system
Given the following post, respond as if you're commenting on Reddit:
human
The weather forecaster just said that summer officially starts at 11:54 am. What determines when a season starts and why isn’t it the start of the calendar day?

Furthermore who makes that determination?
gpt
This start of summer is the moment when the sun reaches its northernmost position in relation to the earth.

Because years are not 365 days long (but approx 365.2425) it shifts slightly between years.

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Winter is when sun in the most south.

Spring and Autumn start when the sun is exactly above the equator.