The Sky Disc – Nebra Lunar New Year’s Clock/Calendar

tl;dr:

It’s a technical instrument for synchronizing a lunar calendar to the seasons.

• Two views show identifiable stars the Moon passes from Pisces to Gemini, during 7 midwinter nights
• Side bands show extents of the ecliptic as it sweeps the horizon during one night (not just the Sun during one year)
• The depicted full Moon (its radius in one view, diameter in the other) shows the width of the band the Moon stays in over months, years, and centuries
• The arc is Vega, kissing the horizon at True North near midnight around the winter solstice
• Hanging the disc from successive perimeter holes maintains proper orientation as the celestial sphere moves
• The crescent’s inner curve, used with a hypothetical addition also hanging from a perimeter hole, approximates Moon phases for 7 nights
• The Moon must cross a “finish line” between Taurus and Gemini, or a leap month is needed
• Should weather be a problem, redundant observations over several nights and during one night, will do
• The Sun, the solstice, and religious beliefs may be tied to the disc, but are not required to account for any design element

It’s all about the last full Moon of the lunar year!

Nebra Disc Decoded

2024 May

According to Wikipedia:

The Nebra sky disc is a bronze disc of around 30 cm (12 in) diameter and a weight of 2.2 kg (4.9 lb), having a blue-green patina and inlaid with gold symbols. The disc was found buried on the Mittelberg hill near Nebra in Germany. It is dated by archaeologists to c. 1800–1600 BCE and attributed to the Early Bronze Age Unetice culture. Various scientific analyses of the disc, the items found with the disc, and the find spot have confirmed the Early Bronze Age dating.

The disc, together with two bronze swords, two sets of remains of axes, a chisel, and fragments of spiral armbands were discovered in 1999 by Henry Westphal and Mario Renner while they were treasure-hunting with a metal detector. The detectorists were operating without a license and knew their activity constituted looting and was illegal. They damaged the disc with their spade and destroyed parts of the site.

https://en.wikipedia.org/wiki/Nebra_sky_disc

We agree with others who have suggested the disc encodes a system for synchronizing a lunar calendar to the seasons, mostly based on the generally accepted identification of the Pleiades star cluster and its importance in known lunar calendar systems. We hypothesize a comprehensive description of how the disc could have been used.

With a high degree of certainty, what follows is a detailed explanation of everything depicted on the Nebra Sky Disc. While there are certainly errors and uncertainty in individual points, the overall weight of the evidence is that the disc shows portions of the night sky with identifiable stars, in multiple views corresponding to different observations made, all consistent with a system for determining and confirming whether a lunar leap month is required or not, to a high degree of reproducibility over a geographic area.

We hypothesize a system related to, but not exactly the same as, a procedure described in ancient Babylon. Our proposal is more suitable for northern Europe and its variable weather, with opportunities for multiple confirming observations over several nights, and during one night. The best time-of-year is midwinter, not spring, and the time is probably midnight, as determined by the star Vega when it kisses the horizon at True North, because midwinter provides long, dark nights for observation, away from busy planting and harvest seasons. The extra month can be added at any time of the year – we guess that summer would be a crowd-pleasing choice. There are other good times-of-night, such as just before dawn, based on the constellations depicted on the disc, and their orientation, scale and proximity to the horizon at dawn of the solstice night.

We are highly certain of what is depicted on the disc, but details of use are more speculative as multiple alternatives work. We provide an example of a lunar calendar system, taking what we judge as the most likely or intuitive choices, but acknowledging they are essentially arbitrary. The goal is to provide a good, likely, working example procedure, consistent with the disc. We cannot offer a definitive solution, but ask you to try it out and decide for yourself.

All of the design elements of the disc: full Moon; crescent Moon; stars of the zodiac; arc of Vega kissing horizon; side-bands marking the extents of not just the Sun on the solstices, but also of the ecliptic every single winter night; and, the perimeter holes for hanging like an astrolabe at various days and hours to maintain proper orientation – are accounted for in a single procedure with a single goal. The disc may have been a revered object with multiple stories and meanings attached to it, but we find no design element inconsistent with a practical technical instrument.

We provide files suitable for printing, decorating, and cutting from cardboard, or ideally for laser cutting, with elements from the sky realigned to modern precision. The result is a working astronomical instrument for identifying constellations, synchronizing a lunar calendar to the seasons, and indoor and outdoor astronomy activities suitable for all ages. We’re having some fun with all this to generate interest, but we’re serious.

