The Early Developments In Astronomy

This part of the book covers the period when observations of the Universe relied entirely on the unaided human eye with its consequent limits and constraints. It starts in ancient times when the earliest civilizations showed a fascination for astronomy, albeit with astrological overtones, and it ends with the Renaissance when the subject was put on to a firm scientific footing and when a new age was heralded by the invention of the telescope.

CHAPTER ONE The Beginning

Before the beginning of years There came to the making of man Time, with a gift of tears;

Swinburne

About five thousand million years ago, a new star was born in our Milky Way galaxy. It was an average star, neither over-bright nor over-faint, and was formed like all other stars by condensation of the interstellar gas under the all-pervading force of gravity. In all such contractions some degree of rotation exists and this causes the material to form into a circulating disk within which condensations occur to form a number of gravitationally bound objects. Whether these objects become a star or a planet depends entirely on the amount of material condensing in the gravitational contraction. If this is large enough, the interior will heat up to the ultra-high temperature needed to ignite a nuclear furnace which generates enormous energy by fusing the most abundant element, hydrogen, into helium. In our Solar System, only the Sun reached this critical mass and the other condensations resulted in the formation of the planets. In more than half of other star formations, more than one body exceeded the critical mass to become a star. This is demonstrated by the fact that more than half of the stars in the sky are multiple, with two or even three stars in orbit about themselves, possibly with some planets.

The interstellar gas from which the Solar System was formed was composed mainly of hydrogen (74 per cent by mass) with 24 per cent helium and all the other heavier elements from carbon to uranium contributing only 2 per cent by mass. These elemental abundances are reflected in the composition of the Sun and the giant planets, Jupiter, Saturn, Uranus and Neptune, but the Earth and the other terrestrial planets, Mercury, Venus and Mars, are rocky in nature, indicating a chemically selective process in their formation which favoured the heavier elements and allowed most of the hydrogen and helium to escape. Yet the heavier elements did not exist at the start of the Universe when the primeval matter produced in the Big Bang was composed entirely of hydrogen and helium, with minute traces of lithium, beryllium and boron; but there was no carbon, nitrogen, oxygen or any of the other 87 elements found on Earth. Hence, the very first stars that were formed in the rapid star-burst era that marked the beginning of our galaxy contained no heavy elements, and any planets formed at that time could not have been even remotely like the Earth. But many of those early stars were more massive than the Sun and, consequently, evolved more rapidly to the extent that they had gone through their whole life-cycles by the time the Solar System was formed. As will be related later in this book, they had successively fused hydrogen into helium, helium into carbon, nitrogen, oxygen, silicon and all the other elements in the periodic table up to iron; then, in an immensely explosive event called a supernova, all the elements heavier than iron, from cobalt to uranium, were formed, and these, together with the lighter elements, were hurled into space in a high-velocity expanding shell. Far from being a contamination of the primeval interstellar gas, this was, as far as we are concerned, a crucial enrichment of it, because it provided those elements essential to life. Indeed, apart from the hydrogen present in the water of our bodies, all the other elements that constitute more than 90 per cent of what we are made of are the result of nuclear processing in the interiors of massive stars and the cataclysmic explosion that heralds the end of their life. We are children of the stars.

These astronomical processes, the nuclear synthesis of the heavier elements in stellar interiors, their ejection into the interstellar medium, and the selective condensation of those elements in the formation of the four terrestrial planets in our Solar System, set the scene for the great miracle—the development of life on one of them, Earth, whose size and distance from the Sun were just right. But the development of life and its evolution was slow.. .very, very slow.

The Earth was formed four and a half thousand million years ago but almost a full thousand million years were to pass before the first micro-organisms appeared and a further thousand million years before marine algae and primitive plants started to generate pure oxygen, not tied up in carbon dioxide, into the atmosphere until, when the Earth was three thousand million years old, it reached a critically important stage for life and the environment when it had an oxygen-bearing atmosphere similar to that of today (except for artificial pollutants). At this point, evolution accelerated greatly and, over the next thousand million years, fish, insects, toothed birds, large reptiles and primitive mammals appeared. Then, 65 million years ago, a catastrophic event, whose cause has just recently been established as a giant meteor or asteroid, led to the extinction of the dinosaurs. It was then that the mammals proliferated and, some 3-4 million years ago, the first human types emerged. But evolution of our own species, Homo sapiens, did not occur until a hundred thousand years ago and the earliest civilizations did not develop until after the most recent ice age had ended about 12000 years ago.

