  In order to probe the mechanism of variations of the Solar Constant on the
inter-solar-cycle scale, total solar irradiance (TSI, the so-called Solar
Constant) in the time interval of 7 November 1978 to 20 September 2010 is
decomposed into three components through the empirical mode decomposition and
time-frequency analyses. The first component is the rotation signal, counting
up to 42.31% of the total variation of TSI, which is understood to be mainly
caused by large magnetic structures, including sunspot groups. The second is an
annual-variation signal, counting up to 15.17% of the total variation, the
origin of which is not known at this point in time. Finally, the third is the
inter-solar-cycle signal, counting up to 42.52%, which are inferred to be
caused by the network magnetic elements in quiet regions, whose magnetic flux
ranges from $(4.27-38.01)\times10^{19}$ Mx.
