All religions have their times and seasons for special ceremonies of worship, and the appropriate time for the greatest of these festivals of solar worship would be at mid-summer, when the sun seemed most brilliant and his rays most energetic. Accordingly we find that at Zimbabwe means had been provided for ascertaining the time of the summer solstice, and that the side of the temple which faced the rising sun at this period of the year was adorned with a decoration symbolical of fertility. But the temples at Zimbabwe seem also to have served a more directly practical purpose than that of mere worship of the powers of nature, and while regulating the festivals held in honour of natural powers, to have provided the means of observing the passage of the seasons and of fixing the limits of a tropical year, and thus providing the elements of a calendar. The duration of a day is clearly marked by an apparent revolution of the sun, and from the most remote antiquity a month has been equivalent to the length of a lunation; but there is no equally obvious astronomical phenomenon to enable the length of the year to be fixed; and although the difference between summer and winter is very apparent in most climates, there is nothing which very obviously defines the limits of these seasons, and the periods of spring and autumn are even less marked. But the dates of all festivals in solar worship would have some relation to the seasons; and, besides, the times for agricultural and many other operations would require to be fixed, and it would thus be doubly necessary to find means of marking the progress of the year. By most ancient peoples twelve lunations were considered to be equal to a tropical year, but it was soon discovered that this was not so, for the several months did not long coincide with their appropriate seasons, and so the history of most ancient calendars tells of devices to make the twelve lunar months of 29½ days each correspond with the tropical year of 365¼ days. At Zimbabwe things seem to have been better arranged, unless there, too, as in ancient Egypt, they had their troublesome civil year measured by twelve revolutions of the moon, in addition to their sacred year measured in the temples by an apparent revolution of the sun among the stars.