How to read this chart
The selected planet contributes its natal longitude as a fixed target. The moving Sun later reaches that target. Every planet in the result table is recalculated for the crossing time; those are real sky positions at a later date, not the natal positions copied into another wheel.
Why there is no Sun option
The birth chart already has the Sun at its own natal longitude. A later return of the Sun to its natal position would be a solar return, a separate chart technique.
Location and time limits
Longitude and event time come from a geocentric Sun, so the chosen place does not change the crossing instant. Birth coordinates affect the Ascendant and houses of the event chart. This calculator keeps those coordinates for the later chart; it does not infer where the person lived at the crossing. Whole-sign houses are used for a clear, stable display.
Without a recorded birth time, the selected natal planet can occupy a range of degrees during the local date. The calculator reports a sampled possible crossing interval and withholds the chart wheel. For the fast Moon, that interval can span many days. Mercury or Venus can station, so the calculation samples every 15 minutes rather than assuming a single noon position.