The observations from which the omens were derived consist of dates for the heliacal rising and setting of the planet Venus. The date was observed at which the planet was first visible in the east, the date of her disappearance was noted, and the duration of her period of invisibility; similar dates were then observed of her first appearance in the west as the Evening Star, followed as before by the dates of her disappearance and her period of invisibility. The taking of such observations does not, of course, imply any elaborate astronomical knowledge on the part of the early Babylonians. This beautiful planet must have been the first, after the moon, to attract systematic observation, and thanks to her nearly circular orbit, no water-clock nor instrument for measuring angles was required. The astrologers of the period would naturally watch for the planet's first appearance in the glimmer of the dawn, that they might read therefrom the will of the great goddess with whom she was identified. They would note her gradual ascension, decline and disappearance, and then count the days of her absence until she reappeared at sunset and repeated her movements of ascension and decline. Such dates, with the resulting fortunes of the country, form the observations noted in the text that was drawn up in Ammi-zaduga's reign.
It will be obvious that the periodic return of the same appearance of the planet Venus would not in itself have supplied us with sufficient means for determining the period of the observations. But we obtain additional data if we employ our information with the further object of ascertaining the relative positions of the sun and moon. On the one hand the heliacal risings and settings of Venus are naturally bound up in a fixed relationship of Venus to the sun; on the other hand the series of dates by the days of the month furnishes us with the relative position of the moon with regard to the sun on the days cited. Without the second criterion, the first would be of very little use. But, taken together, the combination of the sun, Venus and the moon are of the greatest value for fixing the position of the group of years, covered by the observations, within any given period of a hundred years or more. Now if we eliminate the Second Dynasty altogether from the Babylonian Kings' List, it is certain that Ammi-zaduga's reign could not have fallen much later than 1800 B.C.; on the other hand, in view of the ascertained minimum of overlapping of the First Dynasty by the Second, it is equally certain that it could not have fallen earlier than 2060 B.C. The period of his reign must thus be sought within the interval between these dates. But, in order to be on the safe side, Dr. Kugler extended both the limits of the period to be examined; he conducted his researches within the period from 2080 to 1740 B.C. He began by taking two observations for the sixth year of Ammi-zaduga, which gave the dates for the heliacal setting of Venus in the west and her rising in the east, and, by using the days of the month to ascertain the relative positions of the moon, he found that throughout the whole course of his period this particular combination took place three times.[52] He then proceeded to examine in the same way the rest of the observations, with their dates, as supplied by the two tablets, and, by working them out in detail for the central one of his three possible periods, he obtained confirmation of his view that the observations did cover a consecutive period of twenty-one years. In order to obtain independent proof of the correctness of his figures, he proceeded to examine the dates upon contemporary legal documents, which could be brought into direct or indirect relation to the time of harvest. These dates, according to his interpretation of the calendar, offered a means of controlling his results, since he was able to show that a higher or lower estimate tended to throw out the time of harvest from the month of Nisan, which was peculiarly the harvest month.