September 是九月的意思,而autumn则是秋天的意思,区别可以从以下例子看出:

In china autumn is falling in September, but in Australia autumn is falling in April, as Australia is in another side of the world. 中国的秋天是九月,而澳大利亚的秋天是四月,因为澳大利亚在世界的另一端。