┌ ┌ True or Cosmic Sun rising. │ │ ┌ Sun not yet risen, but │ Morning ┤ Apparent or ┤ depressed below horizon │ │ Heliacal │ sufficiently to enable │ └ └ the star to be seen. │ Star at Eastern ┤ ┌ True or Achronic Sun setting. Horizon (Rising) │ │ ┌ Sun set, and depressed │ Evening ┤ Apparent or ┤ below horizon sufficiently │ │ Heliacal │ to enable the └ └ └ star to be seen. ┌ ┌ True or Cosmic Sun setting. │ │ ┌ Sun set and depressed │ Evening ┤ Apparent or ┤ below horizon sufficiently Star at Western │ │ Heliacal │ to enable the Horizon (Setting)┤ └ │ star to be seen. │ │ ┌ True or Achronic Sun rising. │ │ ┌ Sun not yet risen, but │ Morning ┤ Apparent or ┤ depressed below horizon │ │ Heliacal │ 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. But it must not be imagined that, even in Egypt, all stars can be observed the moment they are above the horizon. In the morning, especially, there are mists, so that all but the brightest stars are often invisible till they are 1° or 2° high. On this point I quote Biot:-- "Comme le rapporte Nouet, l'astronome de l'expédition française, on n'y aperçoit jamais à leur lever les étoiles de 2° et de 3° grandeur même dans les plus belles nuits, à cause d'une bande constant de vapeurs qui borde l'horizon.[38] Aussi en expliquant le calcul des levers héliaques dans l'Almageste, Ptolémée a-t-il soin de remarquer[39] que les annonces qu'on voudrait faire de ces phénomènes seront toujours très-incertaines, à cause de l'état des couches d'air dans lesquelles on les observe, et à cause de la difficulté optique qu'on éprouve à saisir la première apparition, comme il dit lui-même en avoir fait l'expérience."[40]