The calendar is such a routine, everyday thing, but how much do you really know about the operation of it. Why is it like that?
A DAY: The Earth rotates at a fairly fixed pace about the imaginary line running between the North and South Poles called the Earth’s Axis. The time it takes to revolve once is called a ‘rotation’ and this takes just under twenty-four hours. However, because the Earth is constantly traveling around the Sun, the exact time from noon one day to noon the next is 3 minutes 56 seconds longer and this makes a day almost precisely twenty-four hours in length.
The actual time from noon to noon varies depending where the Earth is on its celestial course around the Sun, but if you average the days in a year out, it comes to precisely twenty-four hours.
A YEAR: All nine planets in our solar system travel around the Sun in approximately perfectly circular routes called orbits. Each journey around the Sun is called a revolution and all the planets revolve around the Sun in the same direction. The course the Earth takes can be verified by noting its position against the background stars.
Since you cannot see the Sun and the stars at the same time, it is obligatory to note the position of the Sun in the morning and the see which stars appear there in the night. You will see that the Sun appears to pass through the twelve constellations of the zodiac during a year.
Earth’s journey around the Sun, which seems like the Sun travelling through the zodiac takes about 365.25 days. This is different from year to year, so astronomers add or delete a second in some years to keep their time accurate with the Earth’s motion.
THE SEASONS: The seasons indicate the variation in the pattern of daylight over the span of a year. Because the Earth is tilted off centre, different parts of it receive different amounts of sunlight on different stages of its path around the Sun, a path that we call a year. So, between about the 21st September and late March, the Earth’s Northern Hemisphere is tilted away from the Sun, which creates Autumn and Winter, giving less than twelve hours of daylight per day.
From April to the 20th September, the Northern Hemisphere receives more than twelve hours of daylight a day, producing Spring and Summer. The exact opposite occurs in the Southern Hemisphere.
The Equinoxes take place at the points in the year when there is precisely twelve hours of sunlight and darkness in the day. So, the vernal or Spring equinox is on or around the 21st March and the autumnal equinox is on or around the 21st September. Summer officially begins on the day with the greatest amount of daylight, the 21st June or summer solstice.
The winter solstice is on the shortest day, the 21st December. ‘Solstice’ is a combination of two words meaning ‘sun standing still’ and the days are so named because they are the days when the apparent movement of the Sun reaches its limits and reverses direction again.
In the West, we tend to think that there is only one calendar, but there are dozens of them around the world. And what is more, there were probably hundreds of them before. All non-operational now either because ours is more precise or because theirs did not fit in with our business way of life.
But that does not mean to say that people do not still use those old-fashioned, redundant calendars. Oh, no! Governments have given up their old, traditional national calendars, but in general, country folk still use to them, even if they can no longer get hold of a printed version. I cannot go into all the calendars here, but I will mention half-a-dozen of them.
Lunar Calendar – There is some indication that early man used marks on bone to track or indicate the passage of time 25,000 years ago, probably calibrated by the Moon’s phases. A calendar can be developed based on the lunar cycles; it produces a year of twelve months (the word ‘month’ is from the word ‘moon’), but only 354 days, that is, eleven short of the time it takes the Earth to revolve around the Sun. The Chinese still use a version of the lunar calendar but they resolve this issue by adding extra moths every now and then to bring ‘time’ back into alignment with the Sun.
Solar Calendar – The ancient Egyptians were the first people to employ a Solar Calendar, although it could justifiably be called a stellar calendar. The new year began for them when Sirius, the Dog Star, the brightest star in the night sky, rose in the same place as the Sun. This usually coincided with the flooding of the Nile.
This calendar was of 365 days; twelve months of thirty days and five festive days. Therefore, it was only one quarter of a day off the true year. However, that meant that gradually, the new year did not coincide with the flood. Scientists have worked out that this calendar was taken up in either 4241 BC or 2773 BC.
Julian Calendar – In 46 BC, Julius Caesar realized that various provinces of the empire were using different calendars, so he ordered the dating system to be unified. Sosigenes came up with a calendar of 365 days with an extra day every four years. Therefore, in 46 BC, the longest year on record, Caesar added days to the year to bring it back into alignment with the seasons. 46 BC was 445 days long! The immensity of the Roman Empire ensured that this calendar was the defacto calendar of the Western world.
Julian Day Count – In 1582, Pope Gregory XIII called into being a new calendar, but the year after that Joseph Justus Scaliger introduced a system of counting days, not years. It starts with 1 on January 1st 4713 BC. On this date the Julian and the lunar calendars and the Roman tax dating system all coincided; something that will next occur in 3267. January 1st 2001 was Julian day 2,451,913
Gregorian Calendar – from at least 730 AD, it was spotted that the year from vernal equinox to vernal equinox was short of the 365.25 days in a year. This meant that the date of Easter was moving back. So he dropped 10 days from 1582 by jumping from October 4th to October 15th and proclaiming that century years would only be leap years if they were divisible by 400. Therefore, 1900 was not a leap year, but 2000 was. This is the calendar we still use today.
Thousands of years ago, ancient Greek astronomers calculated that the track of the Earth’s axis is constantly, even if in a very slow way, shifting in a uniform pattern. The variation is very similar to the manner a spinning top slowly leans one way and then another as it slows down. It is a wobble that happens as its axis alters direction.
This odd movement of the planet is due to a couple of factors, the most important of which is something called ‘precession’. Precession arises from the fact that the Earth is not a perfect sphere. It is in fact about twenty-seven miles longer around the Equator that it is around the Poles. The Earth then is oblate, or fat around the middle like middle-aged spread, but it is due to the rotation not to its age.
If you picture the Earth with its Poles off centre. Then rotate that image and you will find that any point, except the very centre of the axis, will travel in a circle. But very, very gradually. So slowly that it takes 26,000 years to go full circle and get back to where it started from.
This point then, any point you choose, is very gradually shifting its position in relation to the stars because the axis is rotating too. The result of this is that, what we call the North Star (formally known as Polaris, which is in fact one degree off true North) will not be over our North Pole one day. In fact, by about 15,000 AD, Vega will be almost over the North Pole, although it will be about four degrees off true North. But even this will not endure, and by 28,000 AD, Polaris will be back above where it is nowadays.
One of the effects of the precession is that seasons vary. They modify the dates that they take place, so that Summer could come earlier or later. The amazing thing about our calendar is that it is corrected for that (with the leap year). If it were not, the vernal or Spring equinox would shift over 13,000 years from March 21st to September 21st., which is the date of the autumnal equinox, precisely half a year later.
It is for this reason that the precession of the Earth is generally referred to as the “Precession of the Equinoxes”. Although the precession of the equinoxes is very lengthy, it can be readily observed. The correct year of 325.25 days is the length of time from one vernal equinox to the next vernal equinox, however, it takes 20 minutes and 24 seconds longer for the Sun to appear in exactly the same place in relation to the stars behind it over the same period. It is for this reason that accurate star maps have to be stamped with the exact time and date to which they relate.