vertical action is a clock

Ruth Angus (Columbia) and I discussed the state of her hierarchical Bayesian model to self-calibrate a range of stellar age indicators. Bugs are fixed and it appears to be working. We discussed the structure of a Gibbs sampler for the problem. We reviewed work Angus and also Melissa Ness (MPIA) did at the 2016 NYC Gaia Sprint on vertical action dispersion as a function of stellar age. Beautiful results! We had an epiphany and decided that we have to publish these results, without waiting for the Bayesian inference to be complete. That is, we should publish a simple empirical paper based on TGAS, proposing the general point that vertical action provides a clock with very good properties: It is not precise, but it is potentially very accurate, because it is very agnostic about what kind of star it is is timing.