A line drawn true north from the centre of the arc at G will pass through the doorway of the small temple and the centre of the arc KB at P. This line points through the outer wall where the gap occurs, and it is probable that the opening which was made in the outer wall to allow of observation along this line, determined its fall at this point. This meridian line is thirty-six feet distant from the other from the centre of the arc AK, and it must have pointed to the same great stone. But if both these lines point to the middle of this stone, which is 680 yards distant, they will incline towards each other about one degree, and the time of the transit of a star over the stone observed by one line will differ four minutes from that observed by the other. This inaccuracy would be so obvious to the observers that we cannot suppose they would have worked in this way. The great stone measures nearly, if not quite, thirty-two feet across, and were the lines directed, not both to its centre, but one to either side, they would be parallel to each other and would both give the same time for a transit of a star. This would imply that stars were observed, not passing over the stone, but disappearing and reappearing behind it, and a star observed at the altar to disappear would at the same instant reappear to an observer at G and P; or if the rock were less than thirty-two feet wide the star would reappear at G and P before it disappeared at the altar. We have thus a sort of double observation of the same meridian transit. If we admit that these meridian lines were used for the observation of stars in this way, and if we can determine what star or stars were observed, the time that has elapsed since they were observed admits of calculation. The apparent altitude of the middle of the stone as seen from the centres of the arcs is 7½°, and the latitude of Zimbabwe is 20° 16′ 30″, so that we want stars having a north polar distance of about 28°. Owing to the changing direction of the pole of the earth, which produces the phenomenon of the precession of the equinoxes, the declinations and right ascensions of all the stars are undergoing a slow but regular change; but there are no stars of the first magnitude which have had approximately this polar distance since any probable date of the foundation of Zimbabwe. Of stars of the second magnitude there are four, and of the third magnitude many more, which may have been used, and they would all serve for widely different periods. In order to enable us to select the proper star from this number we must have its right ascension, and this we may yet hope to get when we have the date of some important yearly festival at Zimbabwe, and the hour at which the star would be wanted on the meridian on the night of this festival.