The 'Ides of March'... In January?

Watching the sun in the sky - solstice and equinox

The graphic on the right shows how an observer would see the sun from Rome at different times of the year looking south at noon. Astronomers call the path which the sun takes through the sky the ecliptic - this is shown as the green line in the graphic. We can still make the same observations as ancient astronomers thousands of years ago. As we do this, we'll notice that the path of the ecliptic moves up and down across the sky during the year. On around June 22nd each year, the sun will have reached it's highest point in the sky - the summer solstice - the day with the most sunlit hours. The winter solstice is the day in the northern hemisphere which has the least amount of sunlit hours, and when the sun makes it's lowest journey through the sky - around December 22nd.

There are two other dates which were important. On around March 22nd and September 22nd each year, the sun is directly over the earth's equator. The red line in the graphics shows where an observor looking from Rome would see the earth's equator if it was a ribbon, hovering thousands of miles above our planet. The word ‘equinox' is latin as well, and means 'equal night'. This is the day of the year when the day and night are of equal length.

The Vernal Equinox marks the beginning of spring, the day on which the sun crosses over the equator and begins it's climb up into the summer sky.

If you live in a city, you probably buy your food in a supermarket. It's easy to forget how important the beginning of spring is - and especially how welcomed it was in early civilisations.

Was the ancient Roman calendar accurate?

In a word, no. As mentioned earlier, the Roman calendar was roughly based on the phases of the moon, which didn't help at all to work out years.

The most ancient Roman calendar had ten months and started in March. We can still recognise some of the month names.

As we can see, this year has 304 days.

In 713 BC the Roman calendar was amended to include two new months, again whose names we can recognise; Ianuarius - with 29 days, and Februarius - with 28 days. These months were added to the end of the year, after December and produced a year of 355 days.

Even after this reform, the calendar was still inaccurate - but the Romans had a system to deal with this. Every few years the Pontifex Maximus, another elected official would add a month of 27 days to this calendar on the date we know as February the 24th. February was shortened, the extra month added on and the resulting year of around 377 days would once again line up with the solstices and equinoxes. Confusing!

Our friend Julius Caesar returns!

In 45 BC Julius Caesar reformed the Roman calendar with the help of an astronomer, Sosigenes of Alexandria. Caesar changed the lengths of some of the months, removed the need for an extra month after February. By using astronomical data, the year was now 365 days long. However, this still wasn't quite right and the first idea of a ‘leap year' came into being but in an odd manner. Every three years, February 24th would be 48 hours long!

A modern calendar

The calendar system put in place by Julius Caesar lasted until 1582 AD! One of the problems with the previous ‘Julian' calendar was that they hadn't quite got it right - an extra day needed to be added - but every four years, not three. And then still a bit more complicated then that!

Hold your breath. On February 24th 1582 Pope Gregory XIII decreed that later in the same year, Thursday October 4th would be followed by Friday October 15th - and this would re-align the solstices and equinoxes. Furthermore, to prevent the calendar drifting out of sync the following formula for leap years was to be followed:

Every year which is exactly divisible by four is a leap year, except for years which are exactly divisible by 100 - and that the years of a new century which are exactly divisible by 400 are leap years as well.

Pope Gregory had some immediate success with his calendar reform. Obviously the countries comprising the Holy Roman Empire at the time adopted the change - alongside a few others. Prussia and Denmark signed up in 1700, Britain in 1752 and Russia held out until 1918.

The Gregorian calendar is still in use today. It is accurate to about one day in 3300 years.

For more information

The World Calendar Organisation
http://www.theworldcalendar.org/

The Roman Calendar and timekeeping
http://www.roman-britain.org/calendar.htm