Astrology is built upon an accurate accounting of time. Fundamental to this accounting is a calendrical system that takes into consideration the irregular manner in which days, months, and years fit together. A calendar in the broadest sense consists of the set of rules that a society uses for deciding which days are ordinary days and which are holidays (a variant of "holy days"). Societies in the past evolved many different kinds of calendars, and a surprisingly large number of them are important for understanding the details of the Western civil calendar.

The famous Aztec calendar can be read in terms of cycles covering thousands of years. Reproduced by permission of Fortean Picture Library.

Origins of the Calendar

There are three natural divisions of time on Earth. The most obvious is the alternation of night and day, and all calendars are organized in terms of the 24-hour day, which is the approximate average length. However, not all calendars are organized in terms of hours with a fixed length. Our ability to measure seconds and even minutes accurately was achieved only in modern times. A medieval sundial divided the hours of sunlight into 12 hours, but, obviously, in northern latitudes an hour during a long summer day might be twice as long as an hour during a short winter day. The Western civil calendar that is now used internationally is based on hours of a precisely defined fixed length, but there are still some local or folk calendars in which the length of an hour is much more flexible.

The next most obvious way to divide time is to use the phases of the Moon. Originally, a month was a "moonth": It represented the period from one full moon or new moon to the next. We cannot know how people measured time during the tens or hundreds of millennia that all human beings existed as bands of hunters and gatherers, following the herds and the ripening fruits and grains in an annual migration north and south. During the last ice age (from roughly 20,000 to 100,000 years ago), when human beings were forced to live in caves and develop new stoneware technology in order to survive, they may have begun tallying the phases of the Moon more carefully than before in attempting to calculate the length of the lunar month. In the Western civil calendar, months are arbitrary groups of days, ranging from 28 to 31 days in length that are not correlated with the phases of the moon. All major religious calendars (Christian, Jewish, Moslem, Buddhist, and Hindu) still depend wholly or partly on having months that are exactly in phase with the Moon.

The third most obvious time division is marked by the seasons—the annual migration north and south of the Sun's rising and setting points. Probably for a long time, years were labeled only relatively, as the regnal year of a king, by the number of years since some memorable event, and so on; and this starting point would be changed with every new generation. Only rather late in the history of civilization did years begin to be numbered from some fixed point in the distant past, such as the first Olympiad, the founding of the city of Rome, or the birth of Jesus of Nazareth.

Constructing a Calendar

As could their predecessors, agricultural villagers today can coordinate their annual activities by word of mouth, but citizens of an empire cannot. It obviously will not work to have the arrival times of people coming to a three-day festival in the capital city spread out over a week. Hence, about 5,000 years ago, the administrators in Egypt and Sumeria were faced with the problem of constructing a calendar that everyone could use to see, on each day, how many more days it would be until some scheduled event. But to construct such a calendar, these people had to deal with four basic questions:,

2. How long is a month? (Or, equivalently, how many days are there in a month?)

4. How many months are in a year?

Being used to our modern answers to these questions, we may think them obvious; but they are not, and adequate answers to them were found only by centuries of ongoing observations, measurements, and calculations.

How Long Is a Day?

The technique of dividing day and night into 12 hours each was devised by the Babylonians, who calculated with a number system that used a base of 12 rather than 10. Hours were introduced into the Roman calendar only rather late in Roman history, when the seven-day week (also a Babylonian invention) was generally adopted. Originally the Romans had divided day and night into watches, each several hours in length.

In modern usage, a day is defined as being 24 hours long; an hour is defined as 60 minutes, or 3,600 seconds, long; and a second is defined as so many vibrations of a specific line in the spectrum of a specific isotope. Naturally, this definition was worked out in a way that makes 24 hours equal to the traditional average length of a day. It was finding this average length that was the problem in the ancient world, for several reasons.

First, where do you measure from? The convention of starting each calendar day at midnight was agreed upon only in modern times. In most ancient calendars, each day began at sunset and ended at the following sunset (some ancient peoples, such as the Egyptians, counted a day as running from one dawn to the next); this is why the "eve" before many traditional holidays is still important and why the Jewish Sabbath celebration begins at sunset on Friday. But exactly when is sunset? It takes about 15 minutes for the Sun to sink completely below the horizon, which appears higher on land than it does at sea. This ambiguity is why the Talmud prescribed that all activities not allowed on the Sabbath should cease two hours before sunset. Some conventional definition—such as measuring from the moment the disc of the sun first touches the horizon—had to be introduced and adhered to.

Furthermore, since the days (in the sense of hours of light) grow longer (how much longer depends on the latitude) during half the year, shorter during the other half, an accurate measurement needs to be correct to within less than a minute to be useful for constructing a calendar. But there were no accurate techniques before modern times—even measuring a quarter hour accurately was difficult—and so the ancient calendars tended to accumulate an error of a day every few years.

