From plotting the stars to pulsars and black holes
INTRODUCTION: REACHING FOR THE STARS
CHAPTER 1: THE FIRST ASTRONOMERS
From seeing to observing • Space and time • From prehistory to history • Astronomy and astrology • Counting the days • Place and navigation • And so to science
CHAPTER 2: THE GREAT SCHEME OF THINGS
Life, the universe, and everything • Space for thought • Our place in space • The centre of all things • The Copernican revolution • The threshold of the modern age
CHAPTER 3: TOOLS OF THE TRADE
Line of sight • Modelling the celestial globe • In the service of Allah • A new way of looking • Lines in the dark and the light • Darkness visible • Going there
CHAPTER 4: EARTH, MOON AND SUN
The Earth in space • Our companion, the Moon • The Sun
CHAPTER 5: THE SOLAR SYSTEM REVEALED
Exploring the planets • How many planets? • Visitors with a bad reputation
CHAPTER 6: MAPPING THE STARS
Tracking stars • Fuzzy stars • Galaxies disputed
CHAPTER 7: REMAKING THE UNIVERSE
Celestial mechanics • The bigger picture • Beginning and being • The size of the universe • The end of everything • Other worlds, other universes
CHAPTER 8: THE FINAL FRONTIER
Asking ‘Is there anybody there?’ • ‘Where is everybody?’ • Somewhere to live
INDEX
PICTURE CREDITS
The history of astronomy is a history of receding horizons.’
Edwin Hubble,
astrophysicist, 1936
Just 500 years ago, most people in the Western world believed that the Earth sat at the centre of the universe with everything else revolving around it. They thought humans were created to be masters of this universe and that the heavens were unchanging for all eternity.
We now know that we are an evolved and evolving species, one among millions of plant and animal species living on a planet orbiting a fairly small star in an outpost of an unremarkable galaxy, somewhere in an unknowably vast universe. We know that there are countless billions of other stars and probably billions of other planets and that the history of the universe stretches billions of years into the past. Paradoxically, with greater knowledge has come greater recognition of the limits of our knowledge. We can account for and explain only a tiny proportion of all there is in the universe. We don’t even know whether there is just one universe, or many universes.
The story of astronomy is one of emerging knowledge and emerging ignorance. It tells how we have come to know so much about the universe and our home in it, but shows that there is much more still to discover. It is a story that has barely begun, as we stand on the brink of space exploration.
Our early ancestors attempted to explain what they saw in the heavens, often using mythology alongside their careful observations and measurements. With settled civilizations came written records and mathematics, allowing more detailed observations to be made and maintained over many years. Then, around 2,500 years ago, the Ancient Greeks started to explain the cosmos without recourse to mythology or the supernatural and so began the science of astronomy.
But the separation of astronomy from the supernatural did not come at a stroke. Astronomy only gradually moved from being the province of priests to the pursuit of scientists. For centuries, the observations and calculations of astronomers were directed towards religious and superstitious ends. They were used to fix the times for prayers and religious festivals, to predict conditions and events on Earth in the political or personal spheres, and to seek propitious times to implement plans. Astrology and astronomy remained inseparable for millennia. Even in the 16th and 17th centuries, respectable astronomers often had a foot in the astrology camp. While they didn’t all believe that there was any validity in astrology, they found it could be lucrative nonetheless.
And then, over a period of only one hundred years, starting in 1543, astronomy and our astronomical knowledge changed beyond measure. First, two giant supernovas (exploding stars) appeared within 32 years of each other (in 1572 and 1604); none has been seen since. They demonstrated conclusively that the cosmos is not fixed and unchanging for all eternity. The old dogma had to shift to accommodate this development. Second, the invention of the telescope came just four years after the second supernova. It revealed there is far more in the night sky than we can see with our eyes alone. These events provided the vital evidence needed for a new theory of the universe to gain credence, one in which the sky is not fixed for all eternity and in which the Earth is not central. With the telescope to extend astronomers’ vision, the path was clear for the development of modern astronomy.
‘Astronomy compels the soul to look upwards and leads up from this world to another.’
Plato,
Greek philosopher,
4th-5th century BC
Imagine living in the Stone Age and looking up at the night sky. What would you notice on a clear night? First, the Moon: a bright, shining body that changes shape over the course of around 29 days from new crescent to full circle and back again, and which moves across the sky during the night. Next, there are a lot of bright pinpricks of light. Without light pollution, far, far more stars would be visible than we can see today. You would also see a fuzzy band of dim light that stretches across the sky – the Milky Way.
