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III. 9 the HELIACAL RISING of SIRIUS Teije De Jong the Heliacal Rising of Sirius, the Brightest Star in the Sky, Was Used In

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III. 9 the HELIACAL RISING of SIRIUS Teije De Jong the Heliacal Rising of Sirius, the Brightest Star in the Sky, Was Used In III. 9 THE HELIACAL RISING OF SIRIUS Teije de Jong The heliacal rising of Sirius, the brightest star in the sky, was used in antiquity, both in Egypt and in Mesopotamia, to synchronize the cal- endar to the solar year. On the day of its heliacal rising Sirius is seen again for the first time in the morning twilight sky after having been invisible for about 70 days (at the geographical latitude of Memphis). On that day it appears a few degrees above the Eastern horizon and disappears again after about 15 minutes due to the brightening of the sky just before sunrise. The date of heliacal rising depends on the rel- ative positions of Sirius and the Sun with respect to the horizon and on atmospheric conditions. According to Parker in the ancient Egyptian lunar calendar an addi- tional 13th month was intercalated in the next year whenever the first visibility of Sirius (associated with the Goddess Sothis) occurred during the last 11 days of the last month Wep renpet of the lunar year.1 In this way the Egyptians could make sure that the first month Toth of their lunar calendar always began shortly (within one lunar month) after the first visibility of Sirius. The heliacal rising of Sirius plays a crucial role in Egyptian chronology because it is supposed to fix the zero-point of the Egyptian civil calendar of 365 days by postulating that at the time of its installation the first visibility of Sirius occurred on the first day of the first month. One of the earliest references to the use of Sirius for intercalation in the Mesopotamian lunar calendar is found on Tablet II of MUL.APIN where we are told that if Sirius rises in the month Du"uzu (the 4th month) this year is normal but if it rises in the month Abu (the 5th) an extra month has to be intercalated that year.2 On Tablet I the nom- inal date of the first visibility of Sirius in the lunar calendar is given 1 R. A. Parker, “The Calendars and Chronology”, in: The Legacy of Egypt, J. R. Harris, ed., (Oxford, 1971), 13–26; cf. also below, Chapter III. 10. 2 H. Hunger & D. Pingree, MUL.APIN: An Astronomical Compendium in Cuneiform, AfO Beiheft 24 (1989). the heliacal rising of sirius 433 as “the 15th of Du"uzu” (the 4th month) as part of a list of first visi- bility dates of about 30 stars. This list was based on observations car- ried out in Babylon during the 13th century BC.3 Later (during the 5th to 1st centuries BC) dates of the first appearance of Sirius are given in the Astronomical Diaries.4 Sachs has shown that in the Babylonian 19-year calendar cycle, introduced in the early 5th century BC and used without any further change during five centuries throughout the ancient near-East, the intercalation pattern was arranged in such a way that Sirius always rose in the 4th lunar month Du"uzu.5 Starting with Ptolemy (~130 AD) in his Almagest (Book VIII.6),6 astronomers have discussed the heliacal rising (also referred to as first visibility or first appearance) of stars and planets in terms of the so- called arcus visionis, the distance between a star/planet and the sun, measured in degrees perpendicular to the horizon when it becomes visible again for the first time in the morning twilight sky. In the sec- ond book of his Fãseiw éplan«n ést°rvn ka‹ sunagvgØ §pishmais«n. Ptolemy gives the dates of first and last visibility of some 30 bright stars at different geographical latitude zones (kl¤mata) from which values of the arcus visionis can be deduced.7 For Sirius Ptolemy apparently uses a value of about 11°. In the 1920’s Carl Schoch was the first to attempt to determine val- ues of the arcus visionis for Sirius and for the planets from Babylonian observations.8 He derived a value of 7.7° degrees for Sirius. The recent edition of the Astronomical Diaries by Sachs and Hunger allows a reanalysis of the Babylonian observational material. We now know that the number of genuine observations of the date of first visibility of Sirius in the Diaries is quite small since Sachs showed that almost all dates during the Seleucid Era (last three centuries BC) are computed 3 T. de Jong, “A New Attempt to Date the Observations of Rising Stars in MUL.APIN”, paper presented at the 7th Notre Dame Workshop on the History of Astronomy ( July 2005). 4 Sachs & Hunger, Diaries. 5 A. Sachs, “Sirius Dates in Babylonian Astronomical Texts of the Seleucid Period”, JCS 6 (1952), 105–114. 6 See Toomer, Almagest. 7 H. Vogt, “Griechische Kalender V. Der Kalender des Claudius Ptolemäus”, Sitzungs- berichte der Heidelberger Akademie der Wissenschaften, Phil.-Hist. Klasse, 15. Abh. (1920), 1–61 8 C. Schoch, The Arcus Visionis in the Babylonian Observations (Oxford, 1924); idem, “The Arcus Visionis of the Planets in the Babylonian Observations”, Monthly Notices of the Royal Astronomical Society 84 (1924), 731–734..
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