To give some idea of the timescale of the development of life on Earth, the important milestones are listed in the table which also scales real times to one year, that is, as if the Earth were formed on 1 January and its present age is midnight on New Year's eve. This demonstrates vividly the very slow initial development, and then the very rapid later evolution of life, together with the relatively brief presence of Homo sapiens. Since this story relates the attempts of the human race to study and understand the Universe it lives in, it is confined, in human terms, to the very brief period of civilization which, on scaling to one year, covers only the last two minutes; but in astronomical terms, it goes back to the very beginning when, most astronomers now believe, the Universe started in a single, immense, explosive event—the Big Bang.

The timescale of the development of life on Earth; in the final column the times are scaled to one year as if the Earth was formed at the start of the year, and now is midnight at the year's end.

CHAPTER TWO Ancient Astronomy

Awake! for Morning in the Bowl of Night Has flung the Stone that puts the Stars to flight:

Rubaiyat

About 10000 years ago, the most recent ice age was over, the ice had fully retreated and the resulting warm period led to a spread of forests, vegetation, fish and mammals. The human race, which had hitherto spent its energy and ingenuity on survival—the acquisition of food and provision of warmth—responded to the greatly increased food supply by accelerating the development of tools and the techniques of hunting and gathering of plants. The increased productivity allowed the human race to exploit its greatest gift, a powerful brain, more fully. It embarked on the development of civilization and found that the setting up of organized societies, in the form of tribes or whatever, resulted in greater prosperity and more effective defence. It found that farming the land to produce the crops that it wanted, and the husbandry of animals to produce the meat that it needed, were far more bountiful than gathering vegetation and hunting game which happened to be present naturally.

With the development of agriculture and the human control of the environment, the land area needed to support a community decreased by a factor of about a hundred compared to that for hunting and gathering. This great increase in productivity caused an increased growth in population; new societies grew and developed with their own structures and customs to become more unified but also more separate; trade between them developed but, not infrequently, disputes occurred which often resulted in warfare. The victor usually took everything and subjected the conquered to slavery, a practice that became extensive in all ancient civilizations. One powerful form of society that developed was the city-state, which often dominated its surroundings as an empire; Babylon was to be the first and Rome the greatest.

By circa 5000 BC, food production had reached a level that freed significant time and effort for pursuits beyond those needed for survival alone. The consequent release of human ingenuity caused an acceleration in human progress, with the further development of new tools and techniques, allowing more effective farming (and warfare), the building of cities, the development of language, from spoken to written, the strengthening of social organization and authority, and the creation of wealth. Out of this evolved the first true civilizations: societies with the means and the wish to pursue intellectual, artistic and other creative activities in addition to the most basic needed for survival. The great early civilizations were located in many parts of the globe, usually in the most fertile regions, often watered by great rivers. The first of these was the Sumerian civilization, which was fully established in Mesopotamia by circa 3500 BC, so named because it is the land between two rivers (the Tigris and the Euphrates), and is largely embraced by modern-day Iraq. Others were well established in Egypt by circa 3000 BC, in India (the Indus valley) by circa 2500 BC, in Crete (Minoan) by circa 2000 BC, in China by 1500 BC, and in Central America (the forerunners of the Incas and the Aztecs) by 1000 BC.

Many of the intellectual activities pursued in the early civilizations grew from the innate curiosity of the human race in the natural world in which it existed. This was particularly true of astronomy, where the motion of the Sun, Moon, planets and stars caused excitement and puzzlement. You should realize that life in a modern industrial society is a great impediment to viewing the heavens because of artificial lighting and smog. One of the really beautiful sights in nature is of the clear night sky in the absence of city lights, say on a remote mountain, where the sky has great depth and the Milky Way is a bright lane of light. But this sight was available to everyone in the distant past and its beauty led to the pursuit of astronomical studies in all the early civilizations.

But the early developments in astronomy were fired more by practical and mystical rather than scientific considerations. The development of agriculture required that crops be planted in spring and harvested in autumn; hence the times of the seasons needed to be known. In other words, a calendar was required, and several attempts to establish one were made in the period between 5000 and 1000 BC. Natural time-periods were available in the day, determined by the rising, setting and rising again of the Sun; the month, determined by the time it took the Moon to pass through all its phases; and the year, determined by the seasons, over which the Sun reached its maximum noon altitude in mid-summer (for the Northern Hemisphere), its lowest in mid-winter and back again in mid-summer. The first and simplest astronomical instrument, the gnomon, was able to give some indication of the time of day and the season of the year; it consisted of a straight vertical rod and is based on the same principle as the modern sundial. In the early morning, its shadow would be long and point roughly westwards; as the day progressed, it would shorten and rotate until, at noon, it would be at its shortest and would point exactly due north; it would then lengthen and rotate until, in the late afternoon, it was pointing roughly eastwards. If the length of the shadow is measured at noon, it is found to be shortest in mid-summer and longest in mid-winter, thereby allowing the seasons to be estimated. Another way to tell the time of year was afforded by the night sky. The stars were fixed and unchanging relative to each other, but appeared to rotate completely over one year, so that there were winter and summer constellations.