Stellarium.org is our main citation for all sections. It is great for checking out past, present, and future skies.

Disc Divided

The disc appears to show several distinct observations to be made on or near the winter solstice, for making the single determination of whether to add a “leap lunar month”, or not. Multiple views of the sky are combined into one design, so separating elements into these views is a useful first step. However, some elements appear to do “double duty” in more than one view, while accounting for a few others requires a deeper dive.

While the veracity of some individual elements may be uncertain, the overall degree of alignment with important astronomical and historical elements make for a convincing interpretation of the disc as a star map, record of lunar movements, and instrument in lunar calendar synchronization. It is obvious that the Nebra Sky Disc does not match up to modern precision. Rather, it is consistent with eye-ball observations sketched by hand onto the disc, or some intermediate medium.

Further, it may have been created all at once, or modified over time, perhaps centuries, to add additional observations, and some older elements may have been obscured or moved to accommodate the new. Without further examination of the artifact, it is difficult to tell. We are not convinced, but have mostly followed others in assuming some elements have moved based on bumps and craters, gilded or bare, and look forward to further evidence to confirm or refute. Some supposed “fixes” or “alterations” might have been there from the beginning, perhaps to mark the significance of their special positions.

The disc incorporates design elements and areas showing:

• The night sky and great circle of the horizon.
• Crescent Moon near the horizon, and the full Moon higher in the sky.
• Side-arcs showing extents of sunrise and sunset positions throughout a year, or for those who prefer the dark, extents of the intersection of ecliptic and horizon throughout a single winter night.
• Arc showing the path of the star Vega from dusk to dawn on the winter solstice, and in particular, how it kisses the horizon at True North around midnight.
• 39 holes around the perimeter of the disc, some of which may be used to hang the disc from successive holes to rotate the view of constellations as the Moon moves across the sky over a week, and the other way during a final night.
• Gilded dots in 2 views of stars in or near the path of the waxing gibbous moon, at or near the winter solstice.
  • A view covering a 60° area of the sky, showing prominent stars in the zodiac constellations Pisces, Aries, and Taurus (with an enlarged Pleiades star cluster), as well as nearby Perseus.
  • A view covering a 30° area, showing prominent stars in Gemini, Orion and Taurus. This area has long been recognized as an important boundary in the sky, between seasons, years, and ages. Two dots in particular may show a somewhat arbitrary but well-defined “finish line” for the Moon to cross, aligned with the dawn’s horizon, and the 2 brightest stars in nearby (but always away from the Moon’s glare) Canis Minor.

The multiple views and uses may represent the accumulation and integration of mystical knowledge into one symbolic object. It might equally track different ideas, perhaps developing and being renovated over time. Or perhaps multiple observations were known to be complimentary and cross-confirming when the disc was made. In any case, it could have been a revered mystical object, while simultaneously applied as a tool to the practical purpose of synchronizing a calendar for a society.

In Northern Europe, weather presents a significant obstacle to this purpose. Multiple systems operating over several days and times should improve the odds of good observations and wide agreement over a geographical area.

Because of how the artifact was found, some have suggested it is a fake, which we hope is not true, but we have not uncovered any new evidence one way or the other.

Astronomy Basics

A few astronomy facts and terms are useful in describing and understanding the disc. Click here for a quick review.

The Night Sky

While some elements shown on the disc are observable during the day, on its face the scene looks to be the night sky. Some have speculated the large circle is the Sun, but everything else suggests nighttime and a full Moon.

The artifact is circular, suggesting the dome of the observable sky down to the horizon. In fact, the disc is a slight oval, which may or may not be intentional. There is obvious damage around the edge, but also some suggestions of possible trimming and renovations, but as we are working from photographs only, we cannot say.

Gold dots on what may have once been a dark background appear to indicate stars. In particular, there is wide agreement that the group of seven dots is the Pleiades, or “Seven Sisters”, star cluster. We agree. Counting the sisters is at the limit of human vision, with broad agreement that 6 stars are discernible today, but 7 would have been discernible in the distant past as the stars move.

