What is a leap year? Why is there an extra day every 4 years?

We usually celebrate our birthday once a year. For example, if your birthday is August 21st, there will be an August 21st every year, which means you will celebrate your birthday once a year.

However, you should have heard of the leap year. If you have heard of it, then you should also know that there is such an "unlucky" person who is born on February 29, a date that only appears once every four years. So strictly speaking, he only has a birthday once every four years.

Because there is a leap year every four years, which means there are 366 days in a year, one more day than usual. The extra day is added to February, so there is February 29th.

But why do we have leap years? Wouldn't it be more convenient if every year had 365 days? If that were the case, there wouldn't be such unlucky people who only have one birthday every four years.

To answer this question, we need to first understand what a year is and what a day is. We usually think that a year is the alternation of spring, summer, autumn and winter, and a day is the cycle of sunrise and sunset. This is correct, but not accurate. A more accurate statement is that the time it takes for the earth to orbit the sun is one year, and the time it takes for the earth to rotate once is one day.

We all know that the Earth revolves around the Sun while rotating. Does that mean that the Earth revolves around the Sun once every 365 times, and every four years, the time it takes for the Earth to revolve around the Sun will increase to 366 days?

This idea makes sense, but it is not the actual situation. The time taken by the earth to revolve will not change. Whether it is 4 years or 100 years, it takes 365 days, 5 hours, 48 ​​minutes and 46 seconds for it to revolve once. The days recorded on the calendar are always integers, so we stipulate that there are 365 days in a year. There is no problem with this rule at the beginning, because a few hours difference in a year is nothing. But as time goes by, there will be nearly 6 hours less each year, and almost 1 day less in 4 years, and almost a month less in 100 years. If time is longer, December every year may become summer vacation time.

So, in order to make up for the problem of about 6 hours missing every year, it was stipulated that a normal year would have 365 days, but one day would be added every four years to make up for the accumulated one day of missing time, making a total of 366 days in total.

But is that the end? No. If a year is 365 days and 6 hours, then there is no problem with this orbit. However, the actual situation is not like this, which leads to the new regulations that the recorded time will increase a little bit every 4 years. If such errors accumulate over the years, there will be problems in the end. So the only way is to continue to increase the regulations. Normally, a year is 365 days, and 1 day will be added every 4 years, but 1 day will be recorded less every 100 years.

Of course, even with this track, the problem is still not solved, and there will still be errors. So we can continue to stipulate: keep the previous regulations, and then continue to increase one day every 400 years. In fact, if you are serious, this regulation can be continued indefinitely. However, we don’t need to stipulate so much. The current regulations will only cause an error of one day in thousands of years, which can fully meet our daily needs, so we will not continue to stipulate it.