Chongzhen revived the Ming Dynasty

The sale of new muskets and the enfeoffment of seven princes made everyone once again realize the emperor's sincerity and determination in promoting the enfeoffment system.

The royal family and nobles had mixed feelings about this.

Those who were ambitious were happy because after confirming the emperor's sincerity in granting fiefs, they were ready to establish their own fiefdoms.

I am worried about those who want to enjoy their wealth and prosperity. They don't want to go overseas at all.

Compared with the Ming Dynasty, overseas in this era can really be called a wild and desolate place.

Ocean voyages are especially unsafe, and accidents and deaths are common.

After the transition from near-shore sailing along the coastline to long-distance sailing across the ocean, many people have worries in their hearts.

Zhu Youjian had the same concern. The day after the nobles' gathering, he summoned the personnel from the Imperial Observatory and the Calendar Bureau to inquire about the progress on astronomical instruments.

In the absence of ocean communication technology, star positioning is extremely important. The progress of astronomical instruments will drive the innovation of navigation technology.

Xu Guangqi, the person in charge of the calendar revision and the highest-ranking official among them, reported:

"The calendar bureau has made many new instruments, all of which are based on ancient methods and carefully studied. Many of them are innovative and unexpected. They are extremely large in size, extremely precise in division, and extremely accurate in inspection."

"I, together with Li Zhizao, Johann Adam Schall von Bell, Luo Yagu and others, have selected calendar instruments, explained their methods of manufacture and use, and compiled a 10-volume book entitled The Complete Meaning of Measurement, which I respectfully present to Your Majesty."

Zhu Youjian was very happy to hear this, and took the book presented by Xu Guangqi and flipped through it. He found that it introduced the basic knowledge of area measurement, volume measurement, plane trigonometry, spherical trigonometry, and the manufacture of surveying and mapping instruments, and contained a lot of geometric knowledge.

The part involving astronomical instruments includes ancient Western instruments such as the three-direction parabolic instrument, the six-ring instrument, the astronomical instrument, and the sagittal instrument, as well as advanced Western measuring instruments such as the altimeter, the horizontal theodolite, the equatorial theodolite, the ecliptic theodolite, the sagittal instrument, and the astronomical instrument.

He seemed to understand this, but he didn't have time to learn more about it, so he asked Xu Guangqi directly:

“Is there an accurate way to measure longitude and latitude?”

Xu Guangqi replied:

"Measuring longitude and latitude is not a difficult problem."

"When the eunuch Sanbao sailed to the West, he was able to use the star-pulling board to measure latitude. Now it is even more accurate."

"Based on the calculation that the celestial sphere is 360 degrees, each degree has 60 minutes, and each minute has 60 seconds, Beijing is about 40 degrees."

"To the minute, it's about thirty-nine degrees and fifty-five minutes."

"Taking the meridian of the capital as the center, a difference of thirty degrees from east to west equals one hour."

"The differences in the provinces have not been measured, but according to the Guangyutu Jili estimate, Nanjing Yingtian Prefecture and Fujian Fuzhou Prefecture are both one degree east, Shaanxi Xi'an Prefecture and Guangxi Guilin Prefecture are eight and a half degrees west, and Yunnan Prefecture is seventeen degrees west."

This data is quite close to what Zhu Youjian knows.

What made him particularly satisfied was that Xu Guangqi and others even knew the time difference and did not require him to explain it in detail.

This made him say to Xu Guangqi with admiration:
"Very good! Mr. Xu is very thoughtful."

"From now on, the time of the Ming Dynasty will be based on the meridian passing through Beijing, and will be set as Beijing time."

Then he turned to Xu Guangqi and others:

"The reason why I want to build a new clock tower is not only to tell the time to the people of the capital, but also to determine the Beijing time."

"The meridian passing through the capital is the prime meridian, which is also the zero degree longitude. The west is called the west longitude, and the east is called the east longitude."

