One-Third of a Second

The rather enigmatic title of this entry refers to the accuracy with which the Babylonians were able to predict the length of lunation, the average length of a lunar month. As far back as the fourth century BC, Babylonian astronomers had not only calculated the Metonic calendar – which enabled them to predict lunar and solar eclipses – they had also accounted for the changing speed of the Moon as it orbits Earth.

As you may be aware, lunar months vary in length, which is due to the fact that the Moon’s orbit is elliptical, making its speed increase as it comes closer to Earth (in the same way that your own radial velocity when spinning on a chair can be increased by bringing your arms closer to your body). Many early cultures, relying upon an understanding of the phases of the Moon in order to plan their religious and cultural observances, were happy to settle upon a two-stage model of the Moon’s orbit, accounting for its maximum and minimum orbital speeds. While this was perfectly accurate enough for most uses, the Babylonians were rarely satisfied with ‘accurate enough’.

The astronomer Kidinnu – a truly remarkable individual – replaced this original system with a more precise calculation that took the variable speed into account, and, in the fourth century, had an error of only one-third of a second. Remember, of course, that until Kepler’s laws of planetary motion were written in the early seventeenth century AD, there was no model for establishing this information, and that the Baylonians had no way of measuring the distance from the Earth to the Moon.

In order to understand just how important this was to the Babylonians, we must understand their cultural attachment to the heavens. Like other cultures of the era, they associated their gods with the Sun, Moon and the five known planets (Mercury – Nabu; Venus – Ishtar; Mars – Nergal; Jupiter – Marduk; Saturn – Ninurta), which, being gods, made them responsible for much that happened in Babylonian society. As such, a Babylonian astronomer’s duty was to maintain a ‘diary’ to record not just the details of the sky, but also events upon the Earth.

For example, their diary included information such as: the times of sunrise and sunset; the positions of the moon and planets relative to the constellations; the weather – even rainbows were recorded; local news of importance such as fires and thefts; more significant news, such as conquests or victories in battle; the height of the water in the Euphrates; and even the relative value of the shekel in the markets.

The Babylonians did not merely believe that their gods resided in the stars, but also that they were responsible for the tiniest minutiae of common life. When the stars are the reflection of the gods’ will, and that this influences society on such fundamental levels, one-third of a second is, indeed, an appropriate margin of error.