In regulating the seasonal requirements of a pastoral and grain economy, the determination of the year was of supreme importance. The continuity of careful observations which preceded, and the precision involved in settling the exact length of the year, entitle this achievement to be regarded as one of the half-dozen great cultural feats in the history of mankind. Since the Egyptian priests had already established a year of 365 days by 4241 B.C., we may conclude that the recognition of the year as a unit of time antedates the beginnings of the great calendar civilizations. Associated with the passage of the seasons in the everyday life of neolithic man, two classes of events contributed to the first crude appreciation of the year as a natural unit of time. One is concerned with the behavior of the stars, the other with that of the sun's shadow.
The stars rise earlier every night. If a star is seen rising exactly at sunset on a particular day, it will be seen well above the horizon when the sun sets a few weeks later. If a star is in the west at sunrise and in the east at sunset in March, it will reach its highest point in the sky when the sun goes down, about three full moons later, i.e. at the end of June. After six months it will be already setting in the west at sunset, unless it is very near the pole. If it is a circumpolar star, as are those in the constellations of Cassiopeia and of the Great Bear in our latitude, it will be sloping down towards the northern horizon. A circumpolar star seen at midnight directly above the pole, will be seen directly below the pole ("lower culmination") at midnight six months later.
The majority of the stars are below the horizon at lower culmination. So they are only visible after nightfall during part of the year. At midwinter, in the latitude of London, Orion, with its three bright stars forming the belt, is visible most of the night, rising just after sunset and setting in the early morning hours before sunrise. By March 21st (vernal equinox) it has reached its highest point (crosses the meridian) in the heavens at sunset, and is seen setting about midnight. By midsummer it sets before sunrise and has not yet risen by sunset. So it is invisible in the summer sky.
All these appearances occur with perfect regularity after the lapse of the same number of full moons. Thus the sun's apparent position among the fixed stars is not constant. Since the stars rise earlier every day, the sun, while, partaking of the apparent diurnal rotation of the celestial sphere, also seems to be slipping back a little in the opposite direction, like the moon only not so fast. In the course of a year it slips back through a complete circle to its original position. A common early estimate of the time taken to do so was twelve 30-day months or 360 days, hence the division of the great circle of the sun's track in the heavens into the three hundred and sixty degrees which have persisted to our own time. From the standpoint of an earth-observer, the constellations cross the meridian above the pole at midnight, when the sun occupies a position on the opposite side of the celestial sphere. When the sun is on the same great semicircle joining the celestial poles, they will pass over the horizon of the observer by day. Consequently they will not be visible to the naked eye, being screened by the brightness of the sun.
The successive positions of the sun in the heavens during its annual retreat below the eastern horizon in the circle called the ecliptic were mapped out by the ancient priesthoods in milestones corresponding to the twelve months of the year. These milestones, the zodiacal constellations, were groups of stars whose rising and setting positions roughly corresponded to that of the sun at a particular season. Owing to the slow rotation (precession of the equinoxes) of the equinoctial circle about the ecliptic, the sun's position among the fixed stars at a particular season is not the same as it was in ancient times, here shown. When the sun occupies the position of Aries (i.e. is seen in the same direction as Aries would be seen if visible), it sets and rises with the latter, which is therefore invisible. A month later, when the sun is in Taurus, Taurus rises and sets with the sun and is invisible. Aries is seen rising just before sunrise where the sun rose a month earlier. When the sun was in Aries, Taurus would have been setting for about an hour after sunset where the sun would sink below the horizon a month later. The constellations corresponding to the sun's position during the summer months (Taurus and Virgo, Gemini and Leo, Cancer) had northerly risings and settings, describing large arcs and therefore remaining long above the horizon in the winter night sky. The constellations mapping out the sun's position in the winter months (Pisces and Scorpio, Aquarius and Sagittarius, Capricorn) have southerly risings and settings, describing short arcs above the horizoin and being conspicuous during the short summer nights.
The number of days which elapse between the rising of a star just before sunrise or its setting just before sunset on two successive occasions is the period in which the sun gets back to its same position relative to the fixed stars. The Egyptian year of 365 days was based on the heliacal rising * of Sirius, the brightest star in the sky. Sirius is a winter star, rising at sunset about the beginning of January. Early in March it is already setting by midnight. After being invisible throughout the night in June, Sirius reappears on the eastern horizon a few minutes before sunrise on a day in July. This happened at the time when the flooding of the Nile brought assurance of food and prosperity to the Middle Kingdom. The advanced state of astronomical knowledge in the calendar civilizations of antiquity need not surprise us, when we take stock of the astronomical knowledge of living peoples whose cultural development is in other respects very primitive.