The obvious extension is that the other dots show stars in other constellations – or they are randomly distributed and decorative only. The relative positions of the stars are the main clue as to what constellations might be depicted, but another important clue is that if not random, someone felt they were important enough to depict with gold. Importance probably means brightness or relative brightness in an area, but also proximity to points in the sky of astronomical importance.

Side Arcs

There is a gilded arc on one side edge of the Nebra disc and evidence of a possibly missing arc on the other side. As others have noted, the extents of these arcs correspond to the extents of sunrises and sunsets throughout a year. In particular, the positions and angles of the ends of the arcs correspond to what is seen at or near the latitude of Nebra.

It should be noted that the disc may represent a map of the ground, rather than the sky. One way to first record the observations at the root of the disc’s design is with markers in the ground, probably sticks before permanent timber and stones, aligned with sunrise and sunset from a fixed central position. Intuitively, a circular layout of the site makes sense, perhaps on a hill, perhaps with a ring bank as an artificial horizon. The disc could then be a map of the site with the arcs showing the accumulation of markers on the ground. Thus the disc might be seen as a map of the site, the sky, or both.

One arc is gilded, the other is not, usually assumed to be evidence of damage and loss. Perhaps it is intentional, for example, to distinguish between sunrise (light is coming) and sunset (darkness is coming) at the East and West horizons, or the reverse.

While this element appears to exclusively reference the Sun and the solstices, this is not so. The arcs also show where the ecliptic crosses the horizon. This not only occurs at sunrise and sunset, it happens all night long, and sweeps across the horizon. This is not obvious to casual observers, even modern experts, who might never observe the whole sky for a whole night, but ancient astronomers knew the planets and stars on or near the ecliptic, and could observe them all rise or set at different times and points within the arcs. Thus, the proper extents of the side arcs can be determined in 366 days by morning larks, as no doubt someone did, or a single winter solstice night by a knowledgeable night owl.

Therefore, the arcs obviously reference the Sun, but in no way exclude nighttime use.

The modern understanding is that solstices are instants in time, or at most one particular day or night. Solstices occur on or very near the same date each year. To an early astronomer or farmer living and thinking within a lunar calendar paradigm, this is not the case. The end of the year’s calendar, found by counting, perhaps with a calendar on the wall or beads on a string, would be within a month of the moving winter solstice. Perhaps they were observed and celebrated together, perhaps not.

Joining the ends of the arcs with crossed lines is one way to define a center point and center line for the disc. By itself this cross does not tell us which way is “up” or “north”, and we hypothesize that the disc is meant to rotate from night-to-night and hour-to-hour depending on which observation is being made. Agreeing on a “proper” orientation is useful for description and discussion, but we believe this is fluid during use.

It is not straightforward to verify that our center point is the center of the circle of the disc’s edge, because the disc is oval (at least in all the photographs we have). If using the center point we have found, it appears a large part of the top of the disc has been “trimmed away” (or it was never there). If assuming an equal amount has been “trimmed” from top and bottom, then the center point should be moved down to the same level as the “double dot” at the middle left edge. It is difficult to decide which is better, let alone intended. Finding the center of mass or balance point of the disc, damaged or not, is a simple measurement we urge someone to make.

Vega’s Path

Some interpret the arc at the bottom of the disc as a “solar boat” from Egyptian mythology. It is a very close fit to the path of the star Vega in the constellation Lyra, from dusk to dawn on nights close to the winter solstice.

There is a critical latitude on the Earth, south of which Vega is seen to rise and set, but north of which Vega is circumpolar. Seen from this latitude, Vega kisses the horizon at True North.

Observations near the horizon are subject to distortion caused by the atmosphere, as well as local topography, which complicate matters, but the critical latitude is about 51° N – Nebra’s latitude, and perhaps not coincidentally, Stonehenge’s. Wobble of the Earth’s axis has altered this critical latitude over the centuries, but Nebra has been close for several thousand years.

Circumpolar stars can be used to track the movement of the celestial sphere as if marking a hand on a 24-hour clock face. This is complicated by rotation corresponding to the time-of-year, but several stars come in handy in various seasons.

Dubhe and Merak, for example, mark the end of the Big Dipper asterism (= informal constellation) in the Ursa Major constellation. They are easy for anyone in the northern hemisphere to spot, and they point at the star Polaris, very near the celestial North Pole and center of rotation of the circumpolar disc. They have been recognized as time keepers and season indicators in cultures around the world.