Another group of objects in the night sky were the five planets or wandering stars. As bright as the brightest stars, they moved in the same plane (the ecliptic) as the Sun and Moon, but in odd and seemingly unpredictable ways. Three of them (Mars, Jupiter and Saturn) would advance across the celestial sphere, reverse their motion and advance again; the other two (Mercury and Venus) also moved but were visible only when they were close to the Sun, just after sunset or just before dawn. The puzzle of the planets was to remain unsolved for several millennia and it posed the greatest problem in early cosmologies—the explanation of the motions of the Sun, Moon, planets and stars.

The mystical aspect of studying the heavens, astrology, developed strongly in the early civilizations and soon became the prime driving force. It was believed that the stars and planets controlled human destiny and therefore their study was encouraged as a means of predicting, or explaining, human triumphs and tragedies. Perhaps this is not surprising: if the heavens could say when crops should be planted or harvested, why not when wars should be embarked upon or preparation made for famine or flood? Religious aspects also crept into the interpretation of the heavens, and astronomical studies were often carried out by priests.

The most ancient civilization, the Sumerian, was based in Mesopotamia, now the southern part of modern-day Iraq. It prospered rapidly and by circa 3000 BC had developed a written language which was etched into clay tablets. Unlike the other early civilizations of Egypt and China, it has not retained its name, culture and identity over the centuries, but has changed hands frequently through invasions, migrations and wars. The development of building technology allowed for larger and larger human groupings into city states, and one of these, Ur, became the capital of Sumeria in circa 2500 BC. It lay near the junction of the Euphrates and Tigris rivers in the south of Mesopotamia. It is believed to have been the home of Abraham and his Semitic tribe, who developed a belief in a single, unseen, all-powerful God, a belief that forms the ancient roots of today's three great monotheistic religions. Abraham was to take his tribe out of Ur circa 2100 BC, when it set off on its wanderings (from which the name Hebrew, or "wanderer", derives), which were to take it to Canaan (Palestine), to Egypt and back to Canaan over a thousand years.

In about 2000 BC Ur was conquered by the Elamites, who came from what is now southwest Iran, and a new powerful city-state emerged, Babylon, which was to establish control of Sumeria and extend its empire over the whole of Mesopotamia. Babylon is on the Euphrates about 80 kilometres south of present-day Baghdad. It became very wealthy and very indulgent in the pursuit of pleasure and consumption, giving it a reputation as a magnificent, worldly, wicked city, which still persists today more than two millennia after its demise. Great buildings were constructed, including the Hanging Gardens, one of the seven wonders of the ancient world. The Sumerian cuneiform script was given a syllabic form, thereby greatly increasing its flexibility. Many written tablets still survive which tell us more about the Babylonian Empire than we know about many European countries during the Dark Ages of AD 500-1000.

Many activities were encouraged, one of which was a study of the heavens, entirely for astrological purposes, since they believed their destiny could be read in the stars, but this had the important result of producing the first major set of astronomical observations ever undertaken. Over centuries, the positions of the bright stars were established and the motions of the Sun, Moon and planets charted against the background of those fixed stars. All these data were recorded in cuneiform script etched into clay tablets, many of which are still available today; they allowed some astronomical analysis, as well as astrological interpretations. By about 1000 BC, lunar eclipses could be predicted with reasonable accuracy and the motions of the outer planets had been established over several centuries.

Studies of the heavens also had religious overtones, and the prime celestial bodies were named after gods, of which the Babylonians had several. But in addition to the religious and astrological applications, there was an astronomical requirement—the prediction of the dates of important events such as festivals and the times of sowing and reaping. This required a calendar and one was constructed which had a year of 12 lunar months, and which started with the vernal equinox in spring, the time for sowing. Each month was defined by the Moon and started at sunset on the evening when the new crescent Moon was visible for the first time. Since a lunar month is not an exact number of days, nor is a solar year an exact number of lunar months, this was a somewhat cumbersome basis for a calendar, as many other civilizations discovered. A lunar month is close to 29.5 days, so, with each month being identified by the first appearance of the new Moon, a month was either 29 or 30 days and a year of 12 lunar months consisted of 354 days. The difference between this and the solar year (365.25 days), which determines the seasons, was accommodated with an extra month about every three years. The Babylonians also introduced the seven-day week in deference to the Sun, Moon and five planets, regarded as representing gods.

The Babylonians were responsible for some of the earliest mathematics and they set up the sexagesimal system of angular measurement which is based around the number six and is still used today. They defined a full circle as 360°, corresponding approximately to the 360 days in their calendar; hence 1° corresponds approximately to the angular movement of the Earth during one day in its orbit around the Sun, although the

Babylonians did not look at it in that way. They also set up a simple form of algebraic geometry which had many practical uses and formed a basis for surveying.

