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### 课程: 宇宙学和天文学 > 单元 4

课程 2: 人类的进化史# 历法符号（修正）

修正上个视频算出的时间差异（因为没有0年）. 由 Sal Khan 创建

## 视频字幕

I told you in the last video on calendar notation that regardless of whether you're using BC ad or BCE and CE e that there is no year 0 that we had one BC and then we had that theoretical birth of Jesus and most historians don't think that he was born right exactly on January 1 180 right after that you go from December 31st 1 BC to January 1st 180 there's no Year Zero and despite the fact that I emphasize that in the last video I didn't take that into consideration when I calculated how many years there were between Plato's birth and Columbus discovering the new world and the reason why I didn't take that in consideration is that the Year 1492 whether you want to call it ad 1492 anno domini 1492 whether you want to call it that or whether you want to call it 1492 in the Common Era it's not 1492 years since the theoretical birth of Jesus which we know is not the actual birth he was probably born before that it is 1491 years since the birth of Jesus and to think about it this way let's just assume I I'll keep him something it's a theoretical date that we're talking about this theoretical event this kind of birth of Jesus that our calendars revolve around if we if we talk about January 1 let's think about it this way so January January 1 1 in the Common Era in the Common Era how long is that since the birth of Jesus it's not one year you wouldn't just look at this it's been one year because this is theoretically the the day that he was born so this is 0 years or almost 0 years since that theoretical birth of Jesus another way to think about it is how long after January 1 January 1 the year 1 before the Common Era and I could have called this ad and I could have called this BC what's the time difference but Queen these two dates so the way I calculated before I said oh this is one year after that theoretical birth that's wrong this is during that theoretical birth but if I did it the way I did in the last video I said oh that's one year after one year before you add them together and you would get two but that's wrong because there is no year 0 so January 1 181 in the common error is right over here and then January 1 1 BCE is exactly one year before that so there's only one year one year difference and the reason why the math is strange is because there is no year 0 if there was a year zero then my calculation in the last video was correct so really the way that you would calculate the time between Plato's birth at 428 BC and Columbus sailing across the Atlantic in 1492 you would say okay this is this is 428 years before that theoretical birth of Christ but this isn't 1492 years after that theoretical birth this is 1492 minus one so what you would do is you would add these two numbers this is 428 before this is 1492 minus 1 years after so you would add them and then subtract 1 so the correct answer so this is the correction part it isn't 1920 years between Plato's birth and Columbus it is we want to subtract one from that it is 1920 minus one years so that is 1919 years the same way that the difference between one ad and one BCE you would say you can almost view it as like you know positive and negative numbers you say oh this is positive 1 minus negative 1 that would give me 2 but there is no 0 so you need to subtract another one so this is exactly 1 year difference so that's just want to clarify that that's what that no year 0 does to us