There would not be much to do at night in the Palaeolithic or Neolithic eras, so you might take to observing these objects in the sky with some care, night after night. You might then notice that most of the points of light twinkle, while a few cast a steady light. Those that twinkle move together, rotating during the course of a night around a set point. That point is not directly overhead unless you are standing at the North or South pole. You might notice that the points of light nearest the horizon rise or set over the course of the night and disappear for months of the year, reappearing predictably the following year.
You would probably notice that only a few of these points of light move along their own paths relative to the majority. Most twinkle and stay in fixed positions in relation to one another; they are the stars, originally known as the ‘fixed stars’. Those that move independently and shine steadily are the planets. Long ago they were called ‘wandering stars’ as they seemed to wander among the fixed stars – indeed, the word ‘planet’ comes from the Greek planētēs, meaning ‘wanderer’. As a Stone Age observer, you would notice how they differ from the twinkling fixed stars, but you would not be able to tell that they are fundamentally different bodies.
You might sometimes see a bright light that streams briefly across the sky and disappears – a shooting star or meteor. And occasionally you might notice, if you had been observing carefully in the past, a new star that moves across the sky slowly, night by night, before eventually disappearing. With its dim ‘tail’ of light trailing behind (or, actually, sometimes in front of it), this is a comet – but it’s a rare occurrence.
In the daytime, the sky is dominated by just one body. You would see the Sun rise in the sky in the summer. and follow a predictable path across the sky before setting at a point opposite its rising. Unless you were at the equator, you would notice that the day is longer in the summer than the winter, and the Sun travels higher in the sky summer.
It would not take long for a Stone Age observer to notice that the appearance and disappearance of some of the fixed stars matches the seasons. In the northern hemisphere, the appearance of the group of stars now known as the Orion constellation heralds the start of winter. Its disappearance is a sign that warmer weather and more plentiful food supplies are on the way. Just as the path of the Sun across the sky over the course of a day could be used to measure time, so the phases of the Moon could be used to track a longer period – a lunar month. The rising and setting positions of the Sun and some of the fixed stars could be used to track the course of the year. The first human uses of astronomical observation were almost certainly to keep track of time.
We think of the night sky as pretty much the same every night, yet a Palaeolithic star-gazer would not see quite the same stars as we do. The northern Pole Star would not be Polaris; instead, the brighter star, Vega, would have been close to the North celestial pole (see page 12). However, as the Pole Star changes, following a cycle of about 26,000 years (see box, page 18), some Palaeolithic observers would have seen Polaris as the Pole Star last time round. Some constellations now seen only in the southern hemisphere would have been visible in the north during some months of the year and vice versa. And as all the stars are constantly moving, some would be in very slightly different places in relation to one another. This is called the ‘proper motion’ of the stars (see page 180), and results from each star moving on its own trajectory independent of all but those closest to it in space. But as these changes happen over thousands of years, much would appear the same to observers on Earth as it does now.
Our earliest ancestors tracked the movement of the Sun, Moon, planets and stars and learned how to predict and interpret them, using their knowledge to plant crops at appropriate times and to anticipate events such as annual floods or rains. But they probably also endowed the heavenly bodies they watched with supernatural significance.
The oldest ‘calendars’ are vast archaeological sites that aligned posts or megaliths (giant stones) with the rising of the Sun or Moon on significant dates, such as the summer or winter solstice. The earliest site so far discovered is Warren Field near Crathes Castle in Aberdeenshire, Scotland, found in 2004. It comprises 12 pits arranged in an arc. At least one pit held a post at some time. It seems likely that the monument performed some sort of calendric function. Archaeologists propose that the pits were used to track the lunar cycle, keeping a record of the lunar months. One of the pits (number 6) also aligns with the position of sunrise at the winter solstice 10,000 years ago.
A piece of carved mammoth ivory discovered in a collapsed cave complex in Geißenklösterle, Germany, is thought to be the earliest depiction of an asterism – a pattern of stars. Just 14cm (5½in) long, the carving is 32,000–35,000 years old. One side shows a human or part-human figure, taken to be Orion. The other side shows a series of pits and notches. It has been suggested that the pattern acts as a calendar which could be used to time the conception of a baby. If conception coincided with the arrival of Orion in the sky over Palaeolithic Germany, the baby would be born at a time when mother and infant would benefit from the food and warmth of summer for three months before winter set in again.
