another term for the Ten Commandments).

Given that December should then be the tenth month by virtue of its name, you might have guessed that the first month of the year might have been March at some point in history, and you would be right! The modern calendar that we use can be traced to the Romans.

Before we go on, it helps to talk about the following terms: ‘equinox’ and ‘solstice’, as some readers might not be familiar with them. Let’s consider the term ‘equinox’ first. The term literally means ‘equal night’ (derived from Latin ‘aequinoctium’), and refers to the instant of time where the plane of the Earth’s equator passes through the centre of the Sun (i.e. the centre of the visible Sun is directly above the Equator). On the vernal (spring) and autumnal equinoxes, the duration for day and night are nearly the same throughout the entirety of Earth, which makes sense, when you think about it.

Let’s consider the term ‘solstice’ next. A Google search defines it as ‘either of the two times in the year, the summer solstice and the winter solstice, when the sun reaches its highest or lowest point in the sky at noon, marked by the longest and shortest days. This should also be quite intuitive.

We have all the above phenomena because the Earth is tilted on its axis by about 23.5 degrees. If not for the tilt, the plane of the Equator would pass through the Sun’s centre throughout Earth’s solar revolution, and there would not be any seasons. There are seasons because in different time periods of the solar year, each location on Earth gets a different daily (as in every day and during the day time) exposure to the Sun’s warmth. This explains why a country such as Singapore (near the Equator) will not experience the four seasons. This is because throughout the year, the Sun strikes equatorial regions at approximately the same angle, and so, these regions have an approximately equal amount of day and night throughout the year.

N.B. Some incorrectly believe that the four seasons are due to the fact that Earth’s solar orbit is elliptical (leading to points at which the Earth is closest to and farthest away from the Sun – perihelion and aphelion respectively). The difference in distances between the perihelion and aphelion (about 3.2 million miles) is not THAT significant, and has a negligible impact on the Earth’s climate.

The time between the equinoxes and solstices, and the duration of a solar year were calculated accurately by ancient astronomers. However, with Mother Nature providing such a prominently displayed Moon in the night sky, many cultures preferred to have their calendars based on the lunar cycle. The original Roman calendar (753 BC; the calendar was adopted by the Romans from the Greeks) had ten months:

1. Martius – March (named after Mars, the god of war in Roman mythology; apart from being the god of war, Mars was also deemed the god of agriculture; the beginning of March corresponds to the transition from Winter to Spring)
2. Aprilis – April (the word April is a variant of ‘Avril’ as in ‘Avril Lavigne’; yes, that singer)
3. Maius – May
4. Junius – June (named after Juno, Jupiter’s wife)
5. Quintilis – July
6. Sextilis – August
7. September
8. October
9. November
10. December

This ten-month did create problems though. As established earlier, many societies liked basing their calendars on the lunar cycle because it was easily observable (the lunar cycle is about 29.53 days long). Each of the ten months would have either 30 or 31 days, and the original ten-month year had a total of 304 days, around 61 days short of the actual solar year (325.242 days).

You might be asking: what happened to January and February? Well, those months correspond to the months during winter. Winter was a pretty harsh time for everyone, and so, people just simply took the new moon before the vernal equinox (aka spring equinox) as the start of the next year. Surprisingly, this arrangement worked relatively well (as long as you didn’t need to figure out the day in the interim 61.25 days)!

One day, Numa Pompilius, who may or may not have existed, felt that even numbers would only bring about bring about misfortune. Hence, he removed one day from all the months with 30 days. As such, Aprilis, Junius, Sextilis, September, November, and December all had 29 days, thereby reducing the total number of days to 298 days.

Numa wanted to have 12 lunar cycles in his calendar, with 56 days being added to the 298, thus bringing the new total to 354 days. This is gonna sound ridiculous (bear with me), but Numa felt that having 354 days would bring bad luck! So, he decided to have an extra day, in addition to the 56 days to be added. The 57 days were divided between two months and just dumped at the end of the year. As you might guess, the two months were Januarius (29 days), and Februarius (28 days), and THAT is how February ended up with 28 days!

I know what you’re thinking. 28 is an even number. How did they accept that? Apparently, February was the month of spiritual purification, so they closed one eye (Februarius derives from the Latin ‘Februum’, which means ‘purification’).

Although this new calendar (355 days) was a massive improvement from the original calendar (304 days), it nevertheless still fell short of the solar year (365.242 days)! As you might imagine, it was only a matter of time before all the seasons did not correspond to certain months, and the whole system was messed up.

Here’s where it starts to get confusing. February actually consisted of two parts: the first 23 days, and the rest. Every year, Numa’s calendar would be out of line with seasons by about 10 days (cf 355 vs 365.242). Hence, every other year (usually two), the ‘rest of February’ (the part after the 23 days) would be ignored and replaced by a 27-day leap month called mensis intercalaris. With this new arrangement, the new calendar would have 366.25 days on average (2 years with leap months and 2 years without leap months would give a total of 1465 days; 1465/4 = 366.25). That is about one day more than what we need, but hey, this is a massive improvement.

This could have worked, in theory, because every 19 solar years correspond to 235 lunar months (with a difference of about two hours). So, as long as there is due diligence in adding the leap months appropriately, things will be fine……except when there ISN’T due diligence.

Politicians would ask for leap months to extend their terms, or ‘forget’ them to get their opponents out of office. Things were worse when Rome was at war. Under such circumstances, the leap month might be forgotten for several years. By the time Julius came to power, things had gotten quite confusing. So, he reformed the calendar!

As we all know, Julius had a thing with *cough* Cleopatra. He spent some time in Egypt, which was a highly sophisticated region. They were perhaps the only culture that used a solar calendar with 365 days then. So, in 46 BC, Julius gave us the Julian calendar, a solar calendar which rendered the lunar calendar that had been used hitherto obsolete. January and February had been moved to the beginning of the year.

Remember how the lunar calendar (355 days) was still about 10 days short of the solar year? Julius resolved this by adding ten days to ten different months (without touching February).

Lastly, to account for the nearly extra average quarter-day difference (since the tropical year was 365.242 days), a leap day would be added to February every four years. This would bring the average Julian year to be 365.25 days long!

Of course, we are not using the Julian Calendar at the moment. There was one more calendar reform that took place. That reform took place to bring the average year from 365.25 days to 365.2425 days (much closer to the actual year). That reform was the Gregorian reform. To learn more about that reform, check out an earlier blog post I made.

I hope you have greatly enjoyed reading this post!

Yours faithfully,
Nicholas Loh
3 February 2019

P.S. Quintilis and Sextilis were renamed July and August after Julius Caesar and Augustus Caesar (Julius’ successor) respectively.

References
1. https://www.thefreedictionary.com/words-that-start-with-dec
2. https://www.quora.com/On-what-basis-the-months-of-english-calender-are-named
3. https://www.thefreedictionary.com/words-that-start-with-oct
4. https://www.vectorstock.com/royalty-free-vector/earth-and-lunar-phase-set-vector-18297327
5. https://study.com/academy/lesson/what-is-a-new-moon-definition-phases.html
6. https://www.windows2universe.org/the_universe/uts/equinox.html
7. https://en.wikipedia.org/wiki/Equinox
8. https://www.weather.gov/cle/seasons
9. https://en.wikipedia.org/wiki/Numa_Pompilius#Institutions_attributed_to_Numa
10. https://en.wikipedia.org/wiki/Mercedonius