What a lot measures
A lot is a point derived from other chart positions. First convert each input into a 0–360° longitude, measured from 0° Aries along the zodiac. Then start at a casting point, usually the Ascendant, and add the arc from one significator to another. The result is reduced modulo 360°. This is a geometric construction on the zodiac circle, not a newly observed body. The mathematics can be precise even where the historical name and its interpretation are unsettled.
Al-Biruni’s Book of Instruction in the Elements of the Art of Astrology was composed in the 11th century. In verse 476, the text says he is reproducing lots mentioned by Abu Ma'shar. It distinguishes a beginning, an end, and a casting point, and explicitly states that some lots keep the same order by day and night while others reverse the first two inputs. The seven “planetary” entries presented here all reverse at night. The manuscript tradition and modern transcription have far more than seven lots; the source table also groups many entries under the houses and other topics.
Why the name of the source belongs beside a result
“Arabian Parts,” “lots,” and “Hermetic lots” are often treated as interchangeable search terms, but they do not designate one fixed calculator specification. A victory lot in one author may use different planets or a different casting point in another. The Al-Biruni table is therefore a declared method.
Even within Al-Biruni’s list, headings can be repeated for different contexts and several labels can share one equation. A modern reader should avoid turning a medieval topic heading into a direct prediction. If a point is used to study a historical interpretation, the full chart, source, sect, and quality of the birth record matter more than the bare sign printed in a results table.
How to use this collection
Start with Fortune and Spirit, which are complementary. Then inspect the seven-entry table and its arithmetic example. Finally, check the day/night branch and source variants before comparing against another calculator. The independent tool displays every chosen equation, so a disagreement can be traced instead of hidden behind a single zodiac degree.