Mercury Retrograde Shadow Period: Meaning, Dates and Calculation
Understand pre-shadow and post-shadow, see Mercury’s 2027 dates in UTC, and inspect the data and method behind the calendar.
Astrology calculation guides
Understand the inputs, methods and limits behind a birth chart or retrograde calendar. Each guide connects an explanation to a calculator or inspectable data.
Birth Chart Without a Birth Time: What Can You Calculate?
Which birth chart placements can you use without a birth time? Learn what local noon means, when the Moon sign is uncertain, and why houses need a time.
Why Do Birth Chart Calculators Give Different Results?
Compare birth chart results step by step: birth time, historical time zones, tropical versus sidereal zodiac, house systems, and calculation conventions.
Checking a Birth Chart Calculator Against JPL Horizons
Our 50-position comparison, reference settings, downloadable results and validation limits.
For formulas, data sources and implementation choices, read the methodology.