  We have matched the astrometric data from Gaia Data Release 2 to the sample
of stars with measured rotation periods from Kepler. Using 30,305 stars with
good distance estimates, we select 16,248 as being likely main sequence single
stars centered within a 0.5 mag region about a 1 Gyr isochrone, removing many
sub-giants and unresolved binary stars from the sample. The rotation period
bimodality, originally discovered by McQuillan et al. (2013), is clearly
recovered for stars out to 525pc, but is not detectable at further distances.
This bimodality correlates with Galactic height as well, dropping strongly for
stars above Z>90 pc. We also find a significant width in the stellar main
sequence of $\Delta M_G$ ~0.25 mag, as well as a coherent gradient of
increasing rotation periods orthogonal to the main sequence. We interpret this
as a signature of stellar angular momentum loss over time, implying a
corresponding diagonal age gradient across the main sequence. Stellar evolution
models predict changes in color and luminosity that are consistent in
amplitude, but not in direction, with those required to produce the gradient we
have detected. This rotation gradient suggests that main sequence evolution
produces offsets in color-magnitude space that are significantly more
orthogonal to the zero-age main sequence than models currently predict, and may
provide new tests for both stellar evolution and gyrochronology models.
