Mystic helping to obscure start of regular astronomical event (11)

I believe the answer is:

occultation

I'm a little stuck... Click here to teach me more about this clue!

'regular astronomical event' is the definition.
The definition suggests a singular noun which matches the answer.

'mystic helping to obscure start of' is the wordplay.
I cannot quite understand how this works, but
'mystic' could be 'occult' (I've seen this before) and 'occult' is located in the answer.
'of' could be 'o' and 'o' is found in the answer.
This explanation may well be incorrect...

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