How to spell Occurrence
You must remember there are 2 "C's" on one side of the "U" and 2 "R's" on the other in occurrence, and it ends with ence not ance.
The occurrence of the Northern Lights was what they wanted to see (2 "C's"), but they had to wait two hours (2 "R's") for it to commence.
oCCuRRENCE
OCCURRENCE