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diameters of the craters in pixels of data

However, because of the approximately 5o difference between the two orbital planes, the azimuth of moon rise (or moon set) can oscillate by up to ± 6o 40′ (for an observer at 36o latitude) about the Sun’s rising (or setting) azimuth in the course of a monthly lunar cycle. In addition, the Sun’s gravitational pull leads to a procession of the Moon’s orbital axis, with a period of 18.6 years. This means that every 18.6 years, the rising or setting Moon reaches a northern extreme in rising and setting azimuth at summer solstice, and a southern extreme at winter solstice. These are called major standstills. While such standstills can in principle be determined using horizon observations, as with the summer solstice Sun the Moon’s year-to-year angular displacement along the horizon at summer solstice is very small near standstill. Unlike for the Sun, anticipatory observations must now be carried a few years ahead of the major standstill.

Pictographs on the third story of the square tower at Cliff Palace. The left panel, located on the inside North wall, can be interpreted as a “teaching aid” demonstrating the back-and-forth motion of the rising and setting Moon along the horizon, in the course of one year. The panel on the right is located on the West wall next to the small square third story window, and can be interpreted as a tally of years between major lunar standstills (see text). Redrawn from diagrams in chapter 8 of J. McKim Malville’s “Prehistoric Astronomy in the Southwest”.

From the square tower’s small third story window noted in the preceding slide, a line pointing to the moon set azimuth of the (southern) major lunar standstill passes through the narrow gap between the two large towers of the Sun Temple. It has been suggested that this is an intentional construct aimed at marking major lunar standstills. Further support for this hypothesis comes from the two aforementioned pictographs, reproduced on Figure 1. The first (on the left) can be interpreted as a space-time diagram of the moon set’s daily displacement along the horizon: azimuth runs horizontally and time vertically upward, and the zigzag lines depict the back-and-forth motion of the moon along the horizon, for the rising (left half of diagram) and setting (right half) Moon. Note that there are a total of twelve zigzags depicted in each half of the diagram, which come as close to adding up to a year as it could, given that the year and lunar month are not commensurate.