From three chart positions to one lot
Most familiar lots take one angle and the difference between two other zodiac positions. A degree in any sign is first converted to an absolute longitude from 0° Aries. Add the ascendant to one position, subtract the other, then bring the answer back into the 360° circle. This arithmetic places a point in a chart; it does not locate a physical object in the sky.
The terms matter historically. Greek astrologers used lots before the Arabic and Latin texts from which the later name “Arabian Parts” became widespread. The old method was not one uniform catalogue: authors used different inputs, day/night reversals, and interpretive schemes. Al-Biruni’s medieval catalogue and the Hellenistic tradition associated with Paulus Alexandrinus preserve different sets of names and equations. This calculator displays its selected formula because no single definition covers every named part.
Why the Sun's horizon position matters
Traditional Fortune and Spirit are complementary. For a day birth, Fortune projects the Sun-to-Moon arc from the Ascendant and Spirit projects the reverse. For a night birth, the assignments reverse. The Sun's position above or below the horizon decides which formula receives which name; the Moon's horizon position does not make the day/night decision. A time near sunrise or sunset can therefore change both labels even when the zodiac positions change little.
Birth time and place are essential because the Ascendant changes through the day and depends on the local horizon. A birthday alone cannot supply these lots. Civil UTC offsets also change over historical dates, so a current time-zone setting is not always the offset written into an older record. Compare the entered time and offset with the birth certificate and a historical atlas when possible.
How to use a result
Read the lots next to the Sun, Moon, Ascendant, and house layout. The calculator shows both formulas, the Sun's geometric altitude, and the alternative positions if sect were reversed. A near-horizon warning invites scrutiny of the source record. The reading remains a traditional symbolic framework, not a diagnosis or a test of future luck, wealth, health, or character.
References
- Al-Biruni, Book of Instruction, verse 476, Wright translation; annotated table — traditional lots formulas and night reversals.
- Digital facsimile of Al-Biruni’s Book of Instruction — context for the eleventh-century work and the 1934 translation.
- Astronomy Engine documentation and source — the astronomical library used for planetary positions and the local horizon calculation.
These sources describe a historical method and the astronomical implementation. The site calculates positions independently with Astronomy Engine 2.1.19 and its own birth-chart code.