For Nebra and Stonehenge, Vega is another timekeeper with a special property – it is on the edge of the circumpolar disc. It is most useful around the winter solstice because it kisses the horizon at True North very near midnight. Vega is by far the brightest star anywhere in the vicinity but is not perfectly positioned. In fact, midnight of what we now designate as December 31 is a better fit than the solstice about 10 days earlier.

The arc on the sky disc is angled slightly but clearly. The full path of Vega is a circle around Polaris, but we cannot see it during the day, of course. Therefore, its path is an arc from the point where it first appears at dusk, high in the Western sky, swooping past the horizon, and back up, until disappearing at dawn. The name “Vega” comes from Arabic, and very loosely translated means landing, falling, or swooping eagle or vulture. “Swooping eagle” makes most sense at 51° N, less so in Arabia.

If tracked on December 31, the arc is more or less level. If tracked earlier at the solstice, the arc is tilted by about 10° (~10 days), as shown on the sky disc. The arc appears to show a slight flattening where it meets the horizon – consistent with distortion caused by the atmosphere.

As an arbitrary choice in a possible lunar calendar system, Vega’s kiss is a great indicator, not accurate but precise, of midnight at Nebra’s latitude, allowing precise observations at the darkest time of the night and year. We could choose some other time, such as sunrise or sunset on some given day, but the inclusion of the arc and its angle suggests midnight near the winter solstice.

The path of Vega, and in particular whether it is always visible, kisses the horizon, or disappears briefly, is sensitive to geographic location, and the Earth’s long-term wobble. There are 2 engraved lines along and within the arc. They are consistent with changes in Vega’s apparent path seen from different geographic locations or changes over centuries.

There are many small ticks engraved along both edges of the arc. It is impossible to get an accurate count due to the condition of the disc and resolution of available photographs. However, a rough guess is that the ticks are 1° apart. The celestial sphere rotates about 15° per hour, or 1° every 4 minutes, and about 1° from one midnight to the next. Thus we might speculate that the ticks represent the passage of units of time somewhere between 3 and 6 minutes long, or if interpreted the other way, 1 day. It is easy to accurately mark time intervals of about 4 minutes by singing, dancing, or marching, and that’s always fun at a New Year’s party, don’t ya think?

In any case, the “Harp Star” kisses the horizon at midnight, December 31 (if you are at a good spot), which seems like a good excuse to kiss someone and sing a song. They might think it is because their smartphone ticked over, but you can try the “it’s in the stars” bit.

Crescent Moon

The crescent moon is shown near the edge of the sky disc, and its points are oriented to show a moonrise or moonset. It is at an angle to the center line and disc edge. The diameter of the crescent Moon is shown to be larger than the full Moon, consistent with the well-known “moon illusion” which makes the Moon appear larger when near the horizon.

It takes 7 or 8 nights for the Moon to go from last crescent to full, so a span of time, and a journey across the celestial sphere, is implied. In a lunar calendar, this period is one-quarter of a month or one-quarter of a complete cycle of lunar phases. During this period the Moon is said to be “gibbous” and “waxing” – more than 50% illuminated, and increasing.

It should be noted that the Moon could be “waning” if we turn the disc 180 degrees.

The outer curve of the crescent is very nearly circular, suggesting care and probable use of a drawing compass. The inner curve is certainly not an exact circle nor a fit to the Moon’s astronomical appearance. The curve might indicate what a watchmaker would call a “moon phase complication”. A hypothetical addition, covering or revealing a portion of the large crescent, pivoting at the center of the disc, using the perimeter holes for counting and hanging, approximates the phases of the Moon for 7 nights leading up to the full Moon. “Center of the disc” is the unknown center of mass in this case, assumed to be close to the designated center.

Constellations

If we assume the dots are more than a random distribution, then we are faced with the seemingly daunting task of finding a matching group of stars. However, starting at the Pleiades proves very helpful.

Graphics software allows us to overlay a modern star chart, such as one copied from https://stellarium.org, and a photo of the disc. Aligning a shared pivot point of the Pleiades, there are only two degrees of freedom left – rotation and scale (assuming from the depiction of the Pleiades that their scale has been greatly exaggerated). It does not take too much play to find alignment with other stars in the constellation Taurus, the two brightest stars in Aries, and three stars in Pisces. Of note, these constellations are in the zodiac, and near the paths of the Sun, Moon, and planets.