The Babylonians also initiated the concept of the zodiac, the band in the celestial sphere through which the Sun moves and completes a full revolution in one year. The Moon and planets also move in the same band (but in totally different ways) and we now know that it represents the ecliptic plane in which the planets revolve about the Sun, and the Moon about the Earth. The Babylonians divided it into 12 equal zones of 30° corresponding to the 12 lunar months of their calendar and then assigned each one to the nearest constellation. Constellations were regions of the sky whose stars showed a pattern which could be likened to known people, animals or objects. All the early civilizations identified their own constellations and the 12 signs of the zodiac evolved slowly from ancient roots. The ones that are in use today are those which were recorded by the ancient Greeks; the sequence starts with Aries (the Ram), moves to Taurus (the Bull), then to Gemini (the Twins), to Cancer (the Crab), on to Leo (the Lion), on to Virgo (the Virgin), then to Libra (the Balance or Scales), on to Scorpio (the Scorpion), then to Sagittarius (the Archer), reaching Capricorn (the Goat), then on to Aquarius (the Water Bearer) and to Pisces (the Fishes), which then joins up with Aries to complete the cycle.

During each year, the dates that correspond to each sign of the zodiac are determined entirely by the constellation in which the Sun is located in its migration around the zodiacal plane. The zodiacal year is defined by the time it takes the Sun to complete its full cycle; it is now called the sidereal year, because it is determined by the Sun's movement against the background of fixed stars. In today's terms, it is precisely the time it takes the Earth to complete one full revolution about the Sun. In developing a calendar, the Babylonians knew that a year determined the cycle of seasons, which we now know are caused by the tilt of the Earth's axis of rotation by 23.5° to the plane of its orbit around the Sun. In summer the axis is tilted towards the Sun, causing greater heating because of the longer period of daylight and the greater solar radiation flux per unit area because of the angle of the Earth's surface being less inclined to the direction of the Sun's rays. Mid-summer is defined by the Earth's axis being tilted exactly towards the Sun (the summer solstice), and mid-winter when it is tilted exactly away from the Sun (the winter solstice). (This is the situation for the Northern Hemisphere; the inverse is the case in the Southern Hemisphere.) Spring is defined by the vernal equinox when the Earth's axis is tilted exactly orthogonal to the Earth-Sun line, so that the Earth is equally illuminated from pole to pole and periods of day and night are equal everywhere; similarly, autumn is defined by the autumnal equinox. Hence, a year can also be determined by measuring the direction of the Earth's inclination relative to the Sun, say from vernal equinox to vernal equinox. Such a year, called a solar year, is directly linked to the seasons and is therefore the most important basis for a calendar, the greatest practical value of which is to predict the occurrence of spring, summer, autumn and winter or, more importantly in Egypt, the flooding of the Nile and, in India, the monsoon. Because of this, our modern calendar (whose development I will recount later, starting with the major and leading contribution of the Egyptians) is based on the solar year.

At this point, you may think that the two years I have described—the zodiacal, or sidereal year and the solar year—are identical and simply represent two different ways of measuring the same thing, but this is not so; there is a very small difference between them. The difference is so small that it was unknown to the Babylonians and was not detected until nearly 2000 years after they started studies of the zodiac and their calendar. It is so small that it has a negligible effect on everyday life and, even today, it is not well known outside astronomical circles. However, there is one area of modern activity that should be affected by the small difference between solar and sidereal years but which seems to be largely ignorant of it.

As already said, much of early astronomy was driven by astrology, the belief that human destiny can be foretold in the stars. The zodiac became one of its main tools and, since it is still used today, as can be seen from its presence in the columns of many newspapers, it is appropriate that I bring its story up to date. The simple proposition is to assign to each individual person that zodiacal sign that the Sun was located in when he or she was born. Destinies could then be read at any one time from the positions of the Moon or planets as they moved through the different signs of the zodiac. However, the direction of the Earth's axis is not fixed but, because of its rotation, precesses in a way similar to that of a spinning top; it closely maintains a tilt of 23.5° to its orbital plane but rotates very slowly about it at a rate that will see a complete revolution in 25800 years. Because of this the constellations move slowly westwards along the plane of the ecliptic with respect to the calendar. This is because the calendar is based on the solar year, which, because of the precession of the Earth's axis, is about 20 minutes shorter than the sidereal or zodiacal year. Without precession, these two years would be identical and the zodiacal constellations would not drift through the calendar. Precession was discovered in the second century before Christ by Hipparchus, the most brilliant observational astronomer of ancient Greece. Later, I will describe the method he used, together with his many other achievements.

