  We have converted the former solar electrical plant THEMIS (French Pyrenees)
into an atmospheric Cherenkov detector called CELESTE, which records gamma rays
above 30 GeV (7E24 Hz). Here we present the first sub-100 GeV detection by a
ground based telescope of a gamma ray source, the Crab nebula, in the energy
region between satellite measurements and imaging atmospheric Cherenkov
telescopes. At our analysis threshold energy of 60 +/- 20 GeV we measure a
gamma ray rate of 6.1 +/- 0.8 per minute. Allowing for 30% systematic
uncertainties and a 30% error on the energy scale yields an integral gamma ray
flux of I(E>60 GeV) = 6.2^{+5.3}_{-2.3} E-6 photons m^-2 s^-1. The analysis
methods used to obtain the gamma ray signal from the raw data are detailed. In
addition, we determine the upper limit for pulsed emission to be <12% of the
Crab flux at the 99% confidence level, in the same energy range. Our result
indicates that if the power law observed by EGRET is attenuated by a cutoff of
form e^{-E/E_0} then E_0 < 26 GeV. This is the lowest energy probed by a
Cherenkov detector and leaves only a narrow range unexplored beyond the energy
range studied by EGRET.