One suggestion is that the pits at Warren Field might have been used to tally lunar periods over the course of a year. The priest-astronomers marked each lunar month as it passed, perhaps dropping a stone into a pit or moving a post to the next pit. Reaching the final pit meant the end of the year, and they would start again with the first pit. The midwinter solstice could be used to recalibrate. Each time the winter solstice fell at (say) a full moon, they added an extra month to the ending year. This would happen once over three years. That the winter solstice was marked by the middle pit suggests that it came in the middle of the year for the people who used it, meaning their year started in late June (at the summer solstice).
Time is naturally divided astronomically by the Earth’s orbit around the Sun (a year), the Earth’s rotation (a day) and the phases of the Moon. A lunar month (a full cycle from one new or full moon to the next) is approximately 29½ days long. A year is 365¼ days. Inconveniently, a year is 12.37 lunar months long. For early societies, a lunar month was a useful and countable period of time, one that could be easily observed and checked just by looking up at the night sky. But if you use twelve lunar months as the basis of your year, the calendar will drift out of sync quite quickly. It will be a month out after only three years, and six months out after 18 years. To avoid this, an extra (intercalary) month has to be added every few years.
The earliest structures built to act as calendars, such as Warren Field, seem to be designed to help calibrate the solar year and lunar months. This can be done by picking a day – the winter solstice is the most convenient as it has the longest night – and noting the moon phase on that day. When the same phase next occurs at the solstice, it’s time to add an intercalary month to keep lunar and solar calendars in sync. So if, say, we began a calendar with the winter solstice starting on the day of a full moon in Year 0, a full year – 12.37 lunar months later – the winter solstice would fall about a third (precisely 0.37) of the way through the 13th lunar cycle. The following year (Year 2), it would fall 0.74 of the way through the 13th lunar cycle. The next year (Year 3), it would be at the full moon again, but a total of 37 lunar cycles would have passed. If you were naming the months January to December, you would have got to the end of the fourth January by the time the third year had passed. To avoid starting the new year with February, you would need to add an extra month to the year just ending.
The site seems to have been modified, apparently to adapt to shifting astronomical positions over a period of 6,000 years. The modifications suggest it was used continuously during that time.
As far as we know, Warren Field was a unique structure – it is 5,000 years older than any other known calendar-monument. But it could just be that others haven’t yet been found. After all, Warren Field was only discovered in 2004 and its significance remained obscure until 2013.
The next oldest calendric structure is the Goseck circle, in Germany, constructed around 4,900 years ago – so only half the age of Warren Field. There are many more circular and elliptical structures in Central Europe, ranging through Poland, Germany, Austria, Slovakia, Hungary and the Czech Republic. All were constructed over a period of about 200 years, some 5,000 years ago. Like most of the other later sites, Goseck allows the winter solstice to be determined from the alignment of sunrise and sunset. The site comprised four concentric circles, made up of a central mound, a surrounding ditch, and two wooden palisades. Gates in the palisades faced southeast, southwest and north. At the winter solstice, the rising sun aligned with the southeast gate and the setting sun aligned with the southwest gate.
Standing on Earth and gazing out to space, it looks as though the Sun goes round the Earth against a background of stars. Astronomers call the path the Sun follows over the course of a year the ‘ecliptic’. If we project the Earth’s equator onto the sky, calling it the celestial equator, the Sun will seem to be above the celestial equator for half the year and below it for the other half of the year. There is a difference between the celestial equator and the ecliptic because the Earth tilts on its axis. The tilt is 23.5 degrees, and this tilt gives us the seasons, with days of different lengths.
For early societies, important days in the natural (solar) year were the summer and winter solstices and the spring (vernal) and autumn (autumnal) equinoxes. The solstices fall in December and June, when the Sun is at its furthest point from the celestial equator. The longest day and night occur at the solstices. The equinoxes fall in March and September, when the ecliptic crosses the celestial equator. Day and night are of equal length at these two points (the word equinox means ‘equal nights’).
