ESSAYS FROM ARCHAEOASTRONOMY & ETHNOASTRONOMY NEWS, THE QUARTERLY BULLETIN OF THE CENTER FOR ARCHAEOASTRONOMY
Number 25 Winter Solstice
Heliacal Rising: Definitions, Calculations, and Some Specific Cases
by Brad Schaefer, Yale University
Heliacal rising is a phenomenon where a star is first visible in the morning
sky. On this day, a star will only be briefly and barely visible, since if you
had looked a day earlier, it was too close to the Sun for visibility. . The
heliacal setting of a source is its last evening of visibility as conjunction
with the Sun approaches. Note that the first visibility of the thin crescent
moon is just a special case of heliacal rising.
Heliacal rising is an important phenomenon for ancient calendrics since almost
all old calendars included such events. Here is a partial list of ancient
cultures and the celestial source on which their calendars are based:
Additionally, everyone had months based on the heliacal events of the Moon. I
only know of one case (Egypt for Sirius) where architectural structures were
oriented towards a heliacal event as known from historical/ethnographic
While much less important and rarer, some cultures did note achronal events.
The achronal rising of a source occurs when it first appears in the eastern
evening sky just after sunset. Similarly, the achronal setting is when the
source last appears in the western morning sky just before sunrise.
Unfortunately, this defintion is ambiguous. By choosing sunset, nautical
twilight, civil twilight, or astronomical twilight as the fiducial time, the
achronal date can vary by many weeks. So achronal events are intrinsically
ill-defined, and a date can only be assigned with an accuracy of weeks. As
such, achronal events are useful only with strong historical/ethnographic
support and with an acknowledged low accuracy. I know of only two cases (old
Greek and Amazon) where achronal events of whole constellations or bright stars
are used as approximate seasonal markers. In the Greek case, this is well
documented in Hesiod.