Start with two stations
At the first station, Mercury changes from increasing to decreasing apparent geocentric tropical longitude. At the second, it changes back to increasing longitude. The station instants and their zodiac degrees come from the site's 2026–2027 retrograde calendar. The station-to-station interval is the actual apparent retrograde part of the cycle.
Find the two outside crossings
Take the longitude at the direct station and search backward for the latest earlier direct-motion crossing: that is the pre-shadow boundary. Then take the longitude at the retrograde station and search forward for the first later direct-motion crossing: that is the post-shadow boundary. Each search samples Mercury every six hours, checks half-steps for short loops, and numerically refines the crossing to the nearest second. The result is rejected if it fails to fall in the expected order or misses its target degree.
Why signs can change
A retrograde loop may cross from one sign into the previous sign. The comparison uses full 0–360° longitude with a wrap-safe angular difference, so 29° Pisces and 1° Aries remain adjacent rather than being treated as almost a whole circle apart. The displayed sign is simply a readable label for the computed longitude.
Limits and interpretation
“Shadow” is a convention for repeated zodiac degrees. It does not describe a new astronomical state and does not extend Mercury's backward motion. Longitude equality alone cannot establish an effect on a person or a likely event. Times are UTC; convert with the time-zone rules in force at the event location. This calendar uses one ephemeris, minute-rounded station inputs, and second-rounded crossings, so published dates may differ slightly.
See 2026 dates · See 2027 dates · Search another planet or degree
Sources and boundaries
Station instants come from the site's independently calculated sky calendar; shadow dates are new longitude searches using the same Astronomy Engine 2.1.19 positions. NASA explains apparent retrograde motion. “Shadow” is an astrologers' degree-based label, not a separate physical event. All results use apparent geocentric tropical ecliptic longitude and UTC. Station inputs are rounded to the minute, and degree crossings to the second; another ephemeris may differ slightly.