This was the obliquity of the ecliptic in the year 3700 B.C., and this is therefore the date of the foundation of the shrine of Amen-Rā at Karnak, so far as we can determine it astronomically with the available data; but about these there is still an element of doubt, for, so far as I learn, the recent magnetic readings have not been checked by astronomical observations.
CHAPTER XII.
THE STARS—THEIR RISINGS AND SETTINGS.
From what has been stated it is not too much to assume that the Egyptians observed, and taught people to observe, the sun on the horizon.
This being so, the chances are that at first they would observe the stars on the horizon too, both stars rising and stars setting; this indeed is rendered more probable by the very careful way in which early astronomers defined the various conditions under which a star can rise or set, always, be it well remembered, in relation to the sun.
It must not be forgotten that the ancients had no telescopes, and had to use their horizon as the only scientific instrument which they possessed. They spoke of a star as rising or setting cosmically, achronically, or heliacally.
The cosmic rising meant that the star rose, and the cosmic setting meant that the star set, at the same moment as the sun—that is, that along the eastern horizon we should see the star rising at the moment of sunrise, or along the western horizon a star setting at the moment of the sun setting; but unless certain very obvious precautions were taken it is clear that neither the rising nor the setting star would be seen, in consequence of the presence of daylight. The achronical rising or setting is different from the cosmic in this respect—that we have the star rising when the sun is setting, or setting when the sun is rising. Finally we have the heliacal rising and setting; that is taken to be that the star appeared in the morning a little in advance of the sunrise, or set at twilight a little later than the sun.
It is quite clear that if we observe a star rising in the dawn, it will get more and more difficult to observe the nearer the time of sunrise is approached. Therefore, what the ancients did was to determine a time before sunrise in the early dawn at which the star could be very obviously and clearly seen to rise. The term "heliacal rising" was coined to represent a star rising visibly in the dawn, therefore, before the sun. Generally throughout Egypt the sun was supposed to be something like 10° below the horizon when a star was stated to rise heliacally.
The following table from Biot should make matters quite clear:—
| Star at Eastern Horizon (rising) |  | Morning |  | True or Cosmic | Sun rising. |
| Apparent or Heliacal | Sun not yet risen, but depressed below horizon sufficiently to enable the star to be seen. | ||||
| Evening | | True or Achronic | Sun setting. | ||
| Apparent or Heliacal | Sun set, and depressed below horizon sufficiently to enable the star to be seen. | ||||
| Star at Western Horizon (Setting ) | | Evening | | True or Cosmic | Sun setting. |
| Apparent or Heliacal | Sun set and depressed below horizon sufficiently to enable the star to be seen. | ||||
| Morning | | True or Achronic | Sun rising. | ||
| Apparent or Heliacal | Sun not yet risen, but depressed below horizon sufficiently to enable the star to be seen. |
It is Ideler's opinion that, in Ptolemy's time, in the case of stars of the first magnitude, for heliacal risings and settings, if the star and sun were on the same horizon, a depression of the sun of 11° was taken; if on opposite horizons, a depression of 7°. For stars of the second magnitude these values were 14° and 8½°. But if temples were employed as I have suggested, even cosmic and achronic risings and settings could be observed in the case of the brightest stars.