The meridian is a line running directly overhead across the sky, joining the north point on the horizon to the south point. Technically, it is defined as the complete circle around the celestial sphere that also passes directly underneath, but this definition can be ignored for current purposes. Any observer on earth, unless they are standing at one of the two poles, has a meridian. Every celestial body crosses that meridian once each day, halfway in time between rising in the east and setting in the west, assuming that the eastern and western horizons are level or at a similar altitude; circumpolar stars, which never rise or set, cross it twice. The celestial north pole lies on the meridian for observers in the northern hemisphere, as does the celestial south pole for observers in the southern hemisphere. Outside the tropics, every celestial object reaches its highest altitude, or point of culmination, on the meridian. In particular, the sun crosses the meridian at local noon: at this time, the shadow of a vertical pole is shortest and points due north in the northern hemisphere or south in the southern hemisphere.
The cultural significance of the meridian is that it is an axis of symmetry around which all the celestial bodies swing evenly in the course of their diurnal motion. Anything aligned along this axis on the ground (i.e., due north-south) will appear naturally “in tune” with the symmetry of the heavens. Perhaps this was one reason the Great North Road leading into and out of Chaco Canyon was so closely aligned along the meridian. Meridianal alignments are also oriented, consequentially, upon the celestial pole, which is often seen as the hub of the heavens, the point that keeps the cosmos in place, or the point of connection with the upper world. Aligning earthly structures along the meridian can serve to confirm and reinforce the power of great leaders, whether living, as in the imperial Forbidden City in Beijing, or dead, as with those entombed in the Pyramids of Giza.
How, though, was the meridian determined in practice, especially when reasonable accuracy was desired? After dispensing with modern instruments, the simplest way in the northern hemisphere at the present time might be to orient directly upon Polaris, which would give true north to within a degree; but owing to precession no bright star marked the pole at most times in the past. One possibility is to use two chosen circumpolar stars to determine the north direction, using a plumb line to determine when one passed directly above the other. It has been suggested that, with the right choice of stars, this method would explain the extraordinary accuracy with which the Khufu Pyramid at Giza was cardinally aligned. The continued use of the same stars over subsequent decades would then have resulted in lower accuracy for subsequent pharaohs.
But there is a problem here. We can easily work out which two stars would give the best result because our modern instruments and computer programs tell us very accurately where true north actually is; people in the past did not, by definition, have such a yardstick. How could they have known which two stars were best? In fact, this was not an insoluble problem for them: one possibility is that they marked the direction defined by a vertically aligned pair of stars, then observed the same pair of stars at a different time of year and night when they were upside down (this would happen twelve hours after the first observation but would not generally be ob servable at that time because the sky would be light). The difference between the two directions would give an indication of the accuracy (in fact, the average of the two directions would be true north). Of course, this is only a suggestion, and the actual method used remains speculative; the point is that we have step out of our own shoes in order to recognize that there is a question to be answered.
