The maths behind how long a planet stays in your sign

Most astrology content rounds the numbers. Saturn spends "about 2.5 years" in each sign. Jupiter spends "about a year." These are averages. They are also wrong for any specific transit you actually care about. Saturn spent 2.7 years in Pisces (March 2023 to February 2026). It will spend 2.1 years in Taurus (April 2028 to June 2030). That is a six-month difference. For a planet whose transits define entire chapters of your life, six months is not a rounding error. When I built transit duration tracking into Lunary, the real numbers surprised me. They also changed how I read charts. Orbits are not circles The reason planets spend different amounts of time in different signs is orbital eccentricity. Planetary orbits are ellipses, not circles. A planet moves faster when it is closer to the Sun (perihelion) and slower when it is further away (aphelion). This is Kepler's second law: a planet sweeps out equal areas in equal times. For Saturn, the difference is measurable. Its orbital eccentri