  The giant outburst from SGR 1806-20 was observed on 2004 December 27 by many
spacecraft. This extremely rare event exhibits a striking similarity to the two
giant outbursts thus far observed, on 1979 March 5 from SGR 0526-66 and 1998
August 27 from SGR 1900+14. All the three outbursts start with a short giant
radiation pulse followed by a weaker tail. The tail pulsates with the period of
neutron star rotation of $\sim$5--8 s, to decay finally in a few minutes. The
enormous intensity of the initial pulse proved to be far above the saturation
level of the gamma-ray detectors, with the result that the most valuable data
on the time structure and energy spectrum of the pulse is lost. At the time of
the December 27 outburst, a Russian spacecraft Coronas-F with a $\gamma$-ray
spectrometer aboard was occulted by the Earth and could not see the outburst.
It succeeded, however, in observing a weak reflected signal due to the
$\gamma$-rays Compton scattered by the Moon. This has been the first
observation of a cosmic gamma-ray flare reflected from a celestial body. Here
we report, that the detection of a weakened back-scattered initial pulse
combined with direct observations by the Konus $\gamma$-ray spectrometer on the
Wind spacecraft permitted us to reliably reconstruct the intensity, time
history, and energy spectra of the outburst.
