Lower in sky now, initially rare astronomical sight (4,4)
I believe the answer is:
blue moon
'lower' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'sky now initially rare astronomical sight' is the wordplay.
'sky' becomes 'blue' (blue is a kind of sky).
'now initially rare astronomical sight' becomes 'moon' (I am not sure about this - if you are sure you should believe this answer much more).
'blue'+'moon'='BLUE MOON'
'in' is the link.
Can you help me to learn more?
(Other definitions for blue moon that I've seen before include "Indicator of rarity" , "'If it happens once in it, it's hardly ever (4,4)'" , "Coloured lunar phenomenon, happens rarely" , "Rodgers and Hart song" , "In which once is very seldom" .)