Stonehenge is a large stone circle in Wiltshire, England, comprising a series of upright stones that originally supported horizontal stones (lintels). Some of the lintels remain in place. The uprights and lintels are made of bluestone and sandstone, the latter mined locally but the former hewn from hills in Wales and transported 250km (155 miles) to the site by land and/or water. The largest stone, the heel stone, weighs 30 tons. There is also an altar stone made of red sandstone. It is the most sophisticated stone circle in the world.
The first monument at Stonehenge was a circular earthwork enclosure – a ditch containing a ring of 56 timber or stone posts. This was built around 3000BC. It was used as a cemetery; cremations were carried out there for several centuries. The stone monument was built around 2500BC. Stonehenge is part of a complex of sites that were used continuously for around 2,000 years.
Prehistoric astronomers left no user manuals for their calendric monuments; their uses had to be rediscovered by archaeologists with knowledge of astronomy (archaeoastronomers).
The notion that ancient monuments might be lined up with astronomical landmarks (or skymarks) first surfaced in 1909, when the eminent British astronomer Norman Lockyer (1836–1920) proposed that Stonehenge had been built as an ancient observatory. Lockyer, famous for discovering helium (see page 120) and founding the journal Nature, noticed while on holiday in Greece that some ancient temples had apparently been rebuilt. Closer inspection revealed they had also been slightly realigned. Rebuilding ancient temples is a lot of work, especially for a pre-industrial culture, and would not have been undertaken lightly. Lockyer realized that the reason must have been to align them with the Sun, stars or planets, making a correction for the shifting appearance of the sky over centuries. Turning his attention to Egypt, he found buildings there that were also aligned with celestial markers. At last, he looked at Stonehenge and found it aligned to face sunrise at the summer solstice. Although many of Lockyer’s conjectures are now rejected (that Stonehenge was built by immigrants from the Far East, for example), the alignment of Stonehenge is uncontested and that it might have been used as an observatory of some kind remains a possibility.
Axial precession, also called precession of the equinoxes, is the gradual movement of the Earth’s axis, which results in the slow shift of the apparent positions of the stars.
The Earth’s axis is at a tilt, a feature which produces the seasons as the Earth orbits the Sun. Over time, the axis moves in a circular path (see image, below). As the axis moves, the apparent position of the stars slowly changes, as we are looking at them from a slightly different angle. The positions of the celestial poles change, too. Polaris is currently the Pole Star in the northern hemisphere, and will be in its best possible Pole Star position around AD2100. Around AD3000, Gamma Cephei will take over the role. Polaris will take its turn again around AD27,800.
A much smaller circle, which contains the earliest known depiction of the cosmos, was found near the Goseck circle. It comes from the boundary between the prehistoric and historic eras in Europe. The Nebra sky disc is a bronze disc measuring 30cm (12in) across, dating from around 1600BC. It shows either the sun or the full moon, a crescent moon, and a collection of stars that represent the Pleiades as they would have looked 3,600 years ago. The crescent is not a new moon, but a moon four or five days old. The astronomical features are in gold leaf, and the greenish background colour was achieved by applying rotten egg to bronze. It is a sophisticated artefact that someone has taken care and time to make. The disc was dug up by archaeological looters and passed around the German black market in antiquities for several years before it was seized in a police raid in 2002.
Not just individual monuments line up with celestial phenomena. In 2016, a 15-year-old Canadian schoolboy, William Gadoury, combined his interests in Mayan culture and astronomy and made an astonishing discovery – the ruins of a vast Mayan city hidden in the jungle.
The Mayans occupied an area from southeast Mexico to Honduras and El Salvador, building cities from c.750BC. Gadoury noticed that if he projected 22 Mayan constellations on to a map, the stars in them corresponded exactly with the location of 117 Mayan settlements – a correspondence never noted before. He turned to a 23rd constellation and found that one of the three stars had no known corresponding city. Convinced there must be one, he worked out where it should be and contacted the Canadian Space Agency for help. With the use of satellite imagery through NASA and JAXA, the US and Japanese space agencies, the outlines of buildings buried in the rainforest emerged.
The use of the disc was initially obscure, but was explained in 2006 by a team of German researchers led by Harald Meller. He examined Babylonian astrological texts written about 1,000 years after the disc was made. These explain when to add an intercalary month by checking the position of the four-day-old moon against the Pleiades, just as the disc shows.
