46 As to the Egyptian calendar see L. Ideler, _Handbuch der mathematischen und technischen Chronologie_ (Berlin, 1825-1826), i. 93 _sqq._; Sir J. G. Wilkinson, _Manners and Customs of the Ancient Egyptians_ (London, 1878), ii. 368 _sqq._; R. Lepsius, _Die Chronologie der Aegypter_, i. (Berlin, 1849) pp. 125 _sqq._; H. Brugsch, _Die Ägyptologie_ (Leipsic, 1891), pp. 347-366; A. Erman, _Aegypten und aegyptisches Leben im Altertum_, pp. 468 _sq._; G. Maspero, _Histoire ancienne des Peuples de l’Orient Classique_, i. 207-210; Ed. Meyer, “Aegyptische Chronologie,” _Abhandlungen der königl. Preuss. Akademie der Wissenschaften_, 1904, pp. 2 _sqq._; _id._, “Nachträge zur ägyptischen Chronologie,” _Abhandlungen der königl. Preuss. Akademie der Wissenschaften_, 1907, pp. 3 _sqq._; _id._, _Geschichte des Altertums_,2 i. 2. pp. 28 _sqq._, 98 _sqq._; F. K. Ginzel, _Handbuch der mathematischen und technischen Chronologie_, i. (Leipsic, 1906) pp. 150 _sqq._ 47 Herodotus, ii. 4, with A. Wiedemann’s note; Geminus, _Elementa Astronomiae_, 8, p. 106, ed. C. Manitius (Leipsic, 1898); Censorinus, _De die natali_, xviii. 10. 48 Geminus, _Elementa Astronomiae_, 8, pp. 106 _sqq._, ed. C. Manitius. M22 Thus the official calendar was divorced from the natural calendar, which is marked by the course of the seasons. 49 Diodorus Siculus, i. 50. 2; Strabo, xvii. i. 46, p. 816. According to H. Brugsch (_Die Ägyptologie_, pp. 349 _sq._), the Egyptians would seem to have denoted the movable year of the calendar and the fixed year of the sun by different written symbols. For more evidence that they were acquainted with a four years’ period, corrected by intercalation, see R. Lepsius, _Chronologie der Aegypter_, i. 149 _sqq._ 50 Geminus, _Elementa Astronomiae_, 8, p. 106, ed. C. Manitius. The same writer further (p. 108) describes as a popular Greek error the opinion that the Egyptian festival of Isis coincided with the winter solstice. In his day, he tells us, the two events were separated by an interval of a full month, though they had coincided a hundred and twenty years before the time he was writing. _ 51 Scholia in Caesaris Germanici Aratea_, p. 409, ed. Fr. Eyssenhardt, in his edition of Martianus Capella (Leipsic, 1866). M23 Attempt of Ptolemy III. to reform the Egyptian calendar by intercalation.