anomalistic

Anomalistic is an adjective derived from anomaly, used to describe phenomena that relate to anomalies or irregularities. In English, the term is most often found in technical contexts, particularly in astronomy, where it denotes cycles or measurements defined by orbital anomalies rather than by averages or standard positions. The word comes from Greek roots meaning irregular or uneven.

In astronomy, anomalistic forms of time measurements describe periods tied to the orbital parameters known as anomalies. The anomalistic month is the time between successive perigees (the Moon’s closest approach to Earth) in its orbit around the Earth, rather than between consecutive alignments with the Sun. Its duration is about 27.55455 days. This cycle is influenced by the gradual rotation of the Moon’s orbit within its own plane, a phenomenon called apsidal precession, which causes the perigee to shift over time. The term helps differentiate it from the synodic month (new moon to new moon) and the sidereal month (Earth’s rotation relative to the stars).

The anomalistic year is the time between successive perihelia (Earth’s closest approach to the Sun) and is about 365.2596 days. It contrasts with the tropical year, which is defined by the cycle of seasons. Both anomalistic measures are used in celestial mechanics and historical calendar calculations to model orbital variations more precisely than mean values.

Beyond astronomy, anomalistic can describe anything pertaining to an anomaly or irregularity, though this usage is comparatively rare outside specialized discussions. In general prose, more common terms such as anomalous or irregular are often preferred.