Why do we need leap seconds?
The simple answer is for the same thing, which is needed on February 29, so that the New Year will not shift over the summer.
But where do they come from? The answer is under the cut.
To understand this, it is necessary to remember that the measurement of time must satisfy two contradictory requirements:
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1. To be uniform, that is, the day in winter should be equal to the day in summer, and the second during the day should be equal to the second at night.
2. Time should correspond to natural cycles - it should be cold in winter and warm in summer, light in the daytime and dark in the night.
Take for example the year and day. As is known, a year does not contain an integer number of days, therefore, if it is postulated that a year contains 365 days, an error will occur in one day in approximately four years. If this error is not taken into account, then the New Year will have to be celebrated not in winter, but at another time of year. To eliminate this phenomenon, an additional day is introduced every four years. This is the so-called Julian calendar. It is also not entirely accurate, so now the Gregorian calendar is used in most countries, which also introduces one extra day every four years except for those years that are divided completely into one hundred and the first two numbers do not completely divided into four. Not everyone switched to the Gregorian calendar, so, for example, in Russia there is such a strange event as the Old New Year.
It's easy enough with days, but where do leap seconds come from?
Let's try to figure it out. It is logical to link the day to the change of day and night and determine the beginning and end of the day according to the position of the Sun. The problem is that the Sun moves along the ecliptic unevenly due to the ellipticity of the Earth's orbit. Therefore, the so-called "mean Sun" is introduced, which moves uniformly along the celestial equator. To calculate this average Sun, you must bind it to the rotation of the Earth. As is known, the Earth makes one revolution around its axis in approximately 23 hours 56 minutes and 4 seconds. This is the so-called starry day. Recounting the sidereal day in our usual solar, you can measure time very accurately, based on the stable rotation of the Earth. Dividing the day by 24 hours, every hour by 60 minutes, and every minute by 60 seconds, we get that the days contain 86400 seconds. This time is called UT1.
It was thought so until about the end of the 19th century, when a very precise theory of the motion of the moon was created — the Brown-Hill theory. Like all theories in celestial mechanics, this theory uses the so-called ephemeris time, which is uniformly the current time, which is called ET. Actually, as the General Theory of Relativity teaches us, there is no absolute time and the time assigned to the center of the Earth TDT and the time assigned to the barycenter of our Solar System TDB should be distinguished, but this difference can be neglected in our case. Let's return to the theory of Brown-Hill. This theory took into account many different parameters, such as the non-sphericity of the Earth and the influence of large planets, but its predictions differed slightly from observations. Since it was not possible to eliminate this discrepancy, it was suggested that it was not a matter of theory, but that the Earth rotates unevenly. It was possible to verify this only in the middle of the twentieth century with the advent of very accurate atomic clocks. It was found that the rotation of the Earth slows down, that is, the length of the day increases. There was a problem - to make seconds of different duration and leave 86400 seconds in days or fix a second and enter an additional second when the difference between atomic clocks (the time of which is called TAI) and observations will be about one second. Chose the second option. Such a time when the length of a second is always the same, but as close as possible to our solar time is called UTC.
How are these times intertwined and who is watching all this? This is a special service in Paris, whose website is http://hpiers.obspm.fr/eop-pc . On this site you can find the difference UT1-UTC and the number of leap seconds (which are entered either on December 31 or June 30, if the difference UT1-UTC exceeds half a second). Two interesting points should be noted. First, the future rotational behavior of the Earth is unknown, so the UT1-UTC difference is unpredictable and can only be obtained from observations. Secondly, with very insignificant changes in the angular velocity of the Earth’s rotation, leap seconds can accumulate rather quickly, since a small change in the angular velocity is multiplied by time, that is, there is a cumulative effect.
But where do they come from? The answer is under the cut.
To understand this, it is necessary to remember that the measurement of time must satisfy two contradictory requirements:
')
1. To be uniform, that is, the day in winter should be equal to the day in summer, and the second during the day should be equal to the second at night.
2. Time should correspond to natural cycles - it should be cold in winter and warm in summer, light in the daytime and dark in the night.
Take for example the year and day. As is known, a year does not contain an integer number of days, therefore, if it is postulated that a year contains 365 days, an error will occur in one day in approximately four years. If this error is not taken into account, then the New Year will have to be celebrated not in winter, but at another time of year. To eliminate this phenomenon, an additional day is introduced every four years. This is the so-called Julian calendar. It is also not entirely accurate, so now the Gregorian calendar is used in most countries, which also introduces one extra day every four years except for those years that are divided completely into one hundred and the first two numbers do not completely divided into four. Not everyone switched to the Gregorian calendar, so, for example, in Russia there is such a strange event as the Old New Year.
It's easy enough with days, but where do leap seconds come from?
Let's try to figure it out. It is logical to link the day to the change of day and night and determine the beginning and end of the day according to the position of the Sun. The problem is that the Sun moves along the ecliptic unevenly due to the ellipticity of the Earth's orbit. Therefore, the so-called "mean Sun" is introduced, which moves uniformly along the celestial equator. To calculate this average Sun, you must bind it to the rotation of the Earth. As is known, the Earth makes one revolution around its axis in approximately 23 hours 56 minutes and 4 seconds. This is the so-called starry day. Recounting the sidereal day in our usual solar, you can measure time very accurately, based on the stable rotation of the Earth. Dividing the day by 24 hours, every hour by 60 minutes, and every minute by 60 seconds, we get that the days contain 86400 seconds. This time is called UT1.
It was thought so until about the end of the 19th century, when a very precise theory of the motion of the moon was created — the Brown-Hill theory. Like all theories in celestial mechanics, this theory uses the so-called ephemeris time, which is uniformly the current time, which is called ET. Actually, as the General Theory of Relativity teaches us, there is no absolute time and the time assigned to the center of the Earth TDT and the time assigned to the barycenter of our Solar System TDB should be distinguished, but this difference can be neglected in our case. Let's return to the theory of Brown-Hill. This theory took into account many different parameters, such as the non-sphericity of the Earth and the influence of large planets, but its predictions differed slightly from observations. Since it was not possible to eliminate this discrepancy, it was suggested that it was not a matter of theory, but that the Earth rotates unevenly. It was possible to verify this only in the middle of the twentieth century with the advent of very accurate atomic clocks. It was found that the rotation of the Earth slows down, that is, the length of the day increases. There was a problem - to make seconds of different duration and leave 86400 seconds in days or fix a second and enter an additional second when the difference between atomic clocks (the time of which is called TAI) and observations will be about one second. Chose the second option. Such a time when the length of a second is always the same, but as close as possible to our solar time is called UTC.
How are these times intertwined and who is watching all this? This is a special service in Paris, whose website is http://hpiers.obspm.fr/eop-pc . On this site you can find the difference UT1-UTC and the number of leap seconds (which are entered either on December 31 or June 30, if the difference UT1-UTC exceeds half a second). Two interesting points should be noted. First, the future rotational behavior of the Earth is unknown, so the UT1-UTC difference is unpredictable and can only be obtained from observations. Secondly, with very insignificant changes in the angular velocity of the Earth’s rotation, leap seconds can accumulate rather quickly, since a small change in the angular velocity is multiplied by time, that is, there is a cumulative effect.
Source: https://habr.com/ru/post/292130/
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