What is causing these circle cracks on my tomatoes?

This is cuticle cracking caused by variations in the watering of the tomato. If there is a period of time where the plant does not have enough water then it has plenty, the fruit will grow faster than the skin and it will crack. Some cracks like this are just in the skin and the fruit underneath is still good, just peel it off. Deeper cracks may develop which invite secondary problems that may render the fruit inedible.

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Related Resources:
What Causes Tomatoes to Crack?
Tomato, Fruit Cracking