The Babylonians had adopted the vernal equinox, marking spring, as the start of their year and the zodiac. At the time of the ancient Greeks, the vernal equinox lay in Aries, so this constellation was labelled as the first sign of the zodiac and the vernal equinox is still referred to as the first point of Aries. But when the Babylonians first set up their zodiac, the vernal equinox lay in Taurus; this was therefore the first sign of their zodiac and it accounts for the Babylonian description of Taurus as not only "The Bull of Heaven" but also "The Bull in Front". If our present series of zodiacal signs had been adopted from the earlier Babylonian records rather than the later Greek ones, the first sign of the zodiac would have been Taurus, not Aries, and the astrological discrepancy I am describing would have been even greater. Today the vernal equinox, which occurs on 21 March in our calendar, lies in Pisces; hence, most people who are assigned the astrological sign of Aries should, astronomically, have the sign Pisces. This drift will continue and, during the next millennium, the vernal equinox will enter Aquarius, presumably heralding the "dawn of Aquarius" Hence, if anyone wished to know which constellation the Sun was in on any particular date, past or future, the appropriate astronomical tables would be needed or, more simply, a sidereal rather than a solar year used in which the constellations will not drift. A more simple but approximate calculation can be made from the fact that the constellations drift through our calendar by one zodiacal sign every 2150 years and that the vernal equinox lay in Aries during the great Greek period of a few, say four, centuries before Christ. As of today, this places the vernal equinox in the western part of Pisces.

The astronomical developments in Babylon were led from the temple and were interlinked with religion and the several gods of the time. A primitive cosmology developed, influenced by the nature of Mesopotamia, a land subject to much flooding (as southern Iraq, it is still a marshland today). This said that the gods created the world out of a watery waste and made human beings out of mud to be slaves to them, a world picture appropriate to a society where monarchs had full and absolute power.

Babylon was at its zenith between 1900 and 1600 BC, but for the following thousand years Mesopotamia was like a battlefield, with invasions from all sides. In 1595 BC, the Hittites attacked from the north (modernday Turkey) and looted and pillaged Babylon; their monopoly of iron gave them military superiority, as well as a great advantage in agricultural tools. In 1100 BC Babylon was conquered by the Assyrians and finally, in 539 BC, it fell to the Persians who established the greatest empire then known through most of the Middle East.

The other great early civilizations, such as those in Egypt, India and China, also conducted astronomical studies which were driven by practical, astrological and religious motives. In China there is evidence that astronomical observations were well under way as long ago as circa 2000 BC. The length of the solar year and the lunar month had been determined with good accuracy, allowing a calendar to be developed which could establish the times of festivals and so on. Constellations were identified with emperors, and stars were ranked in status according to their brightness. They had a very simple cosmology: the Universe was the rotating sphere of fixed stars in which the poles were the exalted regions. Astrology was dominant, but little attention was given to the motion of the planets, an important basis of many other astrologies. Instead, changes such as the appearance of a new star or comet were believed to herald calamities on Earth. One of the most dramatic of such celestial changes is a total solar eclipse, and legend has it that two astrologers were executed because they failed to predict one that occurred in 2137 BC. In India, astronomy developed a little later than in China but in a similar fashion. The solar year was measured, a calendar was constructed and the major celestial objects were named after gods and goddesses; astrology was the driving force in these endeavours.

Of course, our knowledge of such ancient activities depends entirely on the evidence handed down to us. The most effective evidence is in written manuscripts, but we must recognize that many of the earliest writings were based on word of mouth transmission through many generations and were therefore written much later (sometimes centuries) after the events they record. Other astronomical developments, of which we do not have a written record, certainly occurred in other parts of the ancient world, and of these we know little or nothing. One example is the megalithic monuments that are found in many countries, especially the British Isles, and which date as far back as the third millennium BC. These represent a considerable engineering feat (some of the stones are as heavy as about 50 tonnes) and their layout, which shows considerable trigonometric knowledge, is clearly dictated by astronomical principles. The most notable of these is Stonehenge in the south of England.

In the above account of astronomical studies in the ancient world, I have given the most detailed description to those in Babylon because they were the most extensive and had the soundest mathematical base. But one other civilization, Egypt, was the first to construct a calendar and, with subsequent developments, provided the basis of the calendar we use today. It is therefore appropriate to tell the story of its origin and evolution.

Life in Egypt depended entirely on a great river, the Nile, which flowed through an immense barren desert, devoid of rain. It had no tributaries and it stretched far farther south than the ancient Egyptians ever travelled. It flooded every year, bringing a rich soil deposit as well as the water needed for agriculture, making the Nile valley very fertile and allowing the early growth of a great civilization, which was confined to a very narrow strip of oasis, bounded by extensive deserts. Since invaders would have to cross these deserts, they played a protective role and Egypt was not subjected to the same change, invasion and strife that marked the Mesopotamian civilizations of Sumer and Babylon.

