'Noon' is only ever 12pm, so if someone says 'at noon' they are saying 'at 12pm' Afternoon is any time between 12:01om and evening (I believe starts at 6pm) so if someone says 'in the afternoon' they are basically saying 'any time between 12:01pm and 6pm. Because 'afternoon' isn't a specific time, the word 'at' cannot be used.

The most common PREPOSITIONS OF TIME are "at, in, on". We use: - "at" for a Precise Time, e.g. at noon / at 3 PM / at 1:45 (one forty-five) - "in" for Months, Years, Centuries and Long Periods, e.g. in April / in 2015 / in the 21st century / in three hundred years - "on" for Days and Dates, e.g. on Wednesday / on April 22