We can deduce what these ancient sites around the world were used for, but not the purpose to which the information gained was put. We can assume the function was for worship, divination, time-keeping, calendar-making for social or agricultural purposes, or sorcery (or a combination of all these), but the way in which the information was used is lost to us now. Perhaps it showed when to plant crops or move herds, or revealed propitious dates for personal rites of passage or when sacrifices or prayers should be offered.
With the beginning of written records – the start of the historical period – we start to see what our ancestors knew about the sky and how they used that information.
The Sumerians developed the first writing system in the world, around 3,200–3,500BC. Called cuneiform, it was made by pressing a wedge-shaped stylus into a clay tablet. Many clay tablets have survived, and some Babylonian astronomy can be pieced together by studying them.
The oldest astronomical text is tablet 63 of Enûma Anu Enlil, a large collection of omens related to astronomical investigations from the 2nd millennium BC when the Babylonians occupied Mesopotamia. Tablet 63 records the first and last risings of the planet Venus over a period of 21 years, and shows that the Babylonians were aware of the periodicity of planetary movements. The text was probably compiled in the Kassite period (1595–1157BC), but based on an earlier version or prototype. It includes names given to the stars by the Sumerians, who occupied the land before the Babylonians.
Because each planet is on its own orbital course around the Sun at the same time as the Earth is orbiting the Sun, its position varies in relation to us. Sometimes a planet is obscured by the Sun, and sometimes it is directly between Earth and the Sun and therefore lost in the Sun’s glare. In between, it is seen to rise (appear above the horizon) either in the early morning (if it has just passed in front of the Sun) or set in the evening (if it has just passed behind the Sun).
When planet-rise is visible, the planet is not seen to set but just fades into the daytime sky, and when planet-set is visible the planet has risen during the daytime when it is invisible. The time at which the planet rises or sets moves further away from sunrise or sunset until it next disappears.
The planets and Moon follow the approximate path of the Sun along the ecliptic (see page 15). Just as sunrise and sunset happen at different points along the horizon, so do the rising and setting points for each planet. The interval from the start of a planetary cycle to its end (its return to the same point) is called the synodic period. For Mercury, that is 116 days and for Venus it is 584 days. This is not the same as the length of the planet’s orbit, which is called its sidereal period.
The exact date of Enûma Anu Enlil is not known, but it’s possible that the oldest astronomical record is not represented by Babylonian clay tablets at all but by an oracle ‘bone’ – a piece of carved turtle shell – from China. It records an eclipse of the Sun that took place on 5 June 1302BC, in the middle of the period during which Enûma Anu Enlil could have been created. It mentions ‘three flames at the Sun, and big stars were seen’. Oracle bones were used for asking questions of the gods, so this was recorded in a context of superstition and religious belief rather than scientific interest.
The most famous and significant Babylonian text is known as MUL.APIN after its opening logograms, meaning Plough Star. (By convention, Sumerian logograms are shown in capital letters, separated by a full stop.) Preserved on tablets made in 686BC, though probably compiled around 1000BC, it is an expansion on and improvement of the earlier ‘Three Stars Each’ catalogues (see page 154) which simply list the stars, and is clearly based on better observations. It’s apparent from the tablets that Babylonian astronomers had a theory of planetary motion, though there is not enough information to piece it together in detail.
What is clear from the presentation of information about Venus in the context of omens is that the Babylonians used astronomy for an astrological purpose. Those responsible for compiling and using the tablets were priests. There was an increase in astronomical activity during the reign of Nabonassar, or Nabû-nāsir (747–734BC), with records of ominous events and detection of the 18-year cycle of lunar eclipses. Even though phenomena were being recorded and predicted for astrological purposes, the detail and accuracy with which they were observed led the Greek-Egyptian astronomer Ptolemy, 800 years later, to consider this to be the period in which astronomy started, with the first usable data. Later Babylonian tablets (350–50BC) show astronomers occasionally using geometrical methods to calculate the position of Jupiter, but most of their calculations, and all the earlier ones, were arithmetical and based on extrapolating from previous observations.
The arithmetical method of calculating the positions of heavenly bodies works by collecting a lot of observational data, identifying patterns or averages and extrapolating into the future to make a prediction. For example, if a comet is observed at intervals of 50 years, an arithmetic prediction that it will appear in another 50 years can be made without needing to know anything about the nature of the comet’s orbit.
