laitimes

Why is there a "leap month" in the lunar calendar, and why is there a "leap month" this year? Read in one article!

February 21, 2023 is the second day of the second lunar month. "February 2" has been a very important day since at least the beginning of the Yuan Dynasty. As the old saying goes, "On the second of February, the dragon raises its head." ”

Thirty days later, on March 23, 2023, guess what? Look at the month card: this day is still the second day of February, and the dragon may still have to be burdened to work overtime and raise his head again.

You probably already know that after one month, you can renew for another month for free, which is called a leap month. But why does this leap month fall in February instead of leap January or March? If you want to fully understand what is going on in leap months, please listen to me from the beginning.

01

One month of the lunar calendar

Not necessarily thirty days

To understand leap months, we must first understand the month of the lunar calendar. As you may know, the moon revolves around the earth, and one revolution is basically a month in the lunar calendar. However, due to the faster rotation of the Earth, the moon also rises and sets in the east and west every day. How do we know when the moon has circled the Earth?

Looking at the sky, there is a super large reference, and that is the sun. In particular, as seen from Earth, the orbits of the Moon and the Sun in circles are very close. Therefore, there is a moment in each month, the moon almost turns between the earth and the sun (generally a slight angle difference, otherwise it will be an eclipse), and from the earth, basically only a black shadow can be seen. This moment is called shuò. Note that "Shuo" here refers to a moment, theoretically only a moment. The day where "Shuo" is located is the first day of the first month of the lunar calendar.

The moon then turns, and the half illuminated by the sun is revealed little by little. From Earth, the crescent is getting bigger and bigger. Half a month later, there was another moment, changed to the earth almost staying between the sun and the moon, the bright side of the moon perfectly displayed in front of the earth, this moment also has a name, called "look".

As the saying goes, "the middle of the day moves, and the moon is full, and the moon loses", and after the "hope", the moon will darken a little bit, until finally it returns to "shuo". This day is the first day of the lunar calendar next month.

Why is there a "leap month" in the lunar calendar, and why is there a "leap month" this year? Read in one article!

From one "synodic month" to the next, this cycle is called a "synodic moon". We very much hope that when the moon rotates in a circle, we can bring a watch and turn on time every thirty days, but unfortunately the moon ignores our set.

In fact, it is affected by a complex physical effect called perturbation (how complicated?) Ever heard of the "three-body movement"? The "three-body movement" is a relatively simple perturbation. But the three-body star is actually the "four-body movement", that's another topic... The speed of the moon circling the earth is not stable, one moment fast, the other slow, running a combination of work and rest, so angry that countless genius astronomers and mathematicians have lost their hair for hundreds of years. Therefore, the length of the "synodic moon" is not certain, at most it can be more than half a day.

However, no matter how much the difference is, it can also be counted as an average. Take the data of recent years, on average, a "synodic month" is about 29.530588 days, about 29 days, 12 hours, 44 minutes and 3 seconds. Rounding up, we can count it as 29 and a half days. Of course, every day has to belong to a specific month, and it cannot be said that a certain day is both the Chinese New Year's Eve and the first day of the first lunar month, which is chaotic.

As a result, about half of the lunar month has 30 days, which is called the "big moon". The other half is only 29 days, guess what it's called? What else can you call, "Satsuki". Whether each month is a small month or a large month is mainly determined by the morning and evening of the first day of the first day of the first year of the "Shuo" moment.

The ancients' calculations of the synodic moon were not as fine as they are today, but there was also a gradual process of refinement. Let's take the famous "quaternary calendar" in the Later Hanshu Law Chronicles as an example. In this calendar, a synodic month is 29+ (499/940) days, which is equivalent to about 29.530851 days, which is not the same until the fourth decimal place compared to today's determination, which is still very powerful.

Why is there a "leap month" in the lunar calendar, and why is there a "leap month" this year? Read in one article!

Jiguge's "Seventeen History" and "Book of the Later Han"

A "synodic month" is 29.530588 days. 12 months in a year adds up to 29.530588×12=354.367056 days. The problem is that this number is too far from the time of a year.

02

Year of return: one circle in heaven, one year on earth

Theoretically, a month is when the moon circles the earth; One year, that's the Earth orbiting the sun.

The problem is again: the moon rotates, and we use the sun as a reference; The earth is orbiting, what can we use as a reference? In fact, our reference is still the sun, specifically, the shadow left by the sun. Why can shadows be used as references? By the axis of the earth is oblique.

Why is there a "leap month" in the lunar calendar, and why is there a "leap month" this year? Read in one article!

The Earth rotates skillfully, running crookedly around the Sun

The Earth's axis is the straight line formed by the north and south poles, that is, the axis of the Earth's rotation. Relative to the "ecliptic plane" in which the Earth revolves around the Sun, the Earth rotates "obliquely".

