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We present the first proper motion (PM) measurements for the dwarf ellipticalgalaxies NGC 147 and NGC 185, two satellite galaxies of M31, using multi-epochHST imaging data with time baselines of $\sim 8$ years. For each galaxy, wetake an error-weighted average of measurements from HST ACS/WFC and WFC3/UVISto determine the PMs. Our final results for the PMs are $(\mu_\mathrm{W},\mu_\mathrm{N})_\mathrm{N147} = (-0.0232, 0.0378) \pm (0.0143,0.0146)\,\mathrm{mas}\,\mathrm{yr}^{-1}$ for NGC 147, and $(\mu_\mathrm{W},\mu_\mathrm{N})_\mathrm{N185} = (-0.0242, 0.0058) \pm (0.0141,0.0147)\,\mathrm{mas}\,\mathrm{yr}^{-1}$ for NGC 185. The 2-dimensionaldirection of motion for NGC 147 about M31 is found to be aligned with its tidaltails. The 3-d positions and velocities of both galaxies are transformed into acommon M31-centric coordinate system to study the detailed orbital histories ofthe combined M31+NGC 147+NGC 185 system via numerical orbit integration. Wefind that NGC 147 (NGC 185) had its closest passage to M31 0.3-0.5~Gyr($\gtrsim 1.6$~Gyr) within the past 6 Gyr at distances of $\sim 70$ kpc (70-260kpc). The pericentric times of NGC 147/NGC 185 correlate qualitatively wellwith the presence/absence of tidal tails seen around the galaxies. Our PMs showthat the orbital poles of NGC 147, and also NGC 185 albeit to a lesser degree,agree within the uncertainties with the normal of the Great Plane of Andromeda(GPoA). These are the first measurements of the 3-d angular momentum vector ofany satellite identified as original GPoA members. Our results strengthen thehypothesis that the GPoA may be a dynamically coherent entity. We revisitprevious claims that NGC 147 and NGC 185 are binary galaxies and conclude thatit is very unlikely the two galaxies were ever gravitationally bound to eachother.

HST Proper Motions of NGC 147 and NGC 185: Orbital Histories and Test of Dynamically Coherent Andromeda Satellite Plane