Astronomy is a useful aid in discovering the Dates of ancient Monuments. Thus, on the ceiling of a portico among the ruins of Tentyris are the twelve signs of the Zodiac, placed according to the apparent motion of the sun. According to this Zodiac, the summer solstice is in Leo; from which it is easy to compute, by the precession of the equinoxes of 50″·1 annually, that the Zodiac of Tentyris must have been made 4000 years ago.
Mrs. Somerville relates that she once witnessed the ascertainment of the date of a Papyrus by means of astronomy. The manuscript was found in Egypt, in a mummy-case; and its antiquity was determined by the configuration of the heavens at the time of its construction. It proved to be a horoscope of the time of Ptolemy.
This poetic designation dates back as far as the early period of Anaximenes; but the first clearly defined signification of the idea on which the term is based occurs in Empedocles. This philosopher regarded the heaven of the fixed stars as a solid mass, formed from the ether which had been rendered crystalline by the action of fire.
In the Middle Ages, the fathers of the Church believed the firmament to consist of from seven to ten glassy strata, incasing each other like the different coatings of an onion. This supposition still keeps its ground in some of the monasteries of southern Europe, where Humboldt was greatly surprised to hear a venerable prelate express an opinion in reference to the fall of aerolites at Aigle, that the bodies we called meteoric stones with vitrified crusts were not portions of the fallen stone itself, but simply fragments of the crystal vault shattered by it in its fall.
Empedocles maintained that the fixed stars were riveted to the crystal heavens; but that the planets were free and unconstrained. It is difficult to conceive how, according to Plato in the _Timæus_, the fixed stars, riveted as they are to solid spheres, could rotate independently.
Among the ancient views, it may be mentioned that the equal distance at which the stars remained, while the whole vault of heaven seemed to move from east to west, had led to the idea of a firmament and a solid crystal sphere, in which Anaximenes (who was probably not much later than Pythagoras) had conjectured that the stars were riveted like nails.