  The amplitude of the Compton-Getting (CG) anisotropy contains the power-law
index of the cosmic-ray energy spectrum. Based on this relation and using the
Tibet air-shower array data, we measure the cosmic-ray spectral index to be
$-3.03 \pm 0.55_{stat} \pm < 0.62_{syst}$ between 6 TeV and 40 TeV, consistent
with $-$2.7 from direct energy spectrum measurements. Potentially, this CG
anisotropy analysis can be utilized to confirm the astrophysical origin of the
``knee'' against models for non-standard hadronic interactions in the
atmosphere.
