We present a new homogeneous sample of 32 spectroscopic binary orbits in the young ( \sim 150 Myr ) main-sequence open cluster M35 .
The distribution of orbital eccentricity vs. orbital period ( e - \log ( P ) ) displays a distinct transition from eccentric to circular orbits at an orbital period of \sim 10 days .
The transition is due to tidal circularization of the closest binaries .
The population of binary orbits in M35 provide a significantly improved constraint on the rate of tidal circularization at an age of 150 Myr .
We propose a new and more robust diagnostic of the degree of tidal circularization in a binary population based on a functional fit to the e - \log ( P ) distribution .
We call this new measure the tidal circularization period .
The tidal circularization period of a binary population represents the orbital period at which a binary orbit with the most frequent initial orbital eccentricity circularizes ( defined as e = 0.01 ) at the age of the population .
We determine the tidal circularization period for M35 as well as for 7 additional binary populations spanning ages from the pre main-sequence ( \sim 3 Myr ) to late main-sequence ( \sim 10 Gyr ) , and use Monte Carlo error analysis to determine the uncertainties on the derived circularization periods .
We conclude that current theories of tidal circularization can not account for the distribution of tidal circularization periods with population age .