Accurate calendars would, of course, have been useful for many purposes such as choosing when best to plant crops, but it is hard to believe that this was the Nebra Sky Disc’s only purpose. Creation myths from around the world offer wildly varied accounts of the origins of the sun, moon and stars and the significance of their behaviour. The extraordinary imaginative energy they display plainly arose from deep concerns about our place in the universe and the meaning of life and death. Such concerns must surely also have influenced the designer of the Disc. It has been suggested that the curved piece of gold at the bottom of the Disc represents a boat – perhaps one that safely carries the sun across the ocean after it has set. It might equally refer to the passage of the soul to the afterlife. We cannot help sensing that this extraordinary object, like the many prehistoric structures that are aligned with the heavens, embodies profound, if mysterious, spiritual beliefs.
Until very recent times the heavens shaped the patterns of everyday life. The farmer judged when to sow his crops by looking at the night sky, and the sun and stars told him the time of day, long before the first mechanical clocks were invented. One of the western portals of the great thirteenth-century cathedral of Amiens in northern France is decorated with the signs of the zodiac, each one accompanied by a depiction of the activities appropriate to the month with which it was associated – such as sowing, reaping, cutting hay and treading grapes. Similar motifs appear on many other medieval buildings. But people did not rely on the heavens only to plan their communal activities; they also thought that the sun, moon and stars could foretell what lay in store for them as individuals or nations. This belief was – and is – so widespread as to qualify as a cultural universal.
The skies were, of course, especially important for sailors. The moon enabled them to predict the height of the tide; ‘full’ and ‘new’ bring the ‘spring’ tides, which have the widest range and produce the strongest currents, while the ‘half’ moon signals the ‘neaps’, with the narrowest and weakest. By day the sun, rising in the east and setting in the west, told mariners roughly which way they were steering, as did Polaris – much more simply – by night. For navigators in the northern hemisphere, the height of Polaris was the crucial measure of latitude – the only one, in fact, until astronomers were able to produce accurate tables of the sun’s varying declination at the end of the fifteenth century.
Fig 3: Diagram illustrating the equivalence of Polaris altitude and latitude.
As this diagram makes clear, the height of Polaris above the northern horizon is equivalent to the observer’s latitude. Thus if Polaris is vertically overhead, you must be at the geographical North Pole, while if it appears right on the horizon you must be on the equator. In this diagram, its height above the horizon is 50 degrees and it follows that you must be 50 degrees north of the equator – in other words your latitude is 50 degrees North.fn2
So vital was Polaris to the seafarer that the English simply called it ‘the Star’, and Shakespeare knew enough to press it into service in Sonnet 116:
Let me not to the marriage of true minds
Admit impediments. Love is not love
Which alters when it alteration finds,
Or bends with the remover to remove:
O, no! it is an ever-fixèd mark,
That looks on tempests and is never shaken;
It is the star to every wandering bark,
Whose worth’s unknown, although his height be taken … fn3
In medieval Latin, Polaris was Stella Maris – ‘star of the sea’ – a term that was applied also to the Virgin Mary, whose sky-blue cloak is emblazoned with a star in many early European paintings. The theologian Alexander Neckham (1157–1217) likened Mary to the Pole Star standing at ‘the fixed hinge of the turning sky’ by which the sailor at night directs his course. Polaris must have seemed a perfect symbol of the Mother of God, the immaculate spiritual guide and intercessor. The name ‘Stella Maris’ was sometimes also applied to the ship’s steering compass on which the thirty-two ‘points’ are still often marked in the form of a star.7 Even today ‘Stella Maris’ is a common name for fishing boats and ‘true north’ – solidly reliable, unlike its variable magnetic cousin – was marked on old charts with a star. (True north marks the direction of the geographical north pole, which is fixed,fn4 whereas the magnetic north pole – like its southern counterpart – wanders and is at present several hundred miles distant from it.)
*
Before the end of the thirteenth century the Venetian explorer Marco Polo recorded that he had measured the altitude of Polaris on the coast of India, though how he did so is unclear. The earliest references to the measurement of the height of Polaris by Portuguese navigators date from the mid-fifteenth century, but it seems unlikely that they were using purpose-built instruments to make their observations.8 In theory they could have used the astronomer’s astrolabe, an elaborate device that permitted the observer to find the time of day, as well as solving other astronomical problems. It had by then achieved a high level of sophistication but it is doubtful that many ordinary sailors would have known how to operate one, and such a complicated and costly instrument would not have been required merely for the purpose of measuring the altitudes of heavenly bodies.
A radically simplified version, known as the mariner’s astrolabe, was however widely adopted for use at sea during the sixteenth century,9 by which time solar declination tables were available. It was a metal disc or hollow circle with a scale of degrees engraved on its circumference. While it was suspended from its ring, the navigator would adjust the ‘alidade’ (a revolving bar with peep-hole sights) so that it was aligned either with the sun or with Polaris and its height could then be read off from the scale. The motion of the ship, however, and the effects of the wind limited the usefulness of any instrument that had to be freely suspended and it would have been much easier to obtain accurate readings from it on dry land.
The Portuguese poet Luís Vaz de Camões (c.1524–80), who sailed out to India in 1553, witnessed the use of an astrolabe at first hand – though it is not clear which kind – after his ship anchored off the west African coast. In his great epic The Lusiads (first published in 1572) he describes the scene:
Like clouds, the mountains we spied
Began to reveal themselves;
The heavy anchors were readied;
Now arrived, we took in the sails.
And so that we would better know
Where we were in these remote parts,
Using that new instrument, the astrolabe,
An invention of subtle skill and wisdom,
We then landed on an open shore
Where the crew scattered, wishing
To see strange things in the land
Where no other people had trod.
But I, with the pilots, on the sandy beach
To find out where I was,
Remained to take the height of the sun
And measure the painted universe.
We reckoned that we had already passed