[554] The earliest Chaldæan astronomical observation, known to the astronomer Ptolemy, both precise and of ascertained date to a degree sufficient for scientific use, was a lunar eclipse of the 19th March 721 B. C.—the 27th year of the era of Nabonassar (Ideler, Ueber die Astronomischen Beobachtungen der Alten, p. 19, Berlin, 1806). Had Ptolemy known any older observations conforming to these conditions, he would not have omitted to notice them: his own words in the Almagest testify how much he valued the knowledge and comparison of observations taken at distant intervals (Almagest, b. 3, p. 62, ap. Ideler, _l. c._ p. 1), and at the same time imply that he had none more ancient than the era of Nabonassar (Alm. iii, p. 77, ap. Idel. p. 169). That the Chaldæans had been, long before this period, in the habit of observing the heavens, there is no reason to doubt; and the exactness of those observations cited by Ptolemy implies (according to the judgment of Ideler _ib._ p. 167) long previous practice. The period of two hundred and twenty-three lunations, after which the moon reverts nearly to the same positions in reference to the apsides and nodes, and after which eclipses return nearly in the same order and magnitude, appears to have been discovered by the Chaldæans (“Defectus ducentis viginti tribus mensibus redire in suos orbes certum est,” Pliny, H. N. ii, 13), and they deduced from hence the mean daily motions of the moon with a degree of accuracy which differs only by four seconds from modern lunar tables (Geminus, Isagoge in Arati Phænomena, c. 15; Ideler, _l. c._ pp. 153, 154, and in his Handbuch der Chronologie, vol. i, Absch. ii, p. 207).