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. The following extracts from Nilsson† monograph Primitive Time Reckoning, are instructive:
Time-indications from the phases of the climate and of Nature are only approximate: they themselves, like the concrete phenomena to which they refer, are subject to fluctuation. . . . In general, primitive man takes no notice of these variations: the Banyankole, for instance, are indifferent as to whether the year is one or even three weeks longer or shorter, i.e. whether the rainy season opens so much earlier or later. The days are not counted exactly, but the people are content with the concrete phenomenon. More accurate points of reference are, however, especially desirable for an agricultural people, since, although the right time for sowing can be discerned from the phenomena and general conditions of the climate, yet a more exact determination of time may be extremely useful. The possibility of such a determination exists -- and that at a far more primitive stage than that of the agricultural peoples -- in the observation of the stars, and especially in the observation of the so-called "apparent" or, more properly, visible risings and settings of the fixed stars, the importance of which has already been explained. The observation of the morning rising and evening setting is extraordinarily widespread, but other positions of the stars, e.g. at a certain distance from the horizon, are also sometimes observed. The Kiwai Papuans also compute the time of invisibility of a star. When a certain star has sunk below the western horizon they wait for some nights during which the star is "inside"; then it has "made a leap," and shows itself in the east in the morning before sunrise. . . . Stellar science and mythology are therefore widespread among the primitive and extremely primitive peoples, and attain a considerable development among certain barbaric peoples. Although this must be conceded, some people are apt to think that the determination of time from the stars belongs to a much more advanced stage: it is frequently regarded as a very learned and very late mode of time-reckoning. Modern man is almost entirely without knowledge of the stars; for him they are the ornaments of the night-sky, which at most call forth a vague emotion or are objects of a science which is considered to be very difficult and highly specialized, and is left to the experts. It is true that the accurate determination of the risings and settings of the stars does demand scientific work, but not so the observation of the visible risings and settings. Primitive man rises and goes to bed with the sun. When he gets up at dawn and steps out of his hut, he directs his gaze to the brightening east, and notices the stars that are shining just there and are soon to vanish before the light of the sun. In the same way he observes at evening before he goes to rest what stars appear in the west at dusk and soon afterwards set there. Experience teaches him that these stars vary throughout the year, and that this variation keeps pace with the phases of Nature, or, more concretely expressed, he learns that the risings and settings of certain stars coincide with certain natural phenomena. . . . Just as the advance of the day is discerned from the position of the sun, so the advance of the year is recognized by the position of certain stars at sunrise and sunset. Stars and sun alike are the indicators of the dial of the heavens. A determination of this kind, however, is not so accurate as that from heliacal risings and settings. Hence the latter pass almost exclusively or at least preeminently under consideration wherever, as in Greece, a calendar of the natural year is based upon the stars: sometimes, however, the upper culmination is given. . . . In order to determine the time of certain important natural phenomena it is therefore sufficient to know and observe a few stars or constellations with accuracy and certainty. The Pleiades are the most important. It has been asked why this particular constellation, consisting as it does of comparatively small and unimportant stars, should have played so great a part, and the answer is chiefly that its appearance coincides (though this is true of other stars also) with important phases of the vegetation. . . . An account of the Bushmen shows how extremely primitive peoples may also observe the risings of the stars, may connect them with the seasons, and -- which is indeed somewhat rare -- may even worship them. . . . Canopus and Sirius appear in winter, hence the cold is connected with them. . . . The Hottentots connect the Pleiades with winter. These stars become visible in the middle of June, that is, in the first half of the cold season, and are therefore called "Rimestars," since at the time of their becoming visible the nights may be already so cold that there is hoar-frost in the early morning. The appearance of the Pleiades also gives to the Bushmen of the Auob district the signal for departure to the tsama field. . . . A tribe of Western Victoria connected certain constellations with the seasons. . . . The winter stars are Arcturus -- who is held in great respect since he has taught the natives to find the pupae of the wood-ants, which are an important article of food in August and September -- and Vega, who has taught them to find the eggs of the mallee-hen, which are also an important article of food in October. The natives also know and tell stories of many other stars. Another authority states that they can tell from the position of Arcturus or Vega above the horizon in August and October respectively when it is time to collect these pupae and these eggs. . . . For example, when Canopus at dawn is only a very little way above the eastern horizon, it is time to collect eggs; when the Pleiades are visible in the east a little before sunrise, the time has come to visit friends and neighbouring tribes. The Chukchee form out of the stars Altair and Tarared in Aquila a constellation named pchittin, which is believed to be a forefather of the tribe who, after death, ascended into heaven. Since this constellation begins to appear above the horizon at the time of the winter solstice, it is said to usher in the light of the new year, and most families belonging to the tribes living by the sea bring their sacrifices at its first appearing. . . . The South American Indians
