Shulou(Shulou.com)11/24 Report--

When I saw Lightyear for the first time, most people were a little confused.

Those familiar units, such as centimeters or units of length, kilograms or units of mass, how come light years are not units of time? According to this logic, is the year of the Tiger the distance that the tiger runs in a year? The year of the Monkey and Ma Yue are both lengths?

There is no comparability, the word "light year" has been given such a meaning, this is called a done deal, but the monkey year has not yet!

But if you know that the word "light year" can actually be written as lightyear, then you will find that it is about the same as centimeter, and there is a multiplicative relationship between the two parts, just as centi means 1%, so 1% times meter gets centimeters (centimeter), and the speed of light multiplied by the year is naturally the length.

Well, light years are the length, indicating the length of the distance light travels in a year.

But there are at least two more questions for you to figure out.

The first question is, which speed of light is the speed of light? In a vacuum?

The answer is yes. The speed of light in a vacuum is a physical constant, expressed as c, whose exact value is 299792458 m / s.

So the first question is not a problem.

The second question is, how long is the year here?

Well, this question looks a little troublesome! Because you know, a year may be 365 days, or it may be 366 days, and when you think about it, the specific length of a day varies. So, how long do you think a year is? It seems hard to tell all of a sudden.

However, as you probably know, the second is the basic unit of time, and its size is fixed. Therefore, if the "year" in a light year is a fixed size, it must contain a certain number of seconds.

Let's take a look at the common days and years, what types are there, and whether the number of seconds they contain is fixed?

Let's first take a look at the sky, that is, the day.

The first is the solar day (Solar day), which is the elapsed time between the two consecutive times when the sun reaches the highest point in the sky. Its value is 86400 seconds.

Isn't that a bit of a coincidence? As you probably know, seconds are defined according to atomic physics. A solar day happens to be 24, 3600 seconds. Why?

Because seconds ago was defined by dividing a solar day into 86400 parts! It was not until 1967 that it was redefined by the method of atomic physics, specifically defined as:

1 second is defined as 9192631770 times the transition period between the hyperfine energy levels of the undisturbed ground state.

Note that although the second is redefined, it does not change the actual size of the second, but is compatible with the previous definition (the long number in the above definition is the result of accommodating the previous definition). In other words, a day is still approximately 86400 seconds. However, the standard for the production and replication of seconds adopts a more sophisticated method of atomic physics.

Of course, the time of the solar day is actually changing. Specifically, there are two changes: one is that there will be ups and downs within a year, but the ups and downs are positive and negative, so they will not accumulate; second, due to the tidal force of the moon, the self-transfer of the earth will gradually slow down, so the solar day will gradually become longer, but the change is very slow. So, on the whole, the solar day is generally considered to be 86400 seconds.

The second is the sidereal day (Sidereal day), which refers to the interval between two consecutive times when a meridian of the earth is facing the same star. Because the stars are extremely distant, this time is the period in which the earth rotates 360 degrees, 23 hours, 56 minutes and 4 seconds. In other words, the sidereal day is the rotation period of the earth.

Due to the tidal force, the period of the earth's rotation will be longer, so the value of the sidereal day is also slightly increasing.

An interesting question: why is the solar day a little longer than the sidereal day?

It can be understood according to the following picture.

There is a sidereal day between 1 and 2, and the earth happens to have completed a complete rotation-a meridian An is facing the star again. However, at the position of 2, the light of the sun does not shine directly from the top of the head at meridian A, and the earth has to continue to walk for a short distance to make the sun face there.

In this process, the earth rotates at a small angle and reaches the maximum height of the sun when it reaches the position of 3, so there is a solar day between 1 and 3. It is 3 minutes and 56 seconds longer than the stellar day.

The term solar day is used more often than sidereal day. Usually we say a day, generally default refers to the sun day, it is 24 hours, that is, 86400 seconds. When we say how many days there are in a year, we also mean the sun day.

After talking about the day, let's look at the New year.

The first is the return year (Tropical year), also known as the solar year (Solar year), which refers to the interval at which the sun passes through the vernal equinox twice in a row. The specific length of each return year is different, each passing year, the return annual meeting produces a 6.14 × day length change. At present, the size of the regression year is 365.24219879 days, that is, 365 days, 5 hours, 48 minutes and 46 seconds.

The second is the sidereal year (Sidereal year), which refers to the actual time it takes for the earth to orbit the sun. This is the time it takes to observe from the earth, starting with the sun and a star in the same position, when it is observed that the sun returns to this position. A stellar year is about 20 minutes and 24 seconds older than a regression year.

It should be noted that sidereal years and regression years are two different time systems. Sidereal years are only used in astronomy. They record atomic time.

Both the return year and the sidereal year are changeable. In other words, the number of seconds contained in these years is uncertain. Since the length of these years is uncertain, the "year" of the light year cannot be any of these "years", otherwise the light year will become an uncertain length.

The "year" in the light year is another redefined year, called the Julian year (Julian Year), which is equal to 365.25 solar days, or 365.25 86400s.

The Julian year originally refers to the length of a year in the Julian calendar, which refers to 365 or 366days, with an average of 365.25 days. Julius was designed by Julius Caesar with the help of Greek mathematicians and astronomers and came into use on January 1, 45 BC. Because it is significantly larger than the actual solar year (365.24219879 days), adding one day every 128years, this error is a bit large, so it was replaced by the Gregorian calendar year in 1582.

Now, the Julian year is only an astronomical unit of time, with a size of 365.25 86400 seconds, or 365.25 solar days. The Julian year does not correspond to a particular historical year, so the Julian year is now not related to any calendar or to the definition of any other form of year.

Let's talk about the year we often talk about in our daily life. It's called the Gregorian year, which is the average length of a year specified in the Gregorian calendar. The Gregorian calendar, also known as the Gregorian calendar, was approved by Pope Gregory XIII and came into use on October 15, 1582.

The size of the Gregorian year is 365.2425 days, or 365.2425X86400 seconds, which is very close to the actual length of 365.24219879 days of the solar year, and will only cause an one-day error in nearly 7, 700 years, which is why the Gregorian calendar can replace the Julian calendar.

You might ask, since the average length of the Gregorian year is closer to the regression year than the Julian year, why not use it to define the "year" in the light-year unit?

To put it simply, the Gregorian year involves too many things, and since the Julian year has retired from the historical stage and is idle at home, using it to define a new unit will not have any bad influence. By the way, I also commemorate this calendar year, which has been used for a long time in history.

In fact, the Julian year is not a unit of measurement in the International system of Units (SI), but a unit used specifically for the International Astronomical Union (IAU) and has become the basis for the definition of light years.

Let's calculate, how long is one light-year?

Yes, 9460730472580800 meters, which is the exact length of a light year. Note that the word "exact" means that it does not have any approximation, and its value is artificially defined.

Therefore, light year, like the speed of light in a vacuum, is also a defined exact value!

This article is from the official account of Wechat: University Physics (ID:wuliboke), by Xue Debao.

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