"If the equator is zero degrees latitude, the north is the north latitude and the south is the south latitude."

"The longitude and latitude coordinates of Beijing are 39 degrees 55 minutes north latitude and 0 degrees longitude."

"What are the longitude and latitude coordinates of Nanjing, Xu Qing?"

Xu Guangqi calculated and said:

"Nanjing's latitude is about 32.5 degrees, one degree east of Beijing."

"The latitude and longitude of Nanjing are approximately 32 degrees 30 minutes north latitude and 1 degree east longitude."

Zhu Youjian clapped his hands in praise and said to Xu Guangqi:

"Very good! It seems that Mr. Xu understands the longitude and latitude."

"In the future, we will send people to the provincial capitals to determine the longitude and latitude coordinates and draw more accurate maps." "The time difference between various places and the capital must also be determined, and Beijing time will be used for all official business."

This kind of thing would be very simple in later generations, but it is very difficult now when communication is inconvenient.

Nowadays, the main timekeeping instrument in various places is the sundial, and the time used is naturally the local time.

Xu Guangqi estimated the difficulty and said to the emperor:
"In this way, we can determine the precise longitude of each place and calculate the difference with Beijing time."

"Then the time measured by the sundial was corrected to Beijing time."

"I am worried that there are not enough officials in the local area who understand astronomy. Time correction will have to rely on the Imperial Astronomical Observatory."

"The Imperial Observatory needs to expand its staff to do this."

Today, the Imperial Observatory belongs to the inner court, and its funds are allocated by the inner court.

Zhu Youjian thought about it and decided to take it slow. After all, his financial situation was very tight and he couldn't invest more money in this area.

There were not many people studying astronomy in the Ming Dynasty, and it would take some time to accumulate talents.

So he ignored the eager eyes of the officials of the Imperial Observatory and said to Xu Guangqi and others:

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"Determine the longitude and latitude first. The map of the Ming Dynasty needs to be more accurate."

"After determining the longitude and latitude, the time difference can also be calculated. Then we can make a table and let all places adjust and report Beijing time."

Xu Guangqi nodded and asked:
"Based on the meridian passing through Beijing, which line is it?"

"Is it the Zi line or the Meridian line? Does it refer to the Zi hour or the Wu hour?"

Zhu Youjian wanted to say midnight without hesitation as the beginning of a new day, but after thinking about it, he found that it should be the meridian.

This is because with current technology, it is easiest to determine noon by looking at a sundial.

Therefore, he combined the time zone division of later generations and said to Xu Guangqi and others:
"Zero degrees of longitude is the central time zone, so noon is of course twelve o'clock noon."

"At 12 noon in Beijing, 15 degrees west is 11 o'clock in the afternoon, and 15 degrees east is 13 o'clock in the afternoon."

"The area between 15 degrees west longitude and 15 degrees east longitude, which is in the noon time range like Beijing, is called the noon time zone, or the central time zone."

"If we use the hourly system, the area from zero to fifteen degrees east longitude is called East Zone 1. The easternmost part of East Zone 1 is one hour ahead of the zero degree longitude."

"Zero degrees to fifteen degrees west longitude is called West Zone 1. The westernmost part of West Zone 1 is one hour behind the zero degree longitude."

"The circumference of the sky has 360 degrees of longitude, and the east and west longitudes have 180 degrees each. It is divided into 12 major time zones and 24 hour zones."

This is different from later generations. The central time zone was set up at the former site of the Greenwich Observatory, ranging from 7.5 degrees west longitude to 7.5 degrees east longitude, and the East Zone 7.5 was from 22.5 degrees to degrees east longitude.

However, the Ming Dynasty still used the hour system, with the middle of the noon at noon and 15 degrees east and west forming a large time zone. The hour zone naturally had to accommodate it, and the East Zone 1 became 0 degrees to 15 degrees east longitude, 15 degrees east longitude, one hour earlier than the prime meridian, and located at the easternmost end of the East Zone