The geometric method requires the astronomer to have a theory about the spatial relationships between celestial bodies. The prediction is derived from geometrical calculations based on those relationships. For example, we now know the orbital periods of the planets and their relation to Earth, so we can calculate their positions relative to Earth at future dates.
The astronomical knowledge amassed by the Babylonians formed the basis of Indian, Greek and possibly Chinese astronomy, the latter by way of India.
Although it’s known that the Indus Valley civilization used astronomy to make calendars in the 3rd millennium BC, the people of the Indus Valley had no written language so we don’t know the extent of their astronomical knowledge. The oldest surviving Indian astronomical text is the Vedanga Jyotisha. It explains how to track the movements of the Sun and Moon, which was important in organizing rituals. It survives in a copy from the 1st or 2nd century BC, but might have been composed around 700BC or later. Its origins lie much earlier: it describes the winter solstice of a date that probably lies between 1150BC and 1400BC. Vedanga Jyotisha resembles Babylonian work, suggesting that the Indian astronomers were familiar with Babylonian texts or methods, either at the time of its original composition or when the text was later revised.
China has always been difficult to reach from the west, with the Himalayas and the Gobi Desert forming natural geological barriers. Further, isolationist policies cut China off from other centres of developing civilization. As a consequence, Chinese astronomy developed largely independently. No external influence can be traced before the Three Kingdoms era (AD220–265) when translated Indian works on astronomy arrived in China along with Buddhism.
The Chinese developed their own accounts of the constellations and stars, with enduring star names that have been preserved on oracle bones from the middle of the Shang dynasty (c.1600–c.1046BC). The earliest known observatory in China, a carved platform 60m (197ft) across, was excavated in Shanxi province in 2005. It was used to locate the rising and setting of the Sun at different times of year, and dates from the Longshan period (2300–1900BC). Eclipses were recorded in China from around 750BC, providing an invaluable record for later astronomers. Detailed astronomical observations began in China during the Warring States period (usually given as 475–221BC), probably around 200BC.
The two principal practical uses of astronomy in the early historical period were time-keeping/calendar-making and location/navigation. But astronomy was also at the centre of religious and superstitious beliefs. Indeed, calendar-making and location often served these ends, too.
Today, scientists draw a very clear distinction between astronomy and astrology. Astronomy is a science, concerned with the movement, nature and history of bodies such as planets, stars, comets and asteroids. Astrology is superstition, a means of (supposedly) predicting events, interpreting the moods of the gods, and drawing correspondences between events in the heavens and events on Earth and in human lives.
It’s easy to see how the idea of a link between earthly and celestial occurrences could come about. A culture observes that when a certain constellation rises above the horizon, the weather changes, and perhaps seeds grow. Whereas today we account for both the change in season and the appearance of the constellation by reference to the Earth’s progress on its orbit around the Sun, a naïve observer could easily think the appearance of the constellation was responsible for the growth of seeds. It’s a small step from there to worshipping the constellation, or beseeching it to intervene if seeds do not grow.
The most potent events in astrological terms were those that could not be as readily predicted as the rise of a constellation or the position of a planet. The appearance of a comet, a shooting star or an eclipse was taken to signal extraordinary events either already happening or about to happen. It could herald a great event that would be celebrated or a catastrophe to be feared.
The earliest known system of astrology was Babylonian, developed around 1800BC. It might have been based on an earlier Sumerian system, but there is insufficient evidence to be sure. Babylonian astrology was of a type now known as ‘mundane’ astrology – it dealt with the fates of nations, cities, states and cultural leaders rather than with individuals. Modern astrology is more often associated with the personal – individual horoscopes and analyses of personality. The tablet-text Enûma Anu Enlil consists of 7,000 celestial omens recorded on 70 tablets.
Babylonian astrology linked gods with the planets and some of the fixed stars, and took the predicted or observed consequences of a planet’s behaviour as an indication of the corresponding god’s mood. It did not, though, link this directly with previous human action. Humans might try to sway or appease a god/planet in the case of an ill omen, but it was not assumed that the presaged event was intended as a punishment for humans. The nature of celestial omens often rested on previous events. So, if a new moon on a day of torrential rain was followed by a good event – an abundant harvest, perhaps – that pairing of new moon and rain would be considered auspicious next time it occurred.
Although rudimentary astrology started early in Babylon, it was only after Nabonassar became king in 747BC