Why is every 15 day a solar term?

Thanks to the ancients, China has been mainly engaged in agricultural production since ancient times. The ancients knew the calendar like the back of their hands. They say that five days is a "wait", three days is a "solar term" and 24 solar terms are a "year" of 15. The most important thing is the five-day "waiting period". The ancients were so awesome. People who have eaten Chinese medicine should know that Chinese medicine gives patients a dose of five days at a time. Why? Because within five days, everything will be different, everything is constantly "changing", and the condition will change every five days.