While imperfect, the alignments are reasonably close for a hand-drawn night-time rendering. Even in modern star charts, there is some distortion due to the particular projection used. While tremendously encouraging, many of the disc’s dots are not aligned with anything in these constellations. Perhaps these dots represent other constellations.

Astronomers who grew up with H.A. Rey’s seminal “The Stars: A New Way to See Them” might open it for ideas, but that’s a mistake because the answer is on the cover. This alignment might have been clearer before dots at the edge of the disc were damaged or moved, and again, the depiction is not perfect, but we can recognize main stars in Gemini, Orion’s club, and the two stars marking Taurus’s horns, as well as a few dimmer stars.

The Stars

Thus it appears that the disc combines two views of the stars in the sky, with a wider 60° view (seen when disc is about 1 diameter from the eye) of the Zodiac mostly on the upper-right, and an adjacent 30° view (2 diameters from eye) of Gemini and surroundings, mostly on the lower-left of the disc. Once identified, the stars chosen to be on the disc reveal some of the thinking of the disc’s maker.

Star 6 in Gemini is notable because it aligns with one of the dots assumed to be in the Pleiades, that is, in the other view. This and several other dots might therefore be doing “double duty” with more than one meaning in more than one view. It is tricky to align highly accurate modern charts for double duty, but with a little artisan’s license, it is possible to achieve economical, recognizable representations covering a lot of sky.

A few dots remain that do not align well with main stars in well-known constellations. It should be said that if we are willing to consider dim stars, it is possible to find a star anywhere we might want to look, especially on dark nights far from modern lights. Clearly, we want to avoid such lazy choices, but if we are to account for all dots we need to consider more than just brightness.

Notably, the ecliptic goes through both depicted views, and we can now plot it. In the “Gemini view”, the ecliptic goes through the center of the disc’s full moon, as detailed below.

There are also stars/dots marking the center line of the disc. Also, some mark a perpendicular horizontal line (as in, horizontal on the disc, but also, parallel with the horizon as stars in the view set) which passes through the 2 brightest stars of Canis Minor, just off the side of the disc.

We mention these lines now and discuss them in detail later, because they explain the extra significance of some of the stars in the following list, i.e., some pairs of dots can be used to locate and draw these lines.

