The Story of Astronomy: From plotting the stars to pulsars and black holes

Reaching For The Stars

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 night sky reveals the stars that have captivated the human imagination for millennia.

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.

From superstition to science

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.

The great divide

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.

The constellation Taurus, from a German astronomical globe of the 1530s.

The Moon has shone above the Earth with the reflected light of the Sun for four-and-a-half billion years.

CHAPTER 1

The First Astronomers

‘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.

From seeing to observing

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.

Time lapse photography shows how the stars revolve around the celestial pole over the course of a night.

Except at the equator, the Sun rises higher in the sky during summer than during winter.

Space and time

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.

THE CHANGING SKY

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.

The celestial poles are found by drawing an imaginary line through the Earth from the North to the South pole and extending it into space.

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 constellation Orion is visible in the northern hemisphere in winter and in the southern hemisphere during summer.

Day to day

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.

BABIES BY ORION

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).

The phases of the Moon over a full lunar month.

LUNAR AND SOLAR CALENDARS

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.

SOLSTICES AND EQUINOXES

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’).

The arrangement of pits at Warren Field. Around 8,000BC, sunrise at the midwinter solstice would have been at the pass between the two central hills.

The Goseck circle in Germany. The entrance points at bottom left and right show the direction of sunrise and sunset at the winter solstice, converging at the circle’s midpoint.

STICKS AND STONE: STONEHENGE

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.

Alignments rediscovered

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