The day started with a conversation with new NYU graduate student Marc Williamson about a project I would like to do in the APOGEE data, looking for time variability in stellar spectra. There should be variations there, because of convection and star spots; the question is whether we can see it, and whether we learn anything about stars from it. I am optimistic. It also connects to ideas I have about improving radial-velocity measurements.
I had lunch with Jo Bovy (Toronto), with whom we discussed what Gaia and APOGEE will jointly reveal about the Milky Way. I like to say that there is great complementarity, because APOGEE is infrared, and its red giants span much of the disk, while Gaia is optical and its sensitivity is best in the halo. However, Gaia is a very sensitive system overall, so it is a detailed question just how much or little Gaia data we will get on the APOGEE giants. Of course I ended up deciding that even if we do get lots of Gaia information about APOGEE targets, that only strengthens my view!