The symbol of Gnosticism is important in studying Hermetic astrology where dates are chosen to align to Sirius/Isis or Alnilam/Belt of Orion/Osiris by intention.
The symbol is a Cross within a Circle like a snipers sights, and point to the positions to align stars.
Gnosticism - Wikipedia, the free encyclopedia
Before we use midnight only sunrise and sunset were used as a fix to align the stars, midnight was included as the clock emerged.