The Part of Fortune (or Lot of Fortune) is the oldest of the "Arabic Parts" — mathematical points calculated from the angular relationship between the Ascendant, Sun, and Moon, rather than tracked bodies in the sky. It's read as a marker of where a person experiences natural flow, wellbeing, and things simply working out with less resistance than elsewhere in the chart.
The formula flips depending on whether someone was born during the day or the night:
- Day birth (Sun above the horizon): Ascendant + Moon − Sun
- Night birth (Sun below the horizon): Ascendant + Sun − Moon
This day/night sensitivity is a deliberate traditional refinement — some modern software skips it and always uses the day formula, which this app does not do.
Because it's built from the Ascendant, the Part of Fortune also depends on knowing an accurate birth time.