How Many Days Are in a Month?

In most ancient calendars, a month was a lunar month, that is, one full cycle of the Moon's phases. We know now that the average length of a lunar month, measured from one astronomical new moon to the next, is 29.5306 days. However, an ancient month began not at the astronomical new moon, which is an invisible event, but at the first visible crescent. Many factors affect when the crescent of the new moon will be visible at a particular location. Usually the interval from each first crescent to the next will alternate between 29 and 30 days—and so the length of the months will alternate likewise—but it is easily possible for two or even three intervals of 29 days or 30 days to fall successively. Hence, it was quite late in history—long after the length of the year was well known—before the average length of the lunar cycle was known with usable accuracy.

How Many Days Are in a Year?

We know now that the average length of the year is 365.2422 days, but this precise value was taken into account only by the Gregorian reform of the calendar in 1582. The Julian calendar (devised by Julius Caesar), which the Gregorian calendar replaced, assumed the year to be 365.25 days long (as we all do for ordinary purposes), and the earlier Roman calendar that Julius replaced apparently assumed the year to be 366.25 days. The Egyptian calendar, which Julius borrowed as the basis for his, assumed the year to be exactly 365 days.

The problem in the ancient world again was finding a fixed point from which to measure the length of (or to begin) the year. The most popular choices were the winter solstice, when the days begin growing longer again, and the spring equinox, when the hours of sunlight and darkness are equal, but many others were also used. Measuring the moment of winter solstice would seem a difficult task for ancient peoples, but it now seems clear that the people who built Stonehenge about 3000 b.c.e. could do so quite accurately. They could also predict all eclipses of the Sun and the Moon. The Egyptians seem to have solved the problem by observing the heliacal rising of Sirius each year: The fixed stars, including Sirius, appear to rotate about the Earth each sidereal day, which is always the same length. Which stars are visible in the night sky depends on where the Earth is in its annual orbit around the Sun. Sirius (and any other star) will always first become visible after sunset (weather and local conditions allowing) on the same day each year relative to the solstices and equinoxes; this is its heliacal rising. In classical times, the Mesopotamians claimed that they had also solved the problem in another way, as early as the Egyptians had, but it is not certain that they had done so before the seventh century b.c.e.

How Many Months Are in a Year?

This is the most difficult of the four questions (and the one that causes the most differences between calendars), because the length of the solar year is not a simple multiple of the length of the lunar month. Hence, if the months are to stay in phase with the Moon, there are many problems.

Twelve months that alternate between 29 and 30 days produce a year of 29.5 x 12 = 354 days, which is 11.2422 days short of an average solar year. Every three years this difference will add up to 33.7266 days, allowing an extra lunar month—of, say, 30 days—to be added. This still leaves a difference of 3.7266 days, which will add up to 33+ days after 27 years, allowing an extra lunar month to be inserted, and so on. It seems clear, however, that people generally would not like to have a feature in their calendars that appears only once in 27 years; for example, what would this extra month be called? Would it contain any holidays?

Only two basic kinds of calendars have succeeded in dealing adequately with the various problems of timekeeping: (1) the lunisolar calendar of the Mesopotami-ans, which added lunar months during years three, five, and eight of eight-year cycles, and (2) the purely solar calendar, devised by the Egyptians. Despite the retention of 30- to 31-day periods that are still termed months, the Western calendar is a solar calendar.

The zodiac bears the imprint of all three means (days, months, and years),of measuring time, but does not correspond precisely with any of them. The astrological year and the solar year, for example, are of equal lengths, but the astrological year begins at the exact moment the Sun enters the sign Aries (the spring equinox) rather than on January 1. Also, the Sun resides in each sign for approximately one month, but neither the lunar months (which vary every year) nor the months of the Western calendar correspond with this residence (the Sun enters each sign between the eighteenth and the twenty-fourth of each month). For these reasons and others, astrologers must use their own calendars, termed ephemerides, to determine the precise positions of the heavenly bodies.


Colson, F. H. The Week. Cambridge: Cambridge University Press, 1926. Hawkins, Gerald S. Stonehenge Decoded. New York: Doubleday, 1965. Hoyle, Sir Fred. Stonehenge. San Francisco: W. H. Freeman, 1976.

Nilsson, Martin P. Primitive Time-Reckoning: A Study in the Origins and First Development of the Art of Counting Time Among the Primitive and Early Culture Peoples. Lund, Norway: Gleerup, 1920.

O'Neil, W. M. Time and the Calendars. Sydney: Sydney University Press, 1975. Parise, Frank, ed. The Book of Calendars. New York: Facts on File, 1982. Wilson, P. W. The Romance of the Calendar. New York: Norton, 1937.

Wright, Lawrence. Clockwork Man: The Story of Time, Its Origins, Its Uses, Its Tyranny. New York: Horizon, 1968.

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