Arabic Parts (Lots)
An Arabic part, or lot, is a point worked out from three positions in the chart — usually the ascendant plus one planet minus another. It marks a place tuned to one topic, and it corroborates what the house ruler already says rather than replacing it.
The fuller picture
The arithmetic is the easy part. Take three longitudes, add two and subtract the third, and the answer is a degree of the zodiac — no body sits there, and nothing about the sky at that spot has changed. Almost every lot is anchored on the ascendant, and most of them reverse by sect: the two planets after the ascendant swap places when the chart is nocturnal. Because the formula is arithmetic, a lot is only ever as good as the three numbers you feed it and the formula you chose, and both of those are worth stating out loud before you read anything from the result.
Their job in a horary is corroboration. Each part answers to a topic the houses already cover — the ninth signifies long journeys, and the Part of Travel signifies long journeys too, so the part supports a conclusion you could already have reached from the ninth-house ruler. That ordering matters more than it sounds. Reach the judgement through the significators, then look at the part to see whether it agrees. A part that contradicts a clear reading of the rulers has not overturned it.
There's a mechanical rule here that gets dropped constantly, and it follows from what a part actually is. A part cannot make an aspect. It has no body, so it radiates nothing for another planet to meet, and every contact runs one way: Saturn squares the Part of Travel, and the Part of Travel does not square Saturn. Applying and separating belong to the planet as well, because the motion is the planet's — the part sits still. The same reasoning exempts parts from the five-degree cusp rule. A planet within five degrees of the next cusp is carried into the house it's entering because it is genuinely travelling towards it; a part is a fixed address, so it stays in the house its degree falls in.
The awkward question is whose formula. Different authorities give the same-named lot different operands, and the disagreement isn't confined to the distant past: one modern working astrologer prints a Part of Marriage in two figures four folios apart, uses a different formula for each, and names neither. The consequence is practical rather than academic. Two astrologers can both say "the Part of Marriage is in the twelfth" and be talking about points thirty degrees apart. So name the formula whenever you cite a lot, and treat a lot with no stated formula as unreadable.
What survives all that is a short list. The Part of Fortune has a real place in the English horary tradition, and it is the only lot that tradition carries. A topic lot can be worth a glance in the question it belongs to, provided you can name its formula and you keep it in the supporting role. Everything else in the Hellenistic catalogue is reference material — computable, occasionally interesting, and not testimony.
In your chart
iHorary computes twenty-three lots on every chart: the seven core lots and sixteen topic lots. Only the Part of Fortune is treated as a point the chart can contact — it goes into the aspect pool with its own restricted orbs, it draws on the wheel, and it's the only lot the planet dossier reports, because a list telling you the Moon shares a sign with Eros, Illness and Travel on a job question is noise rather than evidence. Every lot is assigned to a house without the five-degree bump, since a lot doesn't travel towards a cusp.
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