When a planet retrogrades it does not simply reverse over new ground. It covers a stretch of the zodiac forward, stops, covers the same stretch backward, stops, and covers it forward a third time. That stretch, from the degree where it will later station direct to the degree where it stations retrograde, is the shadow. The word is a modern one, but the observation is old: the retrograde has an approach and a settling, and both belong to the season.
The pre-shadow begins when the planet first reaches the degree it will later turn direct at, and runs until the station retrograde. Everything the planet does across those degrees it will do twice more. The post-shadow begins at the station direct and runs until the planet passes the degree where it first turned. Only then has it left the three-times ground behind, and only then do astrologers consider the retrograde season closed.
For Mercury the shadows are two or three weeks on either side of a three-week retrograde, so the whole season runs about two months. For Venus and Mars the two shadows together run about as long as the retrograde, or longer. For the slow planets the shadow and retrograde together cover most of a year, which is why Saturn, say, spends the better part of one crossing and recrossing a stretch of six or seven degrees.
Astrologers read the three crossings as three chapters. The first pass is the first draft, the sketch, the conversation begun. The retrograde is the review. The third pass is the fair copy, the build, the thing said properly at last. What survives all three crossings is considered settled. Moondark shows the shadow on every passage page as the two outer marks on the timeline, and says on the Now screen the day a planet enters its shadow and the day it is clear.
Moondark defines the shadow by longitude, not by date: the planet is in shadow while it is between the two station degrees, however long that takes. This is the common definition and the one that matches the sky. A few sources use fixed day counts instead; the dates will differ by a day or two, and Moondarkâs are the ones the geometry gives.