  Using the Chandra/High Energy Transmission Grating Spectrometer (hereafter
HETGS) we have detected Doppler motion of Capella's X-ray emission lines in the
6 -- 25AA wave-band. The observed motion follows the expected orbital motion of
Capella's primary. This finding implies that the primary G8 III star, not the
secondary G1 III star in the Hertzsprung gap, has been the dominant source of
hot 10^{6.8} -- 10^{7}K plasma at least in the last six years. In addition, the
results demonstrate the long-term stability of the HETGS and demonstrate small
uncertainties of 25 and 33 km/s in the velocity determination with the HEG and
MEG, respectively.