We now know, but thanks to the discoveries of the nineteenth century, that the source of the Nile is twofold. One is the great lakes of central Africa which overflow in the aftermath of heavy, regular, annual rains—the White Nile—and the other is the melting of the mountain snows of Ethiopia, which feed the Blue Nile. The White Nile carries vegetable detritus from the equatorial swamps and the Blue Nile carries ferruginous mud, containing iron compounds, from the soils of Ethiopia. The White Nile starts its 4000-mile course one month before the Blue Nile, and the two streams combine to reach the lower valley just before the summer solstice (mid-June on our present calendar). The inundation floods the whole of the arable land, bringing to the black Earth, burned dry by the Sun, the renewal of its soil and its watering. By late November to early December, the flood has subsided and the wet fertile soil is ready for the ploughshare and seed. Harvesting occurs in March-April, after which the land lies fallow and, without rain, becomes dry and devoid of all vegetation until the next annual flood in mid-June. These events define the three Egyptian seasons of inundation, agriculture and dryness.

Hence, unlike the other early civilizations, the Egyptian "seasons" were not determined by the times of the seasons, but by the Nile and, particularly, its flooding, which recurs annually and regularly. It so happens that, when the Egyptians were developing their calendar, the flooding of the Nile was heralded by the heliacal rising of the brightest star in the sky, Sirius (Sothis to the Egyptians), which occurred in mid-June as seen from Egypt. The heliacal rising occurs in the eastern sky just before dawn when the star reappears after having been invisible for about 70 days because of being in daylight. This remarkable event—the reappearance of the brightest star in the sky just before the most important event in Egyptian life, the flooding of the Nile—greatly strengthened the belief in astrology. Other striking astronomical events which recurred (but at that time not predictably), such as the conjunction of two planets, were believed to indicate that the human events of the previous conjunction would be repeated. The Egyptians also intertwined astronomical phenomena with their religion and, like many other ancient civilizations, assigned stars and constellations to different gods and goddesses. Sirius, the herald of flooding of the Nile, was the star of the goddess Isis, consort to the great god Osiris, who was represented by the constellation of Orion. Osiris was originally the god of fertility but was slain by his brother, Seth, who dismembered his body into 14 parts and distributed them throughout the land. This led Isis to embark on a lengthy search, in which she retrieved the parts and reassembled them, bringing Osiris back to life. This familiar ancient story of death and resurrection resulted in Osiris becoming the god of the dead, with the power of salvation or purgatory to each individual in the afterlife. His spirit was believed to dwell in the Pharaoh, thereby endowing him with immense authority as the god incarnate. However, it was believed that, if the Pharaoh died naturally of old age, the spirit of Osiris would also weaken and share the same fate. To avoid this, the spirit had to be released from the body when it was still whole and well, this being accomplished by a religious execution. Not surprisingly, with the passage of time, the Pharaohs adopted the ruse of appointing a substitute who knowingly accepted the spirit and enjoyed the full status and privileges of a king for a few days or weeks before being executed with full ceremonial rites.

Out of these beliefs emerged an explanation of the annual flooding of the Nile on the heliacal rising of Sirius, which was an imaginative and romantic combination of religion, mysticism and astronomy. The heliacal rising of Orion occurs before that of Sirius, so, star by star, Osiris was revealed by this most magnificent of constellations straddling the celestial equator. Then, on the later heliacal rising of Sirius, Isis was also revealed and, seeing her husband, she was immediately saddened and went into mourning for his death. Being a very loving and faithful wife, her sorrow was such as to cause the tears she shed to be so profuse that they inundated the valley of the Nile.

The Egyptians were the first to recognize that the naturally available time intervals of a day, month and year were not commensurable and they were therefore the first to appreciate the difficulty this posed for the construction of a calendar. They took the first major step in developing their own calendar by accurately determining the length of a year as 365.25 days by timing the occurrence of the equinoxes over a long period. They were therefore measuring a solar year, and since this determines the seasons, it is the best basis for a calendar and is the basis of the one we use today. To avoid any confusion, it should be stressed that the small difference between the solar year and the sidereal year, discussed above in connection with the zodiac and caused by the precession of the Earth's axis, has no effect on this story of the development of the calendar. The difference can be detected only by reference to the fixed stars, as was the case with the zodiac, and would ultimately have been revealed to the Egyptians because precession causes the heliacal rising of Sirius to occur later as time progresses. Today, some five millennia later, the rising is no longer in mid-June but in August and it can no longer be regarded as the herald of the flood.