As the Earth rotates, the point closest to the Sun on the Earth (generally called the "direct point") also moves back and forth between the two "tropic lines" of north and south. The area between the Tropic of Cancer is called tropic. The northernmost moment of the direct sun is the "summer solstice", and the southernmost moment is called the "winter solstice".

Why is there a "leap month" in the lunar calendar, and why is there a "leap month" this year? Read in one article!

Schematic diagram of the Earth's rotation

According to general life experience, the sun is high and the shadow is short in summer; In winter, the sun is crooked and the shadow is long, which is related to the change of the direct point of the sun. We put up a sturdy stick on the ground, measure which day of the year has the shortest shadow at noon and which day has the longest shadow, and accumulate several years of data to calculate it, we can basically understand when is the northernmost "summer solstice moment" of the direct sun's point and when is the southernmost "winter solstice moment" of the sun.

Why is there a "leap month" in the lunar calendar, and why is there a "leap month" this year? Read in one article!
Why is there a "leap month" in the lunar calendar, and why is there a "leap month" this year? Read in one article!

Summer solstice, winter solstice, shadow length is different

The length between every two winter solstices is called a "year of return". The good news is that compared to the "synodic moon" of the moon's lap, the "return year" of the Earth's lap is much more stable.

Although the return year is stable, it is not very easy to calculate, after all, the shadow length of the day is only a little different, and the earth has been turning, specific to the year you measure, at the summer solstice, winter solstice these key moments, the sun is not necessarily shining in which country. In the "Jinwen Shangshu Yaodian", it is said that Emperor Yao: "Nai Mei Xi, He, Qin Ruohao Celestial Calendar - Sun, Moon and Stars, Honor People." ...... The period is three hundred and sixty days and six days, and the leap month is set at four o'clock. According to this statement, one of the five emperors, "Di Yao" (who the "five emperors" include, the ancients have different sayings, here is the opinion of the "History of the Five Emperors") sent Xi, and the four sons to run all over the world, worked hard for half a day, and finally calculated that a "year" should be 366 days, which is too much.

Why is there a "leap month" in the lunar calendar, and why is there a "leap month" this year? Read in one article!

Jiajing Min's "Thirteen Classics" and "Shang Shu" book shadow

Every year, there is an extra half a day, and the "year" will be chaotic. The ancients were also aware of this problem. In the aforementioned "quaternary calendar" of the Book of the Later Han, the length of the "year of return" is set at 365+ (1/4) days. Today's astronomers estimate the average length of a regression year to be about 365.2422 days, or about 365 days, 5 hours, 48 minutes and 46 seconds. The "quarter calendar" is not too bad.

You may have noticed that when we discuss this "return year" calendar, the moments mentioned are all solar terms such as "spring equinox", "summer solstice", "autumn equinox" and "winter solstice". That's right, the "twenty-four solar terms" are delineated by the twenty-four equal "return years". The "Gregorian calendar" that we use most commonly today, which is almost a "leap day" for almost four years, is also basically delimited with reference to the year of return, so the Gregorian calendar date of the solar term is basically fixed, and the difference between one or two days every year is not enough.

03

Leap Moon: The secret of the "Medium Gas" decision

The "Year of Return" calendar mentioned above is particularly good and particularly useful for agricultural production. Its only drawback is that it does not match the "synodic moon". Let's take the quaternary calendar as an example. In that calendar, the ancients felt that a return year had 365+ (1/4) days, and a synodic month had 29+ (499/940) days, which was obviously inseparable. Specifically calculate:

=235/19

=12+(7/19)

That return year has 12+ (7/19) months. What is (7/19) month? The ancients' approach at that time was: every nineteen years, we cram seven more months, and these months are called "leap months".

The history of leap months is also very long, at least since the Shang Dynasty, so we can find months like "March in November" in Oracle. However, after the Shang Dynasty, we all "set the leap" by going through a certain month from January to December. So, in 19 years, it is said that there are 19×12=228 flat months, which seven months should be picked to work overtime and set up leaps? The simplest rule is to choose the first month in each lunar year without "medium qi" as the leap month of the previous month.

Why is there a "leap month" in the lunar calendar, and why is there a "leap month" this year? Read in one article!

Medium gas is the solar term that distens even digits. "If people do not have medium qi, they will be weak, and if they do not have medium qi, they will have to leap."

Let's say the lunar month that starts on March 22, 2023. It's a small month. The last day of the last month (March 21) happens to be the "spring equinox", so only one "solar term" in the whole month is Qingming, and then wait until the next "medium qi" valley rain is already the next lunar month. This month does not have "medium qi", so it cannot be a March, but can only be a leap February. This delayed next year's Spring Festival for a month, and it was not until February 10.

Seeing this, I think you should have a general understanding of the source and arrangement of the leap set, and the next time someone is curious about this question, you can answer them.

Author: Fan Baizhou

The cover image and the image in this article are from the copyright gallery

The content of the image is not authorized to be reproduced

Read on