Stars and Dots

1) Pollux – Bright star in Gemini. One of the twin stars. Not on the disc, but maybe missing due to edge damage.
2) Castor -Bright star in Gemini. Maybe moved away from the edge or damage.
3) 77 Gem – Pollux’s right shoulder. Not on the disc, but maybe missing from the very edge. Stars on the very edge might be used to accurately align the disc when held up to the sky.
4) 69 Gem – Star in Gemini. On the highest possible path of the Moon – the edge of the Moon’s Ecliptic.
5) 46 Gem – Castor’s left shoulder.
6) 34 Gem – Castor’s left elbow. Does double duty, as the 7th Pleiades sister in the other view.
7) 55 Gem – in Gemini. On the ecliptic.
8) 54 Gem – Pollux’s right knee. There is evidence of either having been shifted slightly, or perhaps of some special meaning. The two overlapping circles making up this “dot” are both on the horizontal center line of the disc and together trace small versions of the disc’s large full and crescent moons.
9) 41 Gem – Dimmer star of Pollux’s calf. Significant because it is near the line defined by off-screen Canis Minor, and near the lowest possible path of the Moon (making it a “goal post” on a finish line to be used in a hypothetical lunar calendar synchronization system described below).
10) 31 Gem – Pollux’s right foot.
11) Alhena – Pollux’s left foot
12) Propus – Poor fit to Castor’s left foot, or star 34 in the other view.
13) HIP 28500 – Not a bright star, and not a close fit, but near a highly significant point in the sky – highest possible Moon position in all of the sky for all years (which moves over centuries). This occurs at midnight, on the winter solstice when the ecliptic is highest, and when the full Moon has wandered furthest North of the ecliptic. It is also on the line of Canis Minor, so it is another “goal post” with star 9. Can be located by imagining an equilateral triangle with the horns of Taurus. Close to the line from star 15 to Polaris. On the modern boundary between Taurus and Gemini. Maybe doing double duty as star 33 in the other view.
14) 69 Ori – Orion’s right hand.
15) HIP 28348 – Dimmer star in Orion’s club. Significant because it is on the lowest possible path of the Moon, and the center line of the disc.
16) Tianguan – A horn of Taurus.
17) Elnath – A horn of Taurus. On the highest possible path of the Moon.
18) Aldebaran – Taurus’s right eye and brightest star in the constellation. Marks the center line near the top of the disc. Almost exactly on the lowest possible path of the Moon. So close that rare occultations by the Moon have been recorded throughout history. This allowed astronomers to determine that this star’s position must have moved since early observations, leading to the modern discovery of the movement of this and other stars. Appears to be part of the Hyades cluster (but is actually much closer), which together with the Pleiades form the “Golden Gate of the Ecliptic”.
19) Alkalbain III – Part of a linear asterism from Taurus’s eyes to its ear and beyond, thereby drawing a line across the Moon’s Ecliptic, together with…
20) Alkalbain V – Close enough to star 19 that perhaps one dot represents 2 stars.
21) 45 Per – in Perseus.
22) 39 Per – in Perseus. Possible evidence of having been moved from under the disc’s side-arc near the edge. Poorer fit of stars in Perseus might be partially explained by the particular projection used to draw it, but without going deeper into perspective or possible observational techniques, suffice it to say that poorer fit towards the disc’s edge is not a surprise, and could be our fault, not the disc maker’s.
23) Maia – Bright star in the Pleiades star cluster (size of cluster greatly exaggerated).
24) Atlas – in the Pleiades.
25) Pleione – Missing 7th sister, which may have been discernible by the human eye around year -100 000 to -70 000.
26) Alcyone – Brightest star in the Pleiades.
27) Merope – in the Pleiades.
28) Electra – in the Pleiades.
29) Taygeta – in the Pleiades.
30) 35 Tau – Taurus’s mouth.
31) 17 Per – Dimmer star in Perseus/Medusa’s hair. May have been moved, or may have some other significance to be found. (We use it with the New Improved Nebra Sky Disc Wizard Stick, which is well into the realm of inventive speculation, but it’s cool).
32) Bharani – in Aires.
33) Hamal – Brightest star in Aires. From about year -2000 to 100 the equinox occurred in Aries in spring, and this star was used as the starting point of the Zodiac and the year. Maybe does double duty as star 13 in the other view.
34) Alpherg – in eastern fish of Pisces. On the highest possible path of the Moon.
35) 85 Psc – in eastern fish of Pisces.
36) 69 Psc – in eastern fish of Pisces.
37) HIP 4979 – Dim star in a relatively empty part of the sky near Pisces (in Cetus). Remarkable because it is on the lowest possible path of the Moon, where the Moon makes its first appearance on the Sky Disc, so it is useful in finding and drawing the edge of the Moon’s Ecliptic.

Full Moon and Ecliptic

The Moon follows the ecliptic but wanders from it by a little over 5° North and South (or above and below, if you prefer). Thus, while the (Sun’s) ecliptic is a line, the “Moon’s Ecliptic” is a band across the sky, with the ecliptic as the center line, and the Moon weaving from side to side over months and years. The edges of this band, the highest and lowest paths the Moon can take, are significant, and locating them represents years or centuries of careful observation.

On the disc, in the Zodiac view, the width of the Moon’s ecliptic equals the radius of the depicted full Moon. One edge of the ecliptic goes through the full Moon’s center, and the other edge is a tangent. The Moon is seen to travel from the lower left to upper right over the course of 5 or 6 nights.

In the Gemini view, the width of the Moon’s ecliptic equals the diameter of the depicted full Moon, a little over 10° of arc in the sky, about 1/3 of the diameter of the disc. Approximately centered on the ecliptic, the edges of the Moon’s ecliptic are therefore close to a tangent to the depicted full moon.

Considering that information collected over many years is required, the size of the Moon’s ecliptic and the depicted Moon is a good fit in both views, meaning they have scales in a ratio of 2:1.

Most people are surprised that an accurately sized Moon on a star chart looks ridiculously small. A Moon scaled properly for the Gemini view, would be close to 1/2 the diameter of the dots depicting stars. The depicted full Moon is about 18 times larger than a Moon drawn to scale.

