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How Long is a Year?

How Long is a Year?

For the last few centuries, children have been taught that a year is the amount of time it takes for the earth to revolve around the sun and that this process takes 365 days. This is not completely true, however. Some people might think that I was attempting to trick them and that a year is 365.25 days, but that is also technically incorrect. So how long is one year really? Well, to answer this question, it must first be made clear that you can not evenly split a year into days. Days are the basis of our time system. They are a consistent length, easy to count, and there’s no dispute as to what defines a day between cultures. However, when you think about it, there is no correlation between the amount of time it takes for the earth to spin and how long it takes to complete an orbit. Counting years with days is innately flawed. So how long is a year, and how do we count them accurately?

If you timed the earth’s orbit with a stopwatch, you would get 365.24220… days, or 365 days 5 hours 58 minutes and 46 seconds. While this is close to 365 days, it is not close enough to be accurate. If we pretended that a year was exactly 365 days, the calendar would shift by a month every 150 years or so. Within less than a thousand years, the seasons would be completely flipped, with winter being warm and sunny while summer is cold and snowy. While I’m sure we all would appreciate a warm and sunny break right now, I like my summers the way they are and so do most calendar makers. To solve this problem, we have invented the leap year.

To offset the inaccuracy of counting years in days, we add an additional day to the calendar every four years. The next time this will occur is next year in 2020. Tacking on February 29th every four years does bring our calendar closer to the actual time it takes to orbit around the sun, but it is not perfect. As a matter of fact, leap years overcorrect for what the 365 day calendar lacks. With the current solution, the calendar is offset by about one day per 100 years. Because of this, we ignore the leap year once per century- specifically on years ending in 00. For example, while 1896 and 1904 were leap years, 1900 was not. This was even closer to an actual year, but it was not close enough for calendar makers. To make a complex system even more difficult, and to keep our calendar accurate for millenia, we ignore the previous rule and keep the leap year once every four centuries. Thus, 1900 was not a leap year while 2000 was. Our current rules bring the offset down to one day in 8,000 years, which calendar makers deemed to be an acceptable level of error and left it at that.

However, if one in eight thousand is not precise enough for you, there is one other factor not yet mentioned: a day is not exactly 24 hours. To be precise, a day is about 24.0000002 hours, or 24 hours and 0.75 milliseconds, plus or minus about half a millisecond. Due to the fact that the earth’s spin is slowing down, every 100 years a day becomes 1 millisecond longer. This discrepancy has an extremely small effect, as it will be billions of years before the difference is noticeable, but it still exists.

To summarize, you’ve been lied to your whole life and everything you thought you knew about time is wrong.


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