A Bronze Sky You Could Hold

Around 1600 BCE, someone buried a bronze disc about 30 centimetres across on a hilltop in central Germany. Inlaid in gold across its blue-green surface are 32 stars, including a tight cluster of seven, a full circle standing for the sun or moon, and a crescent. Along two edges run arcs spanning 82 degrees each, matching how far the sunrise point shifts on this latitude between midsummer and midwinter.

UNESCO has called the Nebra sky disc the oldest known realistic depiction of the sky anywhere in the world, and on that point there is little argument. The cluster of seven stars is generally read as the Pleiades, a group prominent in the spring sky.

A more contested claim, put forward by the State Museum of Prehistory in Halle, concerns a possible rule embedded in the disc: that a slim crescent moon appearing near the Pleiades in spring meant a lunar calendar was running on schedule, while a fuller crescent, roughly four and a half days old, signalled that a leap month needed to be added to keep the moon's year in step with the sun's. Other researchers are less convinced the disc was used this precisely, so the idea is best treated as a serious proposal rather than a proven one.

Twelve lunar months fall short of a solar year by about eleven days annually, a gap every calendar-keeping culture has had to solve somehow. Some still solve it exactly as the theory suggests the disc did, by adding a leap month in certain years. The Hebrew and Chinese calendars both do this today. If the Halle team is right, the Nebra disc shows one of the earliest known versions of that same fix, cast in bronze and gold.