The Moon moves across the disc from lower right to upper left, at about 1 diameter per hour. At 51°N latitude, the night of the winter solstice is about 18 hours long, so the Moon travels about 18 real diameters, or 1 diameter of the depicted full Moon, between sunset and sunrise.

The Moon’s depicted position is higher on the disc than it might be. It appears to be on the horizontal center line of the disc rather than the ecliptic. Perhaps the horizontal center line, the parallel line through 2 of the dots, and/or the parallel line defined by Canis Minor, are spaced to compensate for the Moon’s movement during the night, to allow for multiple equivalent and confirming observations, at dusk, midnight and dawn, for example.

39 Perimeter Holes

There is a baffling controversy over whether there are 39 or 40 holes on the perimeter of the artifact. The artifact is in rough shape, with obvious damage to the edge, but we count 39 without much difficulty. Perhaps part of the problem is that the holes are not spaced as evenly as some might like.

For most of the disc, it appears the holes are spaced appropriately for 40 per complete circle. However, on the right edge, the spacing appears to differ, with the end result of 39 holes. Some scholars have found significance in this number, with some proposing use in tracking the orbits of planets. Without detracting from these suggestions, and acknowledging the very real possibility of multiple uses, it appears there is another possibility.

If the disc is any kind of star map – every telescope owner will confirm that the sky is constantly moving, and a map or app that moves to maintain correct orientation is very helpful. A constellation might be seen to rise “right-side-up” near dusk, but set “upside-down” at dawn. One way to keep oriented is to let the disc hang from a cord threaded through one of the holes, moving along to the next hole at appropriate times. Hanging a properly balanced object is a simple way to keep it plumb and level, like an early astrolabe.

All the holes are precariously close to the edge for use as hanging points, but most would work. Some, especially on the left side, would not. One shows evidence of “pull-through” damage. As noted above, it is hard to tell exactly how the edge was damaged and might have been repaired over, perhaps, centuries.

Lunar Calendar

To understand how the Sky Disc can be used to synchronize a lunar calendar with the seasons, it is helpful to start with the ancient Babylonian calendar, used in Islam today with little change. Click here for more details, but three defined elements and one observation can keep the lunar calendar in sync with the seasons. This can be done with several variations to the time-of-year, time-of-day (or night) and the phase of the Moon. Then, the position of the Moon among known stars of the zodiac is observed. While somewhat arbitrary, the more precisely defined these elements are, the better the odds of agreement among geographically separated observers.

We frame our system as a modification of the Babylonian for convenience because its veracity and utility are well established. We do not mean to imply that one system necessarily evolved from another, as there may be a much older common root, and the sky itself may have guided independent astronomers to make identical or similar decisions. Our system is guided by the sky as much as possible, and a basic knowledge of traditionally recognized constellations.

Time-of-Year

For time-of-year, spring would not be best for everyone, simply because planting demanded attention and labor. A few individuals in a community might have been able to spare the time, but calendars are for syncing entire societies, so it is better if everyone at least has the option of seeing for themselves.

Winter would be a better time of year, because there is more downtime, nights are longest and darkest, and the ecliptic is at its highest in the night sky (lowest during the day), especially for observations made further North.

If it is determined that an extra lunar month is required, the month can be added at any time of year. Early historical accounts suggest midsummer, in which case the Big Dipper’s Dubhe and Merak reaching True North before disappearing at dawn could have served as a reminder. Such a scheme would allow for reasonable advance notice for all to make plans for the upcoming year. Early farmers would surely recognize signs of spring without the stars, but a shared calendar allows farm hands and traders to be at the right place on any specific calendar day, for example.

Time-of-Day

For time-of-day, sunset works, but only allows for a short period for observation before the Moon and the Pleiades also set. Using “nighttime” as an imprecise time-of-day means there is more time, as the Pleiades are visible all night long in the winter.

There is another difference between Babylon and northern Europe. Observing stars at or near sunrise or sunset is tricky everywhere because of the glare of the Sun, which lasts through dusk and dawn. As rules of thumb, star-gazers typically want the Sun to be 6, 12, or 18 degrees below the horizon, depending on how dark (civil, nautical, or astronomical night) they want the sky to be. The further North the trickier this gets, as sunrise, sunset, dusk, and dawn are drawn out, to the point where the sky never gets really dark in summer for some observers.

