The Geometry of Sagittarius Stream from Pan-STARRS1 3$\pi$ RR Lyrae
  We present a comprehensive and precise description of the Sagittarius (Sgr)
stellar stream's 3D geometry as traced by its old stellar population. This
analysis draws on the sample of ${\sim}44,000$ RR Lyrae (RRab) stars from the
Pan-STARRS1 (PS1) 3$\pi$ survey (Hernitschek et al. 2016,Sesar et al. 2017b),
which is ${\sim}80\%$ complete and ${\sim}90\%$ pure within 80~kpc, and extends
to ${\gtrsim} 120$~kpc with a distance precision of ${\sim} 3\%$. A projection
of RR Lyrae stars within $|\tilde{B}|_{\odot}<9^\circ$ of the Sgr stream's
orbital plane reveals the morphology of both the leading and the trailing arms
at very high contrast, across much of the sky. In particular, the map traces
the stream near-contiguously through the distant apocenters. We fit a simple
model for the mean distance and line-of-sight depth of the Sgr stream as a
function of the orbital plane angle $\tilde{\Lambda}_{\odot}$, along with a
power-law background-model for the field stars. This modeling results in
estimates of the mean stream distance precise to ${\sim}1\%$ and it resolves
the stream's line-of-sight depth. These improved geometric constraints can
serve as new constraints for dynamical stream models.
