  We have measured the energies of the strongest 1s-2ell (ell=s,p) transitions
in He- through Ne-like silicon and sulfur ions to an accuracy of better than
1eV using Lawrence Livermore National Laboratory's electron beam ion traps,
EBIT-I and SuperEBIT, and the NASA/GSFC EBIT Calorimeter Spectrometer (ECS). We
identify and measure the energies of 18 and 21 X-ray features from silicon and
sulfur, respectively. The results are compared to new Flexible Atomic Code
calculations and to semi-relativistic Hartree Fock calculations by Palmeri et
al. (2008). These results will be especially useful for wind diagnostics in
high mass X-ray binaries, such as Vela X-1 and Cygnus X-1, where
high-resolution spectral measurements using Chandra's high energy transmission
grating has made it possible to measure Doppler shifts of 100km/s. The accuracy
of our measurements is consistent with that needed to analyze Chandra
observations, exceeding Chandra's 100km/s limit. Hence, the results presented
here not only provide benchmarks for theory, but also accurate rest energies
that can be used to determine the bulk motion of material in astrophysical
sources. We show the usefulness of our results by applying them to redetermine
Doppler shifts from Chandra observations of Vela X-1.
