The oldest calendar appears to have been based on the lunar cycle. The interval between equal phases of the moon (the synodic period), however, is about 29.5 days, and lunar months therefore last either 29 or 30 days. This irregularity, noted already in earliest times, might have prompted the Egyptians to turn to the more regular solar calendar. The old lunar system survived in parallel and was used to fix the dates of some religious festivals (Parker 1978: 708).
Finding a match between lunar and solar dates is a difficult task: first of all, the precise sequence in which 29- and 30-day periods alternated is unclear (see, for instance, Parker 1950; Krauss 1985; Luft 1992). Moreover, the first day of the lunar month corresponded to the disappearance of the last lunar crescent, an event that could easily be missed in situations of imperfect visibility. Moreover, the irregular lunar calendar could not be precisely linked to the most important natural event of the year, the Nile flood. Another celestial body, however, appeared to fit this requirement: Sopdet (Sirius), which ‘‘disappeared’’ (that is, rose when the sun was already up in the sky and was thus invisible) for a period of 70 days. Her first visible rising at dawn, called ‘‘heliacal’’, took place at the end of June, in the same period when the inundation reached Upper Egypt.
To the early third millennium bc date the earliest representations of the star-goddess Sopdet as well as, probably, the introduction of a civil calendar which divided the solar year into 10 months of 30 days each, plus 5 extra days, called ‘‘epagomenal’’ (Wilkinson 2003: 167-8; Shaw2000a: 10-1; Parker 1950). Originally, the beginning of the solar year is likely to have corresponded to the heliacal rise of Sirius. The true length of the year, however, is a few hours longer than 365 days and in fact nowadays, to keep the calendar in place, every four years we assemble the extra hours into an extra day. The ancient Egyptians were fully aware of this shift (they called it ‘‘the wandering year’’) but maintained this time-keeping system unaltered for millennia. It took about 1,460 years to the civil calendar to complete the cycle, and to start again in correspondence of the helical rising of Sirius. This event was celebrated during the reign of the Roman emperor Antoninus Pius in ad 139, thus suggesting that the same correspondence must have taken place around 1320 bc and 2780 bc (Parker 1952; Ingham 1969; Krauss 1985).
The 70-day period of invisibility of Sirius may have also played an important role in the compilation of the earliest diagonal star tables. There are three types of tables, previously known as ‘‘star clocks’’, that were thought to correspond to three stages of a linear evolution of the same system; recent research, however, casts a new light on their function.
The earliest examples of diagonal star tables date to the Middle Kingdom. They record the rising of the stars called ‘‘decans’’ during the course of one night. Every ten days one of the twelve decans disappeared, and a new one appeared, and thus the list ‘‘shifted’’ upwards and sideways, giving to the table a distinctive diagonal pattern. By the time the solar year had passed, 36 decans had appeared and disappeared in the night sky; the five remaining days were marked by a specific group of additional stars (Symons 2007). These tables appear to have been time-keeping systems: the columns represent stars and the rows time-periods, but not necessarily hours, as previously believed (Depuydt 1988). For this reason, the misleading word ‘‘clock’’ has been abandoned in favor of a more neutral and objective definition as ‘‘table’’ or ‘‘list’’.
Spotting a rising star may be difficult, and this led scholars to believe that a ‘‘natural’’ evolution of this method would be looking instead at the transit, or culmination, of stars (that is, their maximum height in the sky; Frankfort 1933; Neugebauer and Parker 1960: 32-42, 113-15). Recent research, however, suggests that the so-called ‘‘transit star clocks’’ of the New Kingdom, rather than being ‘‘clocks’’, are simply embellished lists of stars that share a common characteristic: a 70-day period of invisibility. This span of time, equal to the period of invisibility of Sirius, corresponded also to the time that conventionally elapsed between death and burial of an individual. When someone died, the corresponding disappearing star was chosen: her heliac, rising, 70 days later, marked the time when the burial would take place. As such, these lists may be precursors, rather than successors, of the diagonal star tables (Symons 2002).
Finally, the Ramesside ‘‘star clocks’’ consist of stars placed within a grid, drawn above a kneeling, full-faced figure. According to the traditional interpretation, two people, the observer and the ‘‘target figure’’, would seat facing one another, along a north-south direction. The observer would look at the position of the stars in relation to the ‘‘target figure’’ and, by comparing it with the table, would be able to tell the time (Neugebauer and Parker 1964). This interpretation, however, is based on a number of conjectures, and several questions remain open (Symons 2000).
World History
