The Sky Disc of Nebra – An Astronomical Clock

In early 2006 experts from the research group around the Sky Disc of Nebra (1600 B.C.) presented a new and far-reaching discovery. Astronomer Rahlf Hansen of  the Planetarium Hamburg decoded a calculation rule behind the celestial depiction on the Sky Disc which allowed the disc's Bronze Age users to harmonise the 365-day solar year with the 354-day lunar year. The earliest known record of this rule is found in cuneiform texts from Babylon (7th to 6th century B.C.).

Soon after the recovery of the Sky Disc, it became clear that its final form was the result of several phases of fabrication. Analyses of the working of the materials and of the gold on the disc indicated that the various phases must have been carried out by craftsmen of different periods. Some time after the original construction of the disc, two stars were removed and one displaced to make space for the application of arcs representing the horizon. Still later, the ship was added to the disc. Either simultaneously with or subsequently to this, holes were punched in its rim. The horizon arc that is now missing was presumably removed when the Sky Disc was buried.  

The first phase of its fabrication is decisive in our achieving an understanding the disc. Dr. Wolfhard Schlosser made early mention of the unusual sobriety of its design. For a prehistoric image, the Sky Disc is idiosyncratic, indeed almost modern. In this it differs fundamentally from illustrations of the heavens from antiquity, which are found exclusively in the high cultures. Accordingly, the Sky Disc did not have a narrative function, but in all probability served as a visual reference. Extraordinary value must have been placed not only on the precious metals it was made of, but on the knowledge that was encoded in its design. The information previously unlocked from the image – about 9 March and 17 October, the dates on which the Pleiades disappeared from the sky – by itself seemed too sparse to support the hypothesis of the disc's function as a memory tool.  

The astronomical references worked out in 2006 reveal, in the first phase of the disc's fabrication, a rule through which to reconcile the lunar and solar years. Research findings by Rahlf Hansen from the Planetarium Hamburg explain in detail how it works:

In our era of organisers, GPS and radio-controlled clocks, it is hard to imagine the difficulty of regulating the calendar in earlier ages. To impose order on the passage of time, prehistoric peoples looked to the rhythms of the heavens. The sun determined the days and years, while the moon marked the weeks and months. Agriculture depended on a solar calendar that dictated the seasons. The slender crescent of the new moon signalled the new month, as it still does in the Islamic calendar. But how was the lunar year to be synchronised with the solar year, which was eleven days longer? This problem was solved by periodically adding an intercalary or "leap" month.

Just such a rule is known from the Babylonian cuneiform text Mul-Apin. When in the spring month, the first month of the year, the thin crescent of the new moon appeared with the Seven Sisters, or Pleiades, this indicated a standard year. When in this month the moon did not appear with the Pleiades until the third day, as a wider crescent, an intercalary month had to be added. Precisely this information is represented on the Sky Disc. The width of the crescent moon adorning the disc corresponds with the Babylonian intercalary signal described here.

The first phase of the Sky Disc bore the crescent and the gold circle, as well as the seven stars of the Pleiades and 25 additional ones, or 32 in all. The interpretation of the image as a memory tool for determining the leap month explains the breadth of the crescent, the significance of the number 32, and the meaning of the gold circle.

The lunar month is 29.5 days in duration. When a leap month must be inserted, the moon appears in the Pleiades not on the first day in the shape of a narrow crescent, but two days later as a wider one – meaning that 32 days pass from the previous new moon. The 32 stars on the disc can be seen as representing the 32 days from the new moon of the preceding month until the moon appears in the Pleiades in the first month of the year. The 32 days and the breadth of the moon's crescent are equivalent, parallel signals for inserting a leap month. That is, the intercalary rule is doubly encoded on the Sky Disc:

  1. When in the first month of the year the moon appears with the Pleiades as a crescent the width of that depicted on the disc, a leap month must be added.
  2. When from the new moon of the month preceding the new year 32 days pass until the moon appears with the Pleiades, a leap month must be added.

