The Borana are a group of nomadic cattle herders inhabiting an area that lies partly in southern Ethiopia and partly in northern Kenya. Their calendar, which regulates both subsistence and ceremonial activities, is of vital importance to them and is regulated by experts on sky observation known as ayantu. The Borana calendar is conventional in some respects. It is based upon the phase cycle of the moon, with each new month signaled by the appearance of the new, crescent moon in the evening sky. Both months and days are named, and the ayantu generally know the current month and day from memory. But if there is any uncertainty they make observations of the moon and stars. Seven months of the year are identified by a star or group of stars that rises side by side with the crescent moon when it first appears in the evening sky. The seven stars and asterisms used are Triangulum, the Pleiades, Aldebaran, Bellatrix, Orion’s belt and sword, Saiph, and Sirius. During the other half of the year, at least some of the ayantu watch to see at what phase the moon is side by side with the first star group in the list, Triangulum (although others may use different asterisms for this purpose). “Side by side” is a concept that only makes sense because the Borana live very close to the equator. This means that all celestial objects rise vertically somewhere in the eastern sector of the horizon and set vertically in the western sector. On any particular night, all the stars and asterisms that rise together continue to climb up into the sky “side by side.”
The Borana’s obsession with Triangulum is interesting in itself. It is often argued that the brightest stars and asterisms are likely to have been the most important to prehistoric peoples. Thus, people who seek stellar alignments at prehistoric monuments tend to examine the brightest stars. The Bo-rana show that this is not necessarily the case. Triangulum is a relatively faint group of three stars, yet it is of prime importance to them.
What renders the Borana calendar highly distinctive, and confused anthropologists and archaeoastronomers for over a decade, is that day names do not start afresh with each month but progress in an endless cycle of twenty-seven names. Since the phase-cycle (synodic) month is between twenty-nine and thirty days long, two or three days appear twice in each month, once at the beginning and again at the end. The reason for this seemingly incomprehensible practice becomes clear when we consider the monthly passage of the moon through the stars. Night by night, the moon moves slowly relative to the stars, completing a circuit through the stars in 27.3 days. This is known as the sidereal month. At the equator, this means that if the moon rises on one night side by side with a certain set of stars, by the next it will have progressed and rise level with a different set. After 27 nights, it will be rising side by side once again (more or less) with the original set of stars. The Borana day is determined by the stars that the moon is level with on a given night, regardless of its phase. Twenty-seven day names suffice, with one being repeated in roughly every third cycle as observations dictate.
The fact that the ayantu ignore the sun in determining the time of year seems surprising until we realize the implications of their location close to the equator. Here, the sun’s behavior changes little through the year: the length of the day is effectively the same all year round, and the annual swing in the sun’s rising or setting position between the solstices is relatively small. On the other hand, the vertical motion of the celestial bodies at night makes it natural to notice which stars are level with the moon, especially just after the moon has risen or before it sets.