The next important step taken by the Egyptians in constructing their calendar was the decision not to use lunar months, thereby avoiding the cumbersome route to be followed by other early civilizations such as

The magnificent constellation of Orion, followed by the brightest star in the sky, Sirius. To the ancient Egyptians, Orion represented the great god Osiris and Sirius represented his consort, the goddess Isis. In about 3000 BC the reappearance (heliacal rising) of Sirius just before dawn heralded the most important event in the Egyptian calendar—the flooding of the Nile. This was interpreted as being due to the tears of Isis, shed on seeing her beloved Osiris who had been murdered by his brother, Seth.

Babylon. They divided the year into 12 months of equal duration, 30 days, and added five intercalary days to make a total of 365 days and thereby harmonize the calendar with the seasons. The start of the year was taken to be 15 June, the official first day of the inundation, and the five intercalary days were devoted to festivals marked by the birthdays of principal deities. Some other days were identified by major historic or religious events and, according to the nature of those events, the days were marked as good or hostile and additional advice given; for example, for one date marking a previous battle the advice was to do nothing, another marking a peace agreement was propitious, and another marking the feast of a god was one in which everything seen would be fortunate. Since we are talking of a time about five millennia into the past, this must represent the first horoscope, as well as the first calendar.

Although the Egyptians knew the length of the solar year accurately as 365.25 days, they did not, at first, introduce an extra intercalary day every fourth year, which would have kept their calendar in step with seasons, the flooding of the Nile and the heliacal rising of Sirius (Sothis). Consequently, they had a sliding calendar through which the above events would move smoothly, completing a full cycle every 1460 years. This is a very long time, but the Egyptian civilization was of very long duration and the period of 1460 years was known as the Sothic cycle because it marked the interval needed for the heliacal rising of Sothis (Sirius) to recur on the first day of the first month of their sliding calendar. The Egyptians were therefore fully aware of the true solar year from the timing of the equinoxes, solstices and the reappearance of Sirius, so they effectively had a seasonal calendar (also called the Sothic calendar) allowing the prediction of the above events within their sliding calendar. Carrying two calendars had an advantage, in that the difference between the two told what year it was since the time when the two were previously synchronized (one day difference represented four years). We know from historic evidence that they were in step in AD 139 when the heliacal rising of Sirius occurred on the first day of the sliding calendar. Since this synchronization happens every Sothic cycle of 1460 years, it also occurred in 1321 BC, 2781 BC and 4241 BC; hence, one of these dates probably marks the date on which the Egyptians first set up their calendar. An even earlier date can be ruled out from archaeological and other evidence. Some historians favour the earliest of these dates and, if so, 4241 BC represents the first accurately dated year in history, but 2781 BC seems more likely.

The Egyptians broke the day into 24 parts, thereby creating the basis for today's 24 hours. They assigned 12 parts for daylight and 12 parts for night, and measured the former from the direction and length of the Sun's shadow and the latter from the positions of stellar constellations which they defined for the purpose. Since the duration of day and night varies through the year, this meant that an hour also varied from day to night and through the year. A constant hour was not established until the development of accurate mechanical chronometers some thousands of years later, the first of which were based on the pendulum. But credit for the very first known man-made clock lies with the Egyptians, who supplemented their solar and stellar time measurements, which could only be conducted when the sky was clear, with a water clock which could be used at any time. This consisted of a large vessel from which water leaked through a small aperture at a constant rate. The vessel was translucent, enabling the level of the water to be seen and measured against a graded scale on the outside.

The Egyptians did not abandon their sliding calendar in favour of the much better Sothic calendar, but maintained it as the official calendar, probably due to the opposition of priests to any change. In the aftermath of the conquests of Alexander the Great, there was a strong Greek presence in Egypt, mainly centred on the city of Alexandria, founded in 322 BC. Acting on advice by Aristarchus, a brilliant Greek philosopher and astronomer (of whom more later), the then King of Alexandria (Ptolemy III) proposed in 238 BC the adoption of a seasonal calendar of 12 months of 30 days, plus 5 intercalary days, plus an additional day every fourth year, like our own present-day calendar. However, opposition to this change was strong in Egypt and it was not implemented, the sliding calendar being maintained for establishing the dates of festivals and religious events, and the Sothic calendar for determining the seasons.

In the early days of its empire, Rome adopted a calendar essentially based on that of Babylon, nearly two millennia before. There were 12 months determined by the first appearance of the new Moon, and therefore of 29 or 30 days' duration, giving a year of 354 days. Since this rapidly got out of step with the seasons, it was corrected, as in Babylon, by the insertion of an additional month every three years or so. Like the Babylonians, the Romans started their year at the vernal equinox and named their months by number. Some of these still survive in our calendar today, where September, October, November and December are derived from the Latin words for seven, eight, nine and ten. Of course they do not correspond to those numbers in our calendar today because the first month of the old Roman calendar coincided approximately with our March.

