calendarAstral themes

The sky is the most mysterious part of our everyday experience. Familiarity may make the amazing events going on at ground level seem almost ordinary. Plants and animals grow and die, rain falls, rivers flow. We feel we understand that.

But the sky is beyond comprehension. Two great objects travel through it, one hot and constant, the other cold and changeable. In the daytime it is moody; there may be blazing sun, or racing clouds, or darkness followed by thunder and lightning. And yet on a clear night the sky is the very opposite – predictable, if you look hard enough, with recognizable groups of stars moving in a slow but reliable manner.

Man’s interest in the sky is at the heart of three separate stories – astronomy, astrology and the .

Astronomy is the scientific study of sun, moon and stars. Astrology is a pseudo-science interpreting the supposed effect of the heavenly bodies on human existence. In early the two are closely linked. The sky is the home of many of the gods, who influence life on earth. And the patterns in the sky must surely reflect that influence.

Days, months and years

Compilers of a calendar, attempting to record and to predict the passage of time, are offered an easy first step in the cycle of the moon.

The only two measures of time available to primitive people are the day (the space between two nights) and the month (the space between new moons). The month is a well adjusted length of time for recalling fairly recent events, and it has a magic significance through its loose link with the female menstrual period. A far more important slice of time is the year, a full circuit of the earth round the sun – crucial in human activities because of its influence on seasons and crops. But the length of a year is exceptionally hard to measure.

Primitive societies make do with a broad concept, counting the year as starting when leaves sprout on a particular tree or describing someone as having lived through a certain number of harvests.

The only simple yet accurate way of measuring a year is in relation to the stars (though structures such as the passage grave at Newgrange can record an annual position of the sun, at a considerable cost in effort). The stars appear in the night sky at different times and places depending on where the earth is in its orbit round the sun. A star observed in a given place – on the horizon at dawn, for example – will be there again exactly a year later.

In Egypt the temple priests derive much of their prestige from close attention to the stars, enabling them to give the impression of predicting natural events. The best example is their use of Sirius, the Dog Star. It rises above the horizon just before dawn at the time of year when the all-important flooding of the Nile is about to occur. Priests who can foretell this great event are powerful soothsayers.

This observation of Sirius also enables the Egyptians to become the first people to move from a lunar to a solar calendar.

Lunar and solar years

In Mesopotamia, where the Babylonians are the leading astronomers, the calendar is a simple lunar one. So probably is the first Egyptian calendar. And a lunar calendar is still in use today in Islam. But such a calendar has one major disadvantage.

The length of a lunar month, from one new moon to the next, is 29.5 days. So twelve lunar months are 354 days, approximately 11 days short of a solar year. In a lunar year each of the twelve months slips steadily back through the seasons (as happens now with the Muslim calendar), returning to its original position only after 32 years.

In some lunar calendars an extra month is inserted from time to time to keep in step with the solar year. This happens in Mesopotamia and in republican Rome, and it remains the case today in the Jewish calendar.

But the Egyptian priests’ observation of Sirius enables them to count the number of days in a solar year. They make it 365. They then very logically adjust the twelve months of the lunar year, making each of them 30 days long and adding 5 extra days at the end of the year. Compared to anybody else’s calendar at the time this is very satisfactory. But there is a snag.

The priests cannot have failed to notice that every four years Sirius appears one day later. The reason is that the solar year is more exactly 365 days and 6 hours. The Egyptians make no adjustment for this, with the result that their calendar slides backwards through the seasons just like a lunar one but much more slowly. Instead of 32 years with the moon, it is 1460 years before Sirius rises again on the first day of the first month.

It is known from the records that in AD 139 Sirius rises on the first day of the first Egyptian month. This makes it certain that the Egyptian calendar is introduced one or two full cycles (1460 or 2920 years) earlier, either in 1321 or 2781 BC – with the earlier date considered more probable.

Julian and Mayan calendars: 1st century BC

The Roman calendar introduced by Julius Caesar, and subsequently known as the Julian calendar, gets far closer to the solar year than any predecessor. By the 1st century BC reform in Rome has become an evident necessity. The existing calendar is a lunar one with extra months slipped in from to time in an attempt to adjust it. In Caesar’s time this calendar is three months out in relation to the seasons.

On the advice of Sosigenes, a learned astronomer from Alexandria, Caesar adds ninety days to the year 46 BC and starts a new calendar on 1 January 45.

Sosigenes advises Caesar that the length of the solar year is 365 days and six hours. The natural solution is to add a day every fourth year – introducing the concept of the leap year. The extra day is added to February, the shortest of the Roman months.

Spread through the Roman empire, and later throughout Christendom, this calendar proves very effective for many centuries. Only much later does a flaw yet again appear. The reason is that the solar year is not 365 days and 6 hours but 365 days, 5 hours, 48 minutes and 46 seconds. The difference amounts to only one day in 130 years. But over the span of history even that begins to show. Another adjustment will eventually be necessary.

While Julius Caesar is improving on the solar calendar of 365 days, a similar calendar has been independently arrived at on the other side of the Atlantic. Devised originally by the Olmecs of central America, it is perfected in about the 1st century AD by the Maya.

The Maya, establishing that there are 365 days in the year, divide them into 18 months of 20 days. Like the Egyptians (who have 12 months of 30 days), they complete the year by adding 5 extra days at the end – days which are considered to be extremely unlucky for any undertaking. An unusual aspect of the Mayan system is the Calendar Round, a 52-year cycle in which no two days have the same name.

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