Anytime during the night is precise enough for most years, but sometimes more precision is required. Midnight is darkest, but with no clock, we might look for some indicator in the sky. The bright star Vega in the constellation Lyra is just about perfect. Depending on the observer’s latitude and local topography, Vega either briefly disappears below the horizon, or swoops past True North along the horizon, near midnight at the winter solstice. While not perfectly accurate, this observation is close enough in winter, and most importantly, well-defined and reproducible.

Phase of the Moon

For the phase of the Moon, the first visible crescent at sunset may not be ideal, as noted above. Choosing the full Moon for the phase might be an intuitive next choice, as it has one advantage – the Moon is full when it is opposite the Sun on the celestial sphere.

Early astronomers were aware that the Sun travels around the zodiac once per year, so the constellation the Sun is in is an indicator of the time-of-year, that is, a particular month in the zodiac calendar. However, direct observation of this is impossible as the Sun is too bright. Its position can be inferred in several ways, for example, the constellation the full Moon is in can be observed, and the opposite constellation in the zodiac is the Sun’s position and the time-of-year.

The full Moon has a disadvantage in that the precise time when it occurs is difficult or impossible to determine with the human eye. Modern instruments can find the precise instant of the full Moon, but to most humans the Moon appears full for several days. This is not good for reproducible agreement.

It is much easier to determine when the Moon transitions from crescent to gibbous, that is, when exactly half of the Moon is illuminated and half is dark. Confusingly, this is commonly called a half-moon, but is officially known as the first-quarter moon. While still susceptible to human judgment, disagreement over whether the Moon is a crescent or first-quarter might last for minutes rather than days.

Once the last setting of a crescent Moon is seen, the night of the full Moon can be determined by counting. While not perfect, a count of 7 days from the first-quarter is more accurate and consistent than judging a full Moon by eye.

It turns out, the first-quarter Moon sets near midnight, confirming another one of the three needed elements.

The Call

To stay in sync with the seasons, we need to see where the Moon is, in other words, which constellation of the zodiac – Taurus or Gemini in this case. Most years, this is an easy call. Most years, based on long-term patterns, what will happen is known beforehand, but confirmation is nice, and we also might want to put on a good, annual show. Some years it is a close call, and a precise boundary between Taurus and Gemini should be defined, to keep the fans excited if nothing else.

A modern boundary might be based on measurement of some kind of celestial latitude and longitude, but without sophisticated instruments, this makes little sense. Early astronomers might get close to this by imagining a vertical line from Polaris through some particular designated star between Taurus and Gemini, but this involves a large arc and consequent loss of precision.

Much more natural is to choose a couple of stars near Gemini and Taurus, and use them to define a “goal line” across the moon’s path. They should be close to the Moon, but not too close, or the glare of the full Moon will interfere. Actually, the night of the full Moon is far from the darkest night of the month, defeating some of the advantage of the winter solstice, so the brighter the chosen stars the better. There is an ideal pair just outside of the Moon’s Ecliptic – the two brightest of the little dog Canis Minor – Procyon and Gomeisa.

Looking at the line they make on a sky chart, it appears angled from what modern astronomers might choose, but as the winter night approaches dawn, it is clear this line will be parallel to the horizon. Earlier at midnight, it is easy to find Canis Minor, easy to judge whether the Moon is on one side or the other of the line it defines, and easy to foresee the scene at the horizon at dawn.

Proposed Calendar

One possible working variation of the Babylonian system is:

• Count the days of the previously declared year, till the last (lunar) month.
• Look for a waxing crescent Moon, and the last time it sets (before becoming gibbous).
• Count another 7 nights, noting nightly Moon positions should they be needed due to unfavorable weather.
• When Vega kisses the horizon, if the Moon is still in Taurus the new year needs an intercalary month, to be inserted in midsummer when the end stars of the Big Dipper reach True North before dawn. If in Gemini, no extra month is required.
• If a close call, use the line defined by the 2 brightest stars of nearby Canis Minor as the boundary.
• In any case, when the ball of the Harp star Vega drops and kisses the horizon, it is midnight and a new year, so kiss someone and sing.

P.S.

Connection to Hey Diddle Diddle has not escaped us.

The Sky Disc – Nebra Lunar New Year’s Clock/Calendar by David C. Walley is licensed under CC BY-SA 4.0