To understand how the Nebra Sky Disc can maintain the synchronicity of a lunar calendar to the seasons, it is instructive to look at some examples of lunar calendars. The Islamic calendar, used today with little change since ancient Babylonian times, is a great example because it relies on astronomical observation.
As later calendars developed, calculations took over, but logically, observations had to have come first. As records accumulated, patterns became increasingly obvious. Over centuries, calculations based on patterns become increasingly accurate. If weather prevents proper observations, it is tempting to rely on the calculations. Ultimately, the Gregorian calendar dispensed with observations altogether, which is superior for synchronizing the modern world.
The Islamic calendar reflects Inshallah – an expression meaning “if God wills”. It is mentioned in the Quran which requires its use when speaking on future events (https://en.wikipedia.org/wiki/Inshallah). While early astronomers were more than capable of predicting astronomical events, the Islamic calendar preserved the use of observation, so as not to presume Allah’s intentions.
The Islamic calendar is based on the first sight of the thin crescent moon at sunset, marking the start of a new month. Islam began in a region where weather was not much of a problem, but not always. To keep everyone on the same calendar page, as it were, a committee decides when months start based on the testimony of trusted observers. From there, word is spread among the faithful. As Islam grew, it became impossible for word to spread fast enough, leading to some disagreements, which still happen among Islamic nations with official religious calendars. In any case, differences are automatically corrected the next month or “moon”.
It is a simple matter of counting, to determine that moons usually alternate between 29 and 30 days long, and there are 12 moons in a year. It is a matter of a few decades to determine that simply counting to 12 causes the seasons to drift out of synchronization. Correcting this requires an additional observation, which uses the Pleiades star cluster in the Islamic calendar.
Prehistoric astronomers noted the Pleiades. Also known as “the Seven Sisters”, the cluster has six stars visible to good human observers. Galileo’s telescope revealed many more, and modern measurements of movements within the cluster suggest that seven would have been discernible 100 000 to 70 000 years ago. Common elements in ancient myths from Europe to Australia, about the disappearance of one of the sisters, suggest the possibility of this being the remnant of the world’s oldest story. (https://youtu.be/_qyjKND3dAE?si=CCDy4Ok1_Wvm78RC)
Certainly, the Pleiades have been easily recognizable as long as humans have looked up. Other constellations require a wide view of the night sky, but if the Pleiades are visible at all in a small area of sky, then they are recognizable.
The Pleiades are in the zodiac, close to the ecliptic, so they are visible everywhere outside Antarctica. In northern latitudes, they become visible in the night sky around the autumn equinox and remain visible until about the spring equinox, so they have long been associated with harvest and planting.
In spring, the Pleiades set near sunset, the same time as the observation of the first crescent moon. Thus, seeing the Pleiades near the horizon, together with the crescent moon, is an astronomical indicator of spring. If this observation indicates that the counted calendar is out of sync, then an extra intercalary month is added. Such an intercalary month is required about once every three years. History has proven this to be a sufficient and reproducible method of keeping the lunar calendar in sync with the Sun, stars, and seasons.
Variations
The Babylonian and Islamic versions use slight variations of this spring observation, but they are not the only possibilities. Synchronization of three things – the Sun, the Moon, and the seasons, requires three defined elements and one observation – a time-of-day, a phase of the Moon, and a time-of-year when the Moon’s position is to be observed. While somewhat arbitrary, the more precisely defined these elements are, the more reproducible the observation is, and the better the agreement can be over wide geographic areas. The modern technical mind demands accuracy, but this is not the same thing as precision and reproducible agreement, which are more important in this case.
As noted, the major challenge to reproducibility was weather. For early Islam, skies were generally clear, and in any case, official word could readily spread in a smaller area along active trade routes. For early Europeans, skies would not be as favorable in general, and spreading the word may or may not have been as practical or fast.
Some of the arbitrary decisions of the Babylonian calendar can be modified while preserving the overall goal of synchronization. For example, it is possible to improve the odds of getting good observations and choose the best time of year for a European society, by playing with some of these decisions.