Every four years in the Julian calendar, there is an extra day in February, February 29, which is also known as leap day, and this year is a leap year. The historical origins and accounts of leap days are very interesting, but the reason why there are leap days is because of the physical mechanisms of the Earth-day system.
<h1>The Earth's rotation is not 24 hours</h1>
The Earth rotates around the axis as well as the Sun. The Rotation of the Earth causes sunrise, sunset, moonrise, moonset, coriolis effect (what we often call clockwise vortexes in the southern hemisphere and counterclockwise swirls in the northern hemisphere) and all the stars in the night sky revolve around the poles. On the other hand, the rotation determines the change of the Earth's seasons, for example: when you are in the hemisphere away from the sun, it is the cold and windy winter, the day time is the shortest; when the hemisphere you are in is facing the sun, it is summer, and the daytime time is the longest.
And we also know that due to the rotation of the Earth, a day is 24 hours, while an orbital year is 365 days (366 days in leap years). How did this day come about? In fact, the rotation and rotation of the earth is much more complicated than we think.
The above figure can see that the Earth does not need to rotate for 24 hours a week, more precisely 23 hours, 56 minutes and 4.09 seconds, the Earth can turn from the starting point and then return to the starting point, but because the Earth is rotating in orbit around the Sun, it will move less than 1 ° position every day, in order to keep the sun still above us at the same time the next day, the Earth needs to rotate at an additional angle, so the 24 hours are set as a day.
If we insist on using the number 23:56:04.09 as our whole day, it's not a big deal! Is the sun will not appear in the same position in the sky on time, if today's sun at 12 o'clock on time to the top of the head, then tomorrow's sun to the top of the head will be delayed by 3 minutes and 56 seconds, so that the error will continue to accumulate, one day when you look at the time, it should be night, but the sun is still in the sky, still daytime! To correct the Earth's orbit around the Sun, we need an additional 3 minutes and 56 seconds to correctly position the Sun so that the Sun appears in the same position in the sky on time. That's where 24 hours a day comes from, but what about a year?
<h1>The difference between an astronomical year and a regression year</h1>
We think of a year as the time when the Earth orbits the Sun, from the starting point back to the starting point, which is indeed an astronomy when the Earth has completed an orbit, which we call a stellar year or an astronomical year, but we have to remember that this is not the definition of a year on Earth. Rather, it is the time that the Earth goes through this year's vernal equinox in orbit to the next year's vernal equinox, which we call a year, using a regression year or a solar year.
We mark the calendar on Earth according to the seasons, which means that the winter solstice point from the winter solstice point to the winter solstice point of the next year is a year! At the winter solstice point in our northern hemisphere, the earth's north pole will be as far away from the sun as possible, and the sun is directly hitting the Tropic of Cancer at this time, and then the sun is slowly moving towards the equator, when it is directly hitting the equator, it is the vernal equinox of our northern hemisphere, and then the sun goes all the way north, and when it directly hits the Tropic of Cancer, it is the summer solstice point of our northern hemisphere, and then the sun will return to the equator, indicating that the northern hemisphere has reached the autumn equinox, and when the sun hits the Tropic of Cancer again, our northern hemisphere has entered the winter solstice point, which is the year! This method of measuring the year is the regression year.
The regression year is a little shorter than the year measured by astronomical methods, that is, the time of one week of the Earth's rotation. Why is that?
<h1>The precession of the Earth's axis resulted in a regression year that was about 20 minutes shorter than the astronomical year</h1>
Because the Earth's axis continues to move slowly, the Earth's axis changes, then the ecliptic plane will also change, the ecliptic plane will change The vernal equinox of the Earth in orbit will also change, in fact, the Earth only needs to orbit the sun less than 360 degrees to complete a regression year. The difference is small, one regression year is 359.986 degrees instead of 360 degrees, but this is enough to shorten the regression year by 20 minutes relative to the star year (astronomical year). This difference is also known as precession. The Earth's axis precedes a week of 26,000 years, so what we think of as the North Star is also changing.
<h1>Why there are leap days, the Gregorian calendar rules for leap days</h1>
Combining these three effects of rotation, rotation, and precession, we calculate that the average length of the regression year is 365.242193 days, and if we had 365 days per year, we would have been 24 days less than a month per century. If putting a leap year every four years (one more day) brings us closer to the real year of return, then the average year is 365.25 days, or a little more. This roman Julian calendar, we have used for 1600 years!
Still, this small difference accumulates, and by 1582 we have added an extra 10 days. For this reason, the 10 days from October 5 to October 14, 1582 never existed in Italy, Poland, Spain, and Portugal, and other countries skipped 10 days.
On 1 March 1582, Pope Gregory issued an edict to change the calendar, which read:
The day after October 4, 1582, is October 15, not October 5, but the week number is still calculated continuously: October 4 is Thursday, and the next day, October 15, is Friday. In this way, the old accounts accumulated since 325 AD were written off.
Second, in order to avoid the phenomenon of the spring equinox drifting away in the future, the method of changing the leap year is: Where the number of years in the Common Era can be divided by 4 is a leap year, but when the number of years in the Common Era is followed by a "century year" with two "0s", the year that must be able to be divided by 400 is a leap year.
So 2000 is a leap year, 1900 is not, 2100 will not be, 2400 is a leap year.
The adoption of the Gregorian calendar (Gregorian calendar) gives us a calendar of 365.2425 days a year. Compared to the actual figure of the average regression year of 365.242193 days, we need more than 3200 years to make an error of one day, and this calendar is already quite accurate.
But if we think about it in the long run, there is also the problem that the speed of the Earth's rotation is changing, and long enough time, our definition of "day" will change! Now we know that there are two things that can change the rotation rate of the Earth.
<h1>As the Earth slows down, we will no longer need leap days in the future</h1>
Every time an earthquake strikes the Earth, the mass of the Earth's interior rearranges, tending to be in a tighter state, through conservation of angular momentum, which means that the Earth's rotation rate accelerates a little faster. On the other hand, there are two celestial bodies in the solar system that have a huge gravitational effect on the Earth!
Both the Sun and the Moon exert gravitational pull on the Earth, and the Earth itself is rotating. It doesn't matter if the Earth is just a particle in space; the Earth will make an elliptical orbit around the Sun, and the Moon will orbit the Center of Mass of the Earth, and nothing will change. But the Earth is a sphere, and the Sun and Moon exert greater gravitational pull on the side of the Earth closer to them than the side farther away from them.
In addition to generating tides, because the Earth is rotating, there will be a tidal braking effect, that is, tidal friction, which will cause the Earth's rotation speed to slow down! This deceleration is small, but fairly stable, averaging 14 microseconds per year, much larger than the acceleration effect caused by earthquakes. As geological age goes by, this change will continue to accumulate! By leaving the tidal rhythm in the soil, we can calculate the changes in the Earth's rotation cycle.
620 million years ago on Earth, we found that the day was less than 22 hours at that time! If we push this tidal friction back to the time when Earth first formed, 4.5 billion years ago, we'd find that there were only 23,000 seconds or six and a half hours a day!
The Earth continues to slow down! In 18 months or so, we'll add 1 leap second to the clock. By about 40,000 years, a day will be extended by 56 seconds, which is enough for us to stop needing leap years; there are exactly 365 Earth days a year!
This is the origin of leap days and leap years, so that the seasons of the earth remain unchanged every year at the same time, but our earth is changing all the time, no matter how small, we will no longer need the day, nor will we need February 29th.