When the earth casts no shadow on the entire rear exposure (4,4)
I believe the answer is:
full moon
'when the earth casts no shadow on' is the definition.
I can't tell whether this definition defines the answer.
'entire rear exposure' is the wordplay.
'entire' becomes 'full' (synonyms).
'rear exposure' becomes 'moon' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'full'+'moon'='FULL MOON'
'the' is the link.
Can you help me to learn more?
(Other definitions for full moon that I've seen before include "monthly highlight?" , "Lunar display with full disc illuminated" , "Bright time of the month" , "Wholly illuminated satellite" .)