Since we have assumed that it is a situation at night thus the sun from you (observer) is very far away as compared to the moon (at night). Now about the angle -->
Angle M=90
Angle ESM is the angle of elevation of the sun from earth and as already stated that sun is very far away thus angle of elevation is very less. Thus angle MES (α) is assumed to be 89.5

Since we have assumed that it is a situation at night thus the sun from you (observer) is very far away as compared to the moon (at night). Now about the angle -->

Angle M=90

Angle ESM is the angle of elevation of the sun from earth and as already stated that sun is very far away thus angle of elevation is very less. Thus angle MES (α) is assumed to be 89.5