Institute for Advanced Study Informal Astrophysics Seminar - Special Date

SPECIAL DATE AND TIME. The Evection is a "regular variation in the eccentricity of the Moon's orbit around the Earth, caused mainly by the Sun's attraction" (O.E.D.). It is the signature of a now weakened resonance between the period of apse precession of the lunar orbit and the year. Kinematically "epicycled" by Ptolemy and followers, it proved a source of headaches to Newton, anxious as he was to deploy his newly forged arsenal on the preeminent three-body problem of Sun, Earth and Moon. Two centuries plus down the road, Delaunay, Hill then Brown would bring Newton's program to near completion, all but drowning the Evection in a sea of kindred perturbations. The 90s witnessed a first serious revival of the Evection Resonance to address a paradox in lunar orbital architecture first noted by Goldreich in the 60s. Its workings were then fleshed out both in the context of lunar formation scenarios, and as a critical feature of the hierarchical three-body problem. Recently, we showed that a generalized Evection Resonance can disrupt multi-planet systems in wide binaries, thus further extending the astrophysical reach of this classical phenomenon. In our limited time together, we can only hope to highlight key aspects of this complex and evolving narrative.