Shadow period

Every retrograde runs between two bookends: the degree where the planet stations retrograde and the degree where it later stations direct. The stretch of zodiac between them gets crossed three times, once heading in, once backing up, once passing through for good. That zone is the shadow, and the weeks spent walking into it before the spell, or clearing it afterward, are the pre and post shadow periods.

The idea earns its keep through aspects. Any natal point inside the zone receives three exact visits from the planet, so a story that opens in the pre shadow tends to reverse during the retrograde and settle only on the final pass. Watching the shadow is watching a triple transit from its first crossing instead of its second.

Hold the term lightly. It is a modern coinage rather than classical doctrine, and nothing about the shadow calls for dread; the older sources counted the passes and left it there. The retrograde calendar prints the two station degrees for every spell, and those are the zone's edges.