The number 32 holds a further meaning as well, however, which has to do with reconciling the solar and lunar calendars. In a period of 32 solar years, 33 lunar years pass. If the gold circle is read as the sun, then the 32 gold points on the disc are interpreted as referring to the 32 solar years. However, counting the gold circle with the 32 gold points yields a total of 33 –referring to the only remaining object on the Sky Disc (in its original form), the moon, and the 33 lunar years.

Prior to this reading, Prof. Schlosser viewed the gold circle as the full moon. The more recent findings do not contradict, but rather complement his reading. Here the gold circle is interpreted as both sun and moon. A further observation underscores this thesis.

Going back 3,600 years, the Pleiades disappeared at dusk about twelve days before the beginning of spring. If – in the ideal case – the new crescent moon was observed with the Pleiades on the last day on which they were visible, the full moon coincided with the start of spring. Then, a full moon in the spring month signalled the beginning of spring and, in many cultures, also the start of the new year. Thus the full moon symbolised the new solar year. Accordingly, the gold circle can be interpreted as the full moon in the spring month as well as the sun.

Hence the Sky Disc presents us with a strikingly complex and layered representation.

The 32 gold points stand for the intercalary signal of the 32 days until the coincidence of the new moon with the Pleiades as well as for the 32 solar years.

On the one hand, the gold circle symbolises the full moon with the Pleiades in autumn (as read by Prof. Schlosser) and the full moon in the month in which spring begins. On the other hand, it represents the sun in connection with the gold points symbolising the 32 solar years, as well as the beginning of the solar year with the spring full moon.

When it appears with the Pleiades, the crescent moon directly indicates, by its breadth, the intercalary signal and symbolises with the 32 gold points and the gold disc the 33 lunar years corresponding to 32 solar years.

In the solar calendar, according to Schlosser, the Pleiades indicate exact dates in spring (with the crescent moon) and autumn (with the full moon). In the lunar calendar, together with the fat crescent of the moon, they additionally give the signal for the leap month.

This reading explains the number of gold points, the meaning of the gold circle, the width of the moon's crescent, and why the crescent is larger than the gold circle. For, if the Sky Disc is used as a comparative image for the width of the actual crescent moon with the Pleiades in the spring month, then this is the critical object on the disc and hence is given greater size than the gold disc.

In an epoch before the advent of numerical intercalary rules, the harmonisation of the solar and lunar cycles was the exclusive domain of celestial observation. These observations alone regulated the calendar. This knowledge constituted a great gain for Bronze Age society and can be viewed as the high-tech know-how of its time. It is no wonder, perhaps, that learned members of that society would fix this knowledge in such elaborate form.

It is a fascinating aspect of the intercalary rule encoded on the Sky Disc that it demanded of its user no mathematical knowledge. He needed only know the corresponding constellations in the spring sky.

These new findings present a number of questions which could cast new light on the cultural-historical significance of the Sky Disc.

  1. Where did the disc's Bronze Age fabricator, most certainly a member of the elite of his day, acquire his astronomic understanding? Whether this knowledge could have been generated locally or was more likely imported, directly or indirectly, from the Orient remains an open question. Previous references to the encoded worldview and the rosette figure of the Pleiades on the Sky Disc suggest Mesopotamia as a possible source and are supported by this reading.
  2. Was the fabricator of the disc the sole keeper of this knowledge, or was it widespread among the Early Bronze Age elite? If possessed by him alone, this could explain the loss of the knowledge of the intercalary rule in the Sky Disc's second phase; for the covering of individual stars by horizon arcs only makes sense in these terms. In the case that this understanding were possessed exclusively, the Sky Disc would constitute a demonstrative display of knowledge as power.
  3. Could the Sky Disc's master use his combined solar-lunar calendar as a tool for making external arrangements? If others – perhaps trade partners from distant regions – had the same knowledge, this would have been possible. This is not a necessary justification of the disc, however, since the internal determination of the dates of celebrations or other events could be explained by the use of the intercalary rule.
  4. The perforations along its circumference, from its fourth phase, made the Sky Disc definitively into a cult object – one that could be carried around until finally being laid to rest on the Mittelberg.