Passé composé fits the English description. It is an event that occurred once, started when the sun began to shine “around midday” and has ended “became small”, not “was becoming smaller”. 

“Clear start and end” does not have to be fully and precisely expressed, it can be implicit or contextual. 

https://www.lawlessfrench.com/grammar/passe-compose-vs-imparfait/

le-passe-compose-usage-on-its-own-or-with-l-imparfait