have much greater knowledge of the stars, and in consequence frequently connect stellar phenomena, especially those of the Pleiades, with phases of Nature. In north-west Brazil the Indians determine the time of planting from the position of certain constellations, in particular the Pleiades. If these have disappeared below the horizon, the regular heavy rains will begin. The Siusi gave an accurate account of the progress of the constellations, by which they calculated the seasons, and in explanation drew three diagrams in the sand. No. 1 had three costellations: -- "a Second Crab," which obviously consists of the three bright stars west of Leo, "the Crab," composed of the principal stars of Leo, and "the Youths," i.e. the Pleiades. When these set, continuous rain falls, the river begins to rise, beginning of the rainy season, planting of manioc. No. 2 had two constellations: -- "the Fishing Basket," in Orion, and kakudzuta, the northern part of Eridanus, in which other tribes see a dancing implement. When these set, much rain falls, the water in the river is at its highest. No. 3 was "the Great Serpent," i.e. Scorpio. When this sets there is little or no rain, the water is at its lowest. The natives of Brazil are acquainted with the course of the constellations, with their height and the period and time of their appearance in, and disappearance from the sky, and according to them divide up their seasons. . . . In Africa also the observation of the stars, and above all the Pleiades, is widespread. In view of the dissemination of this knowledge all over the world it is making a quite unnecessary exception to state that it came into Africa from Egypt. Moreover, this assertion does not correspond with the facts, since among the Egyptians Sirius, and not the Pleiades, occupied the chief place. . . . The Melanesians of Banks Island and the northern New Hebrides are also acquainted with the Pleiades as a sign of the approach of the yam-harvest. The inhabitants of New Britain ( Bismarck Archipelago) are guided in ascertaining the time of planting by the position of certain stars. The Moanu of the Admiralty Islands use the stars as a guide both on land and at sea, and recognize the season of the monsoons by them. When the Pleiades (tjasa) appear at nightfall on the horizon, this is the signal for the north-west wind to begin. But when the Thornback (Scorpio) and the Shark (Altair) emerge as twilight begins, this shows that the south-east wind is at hand. When the "Fishers' Canoe" (Orion, three fishermen in a canoe) disappears from the horizon at evening, the south-east wind sets in strongly: so also when the constellation is visible at morning on the horizon. When it comes up at evening, the rainy season and the north-west wind are not far off. When "the Bird" (Canis major) is in such a position that one wing points to the north but the other is still invisible, the time has come in which the turtles lay eggs, and many natives then go to the Los-Reys group in order to collect them. The Crown is called the "Mosquito-star," since the mosquitoes swarm into the houses when this constellation sets. The two largest stars of the circle are called pitui and papai: when this constellation becomes visible in the early morning, the time is favourable for catching the fish papai. The natives of the Bougainville Straits are acquainted with certain stars, especially the Pleiades: the rising of this constellation is a sign that the kai-nut is ripe: a ceremony takes places at this season. On Treasury Island a grand festival is held towards the end of October, in order -- so far as could be ascertained -- to celebrate the approaching appearance of the Pleiades above the eastern horizon after sunset. In Ugi, where of all the stars the Pleiades alone have a name, the times for planting and taking up yams are determined by this constellation. In Lambutjo the year is reckoned according to the position of the Pleiades. . . . When the stars indicate this or that event, the primitive mind, as so often happens, is unable to distinguish between accompanying phenomena and causal connexion; it follows that the stars are regarded as authors of the events accompanying their appearance, when these take place without the interference of man. So in ancient Greece the expressions (a certain star) "indicates" or "makes" certain weather were not kept apart, and the stars were regarded as causes of the atmospheric phenomena. A similar process of reasoning is not seldom found among primitive peoples, and a few instances have already been given, such as the warning-incantation of the Bushmen against Canopus and Sirius, the name given to the Pleiades among the Bakongo ("the Caretakers-who-guard-the-rain"), and the belief that the rain comes from them, the myth of the Euahlayi tribe that the Pleiades let ice fall down on to the earth in winter and cause thunderstorms, in other words send the rain, and the belief of the Marshall Islanders that the various positions of certain stars cause storms or good winds.