why was it decided to make 12:01 Am in that particular time ofthe earths orbit? y not have day start at sunup

Likely it was always easy with simple implements (sundials?) to identify 12 noon which arrives each (sunny) day due to the earth's spin rather than its orbit about the sun. The shadow is longest at noon. Early man worshipped the sun (as do many beach bunnies to this day). Mid-day is 12 noon and mid-night is well 12 midnight (easier than say 3:30 midnight?). Of course on a 24 hour clock, 12 is still noon.

Which day is "12:00 a.m."? Is "12:00 a.m." the last moment of the day that includes 11:59 p.m. (that is, 11:59 + 1 = 12:00). Or the first moment of the day that includes 12:01 a.m. (that is, 12:00 + 1 = 12:01). How do you know? How will you compellingly convince me?

For that matter, how do I know whether "12:00 a.m." is 12 midnight? Why isn't it 12 noon (that is, 11:59 a.m. + 1 = 12:00 a.m.)? How do you know? How will you compellingly convince me?

One reason is that the sun rises at a different time every day. Midnight and noon are in the middle of the night/day and remain unchanged by the length of daylight or darkness.

I think we shoud change it, so on the first day of spring it would be 12:00 AM when the sun starts to rise. That would be cool, because you could set your watch by the sun.

no, because the earth finishes it orbit at that certain time!