The Roman calendar, based on lunar months, was very unsatisfactory and untidy, and could give different dates in different parts of the empire, because of the uncertainty in identifying the day of the new Moon. Accordingly, Julius Caesar decided to introduce a new calendar and, having been to his wars in Egypt and having noted the advantages of its calendar, he invited Sosigenes of Alexandria to advise on its construction. Sosigenes proposed the same calendar as that proposed by Ptolemy III nearly a hundred years before—12 months of 30 days, plus 5 intercalary days, plus an additional day every fourth year. He also proposed that the year should start with the five intercalary days or, in a leap year, six. These proposals were modified slightly in Rome; the five intercalary days were spread through the year with months being formed alternately of 30 and 31 days. Since this would have made 366 days, one day too many in a year, one month (February) was assigned one day less (29) but would have 30 in a leap year. (Retrospectively, it would have been tidier to remove the day from a month with 31 days.) Sosigenes had also proposed to start the year at the winter solstice and after the intercalary days the first day of the first month would begin. This was accepted but, in an oversight, no account was taken of the dispersal of the intercalary days throughout the year, with the result that the first day of the first month, which now became the first day of the new year, started five days after the winter solstice and placed the vernal equinox at 25 March. This Julian calendar was introduced in 46 BC, two years before Caesar's assassination, but a small modification was made by Augustus who became the first Roman emperor in 27 BC. The seventh month had been named July in honour of Julius, so it was natural that the next month be named August after Augustus. But July had 31 days and August only 30 and, since this may have been taken as a measure of their relative greatness, Augustus stole another day from February (giving it 28) and added it to August to make it 31 and therefore equal to July. In order to resume the 30-31 sequence, the subsequent month, September, was assigned 30, and so on. This explains the somewhat illogical distribution of days through the months of our present calendar.

The Julian calendar was used throughout Europe until AD 1582, when an adjustment was made by Pope Gregory XIII to create the Gregorian calendar. The solar year is not exactly 365.25 days, but is about 11 minutes shorter. This is not much, but in the 16 centuries since the initiation of the Julian calendar, it had built up to 14 days and it was resolved to correct for this and bring the calendar back into step with the seasons. To do this would require the removal of 14 days, but the Church, for reasons connected to the timing of the great Christian festival of Easter, which was dated by the Jewish lunar month calendar, wished the vernal equinox to be dated as 21 March and not the 25 March of the Julian calendar. Hence, the required adjustment became ten days and Pope Gregory decreed that, in the year AD 1582, ten days would be removed from the month of October and 4 October was followed by 15 October. This adjustment of the vernal equinox by four days also moved the winter solstice backwards in the calendar by the same amount. These four days, added to the five days caused by the error in not accounting for the removal of the intercalary days from the start of the Julian calendar, explains why, in our present calendar, the first of January is nine days after the winter solstice.

In order to avoid any future adjustments to the calendar caused by the solar year being slightly less that 365.25 days, a fine tuning of the leap-year rule was introduced in which century years not divisible by 400

would not have an extra day added, even though they meet the normal four-year rule. Hence, 1700, 1800 and 1900 were not leap years but 2000 will be. The Gregorian calendar was immediately adopted by all Roman Catholic European countries, but in those early post-reformation years, the Protestant states of northern Europe and the orthodox Christian countries of eastern Europe did not follow suit. Britain waited until 1752, when 11 days had to be deleted from the Julian calendar, and Russia did not change until the twentieth century, when 13 days had to be deleted. This explains why the Bolshevik revolution of 1917 is called the "October" revolution; it fell in that month on the 25th in the Julian calendar although it now falls on 7 November in the Gregorian calendar.

Although two developments in ancient astronomy, the zodiac (see Colour Plate 1) and the calendar, have been brought up-to-date, this narrative is only approaching the first millennium before Christ. During that time, as has been stressed, the driving force for studying the sky came from astrology, with astronomy playing a very subsidiary role but benefiting from the fall-out. From then on, this situation was to change, albeit slowly, with astronomy playing a greater and greater role relative to astrology and, although the coupling of these two completely incompatible activities weakened with time, it was not finally broken until the closing stages of the Renaissance. But, as far as this story goes, the connection is being severed now because it is relating the human attempt to understand the Universe we live in, something to which astrology, apart from the ancient stimulation of astronomical observations, has made no contribution. No further reference will be made to it, and the author defers to Shakespeare:

This excellent foppery of the world, that, when we are sick in fortuneD often the surfeit of our own behaviourDwe make guilty of our disasters the Sun, Moon and the stars, as if we were villains by necessity, fools by heavenly compulsion, knaves, thieves, and treachers by spherical predominance, drunkards, liars, and adulterers by an enforced obedience of planetary influence; and all that we are evil in, by a divine thrusting on. King Lear

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