As seen from Fig. 9-15, as xp increases beyond 90 deg, a, will exceed a2 and there exists a value of tf> such that

and a, = a2 = 90 deg This is the condition for which the central body is half lit.

The conditions for the gibbous central body may be obtained by inspection. Note that for a, > a > a2, a triggering must occur at the terminator (the corresponding horizon is dark) and the indeterminate case is absent. Table 9-3 summarizes the results for terminator identification.

Table 9-3. Definition of Triggering Event Classification in Terms of a

central body light mc


range op a


termina tor



fully lrr

tap®»» >9cP-p*ot

«3 * o >a0

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