Constant calendrical_calculations::astronomy::MEAN_SYNODIC_MONTH

pub const MEAN_SYNODIC_MONTH: f64 = 29.530588861;

The mean synodic month in days of 86400 atomic seconds (86400 seconds = 24 hours * 60 minutes/hour * 60 seconds/minute)

This is defined in Calendrical Calculations by Reingold & Dershowitz. Reference lisp code: https://github.com/EdReingold/calendar-code2/blob/9afc1f3/calendar.l#L3880-L3882