What exactly defines the modern era in baseball and who what when where and why is there this division in baseball history?

Answer 1

By Baseball convention the "Modern Era" began in 1900, roughly when the rules and conventions were stabilizing. The American League was founded in that year. It was also the first year where home plate acquired its current pentagon shape. The strike zone was only defined in 1887, and the four ball walk appeared 2 years later. In 1893 the pitcher's mound was moved to its current location of 60 feet, 6 inches from home plate. So until very late in the 19th Century, the games rules were undergoing very significant changes, and Major League Baseball thus does not recognize records and statistics compiled in that era to be comparable to the statistics achieved today. 1900 was determined to be a simple, clean date to separate between baseball's formative years, and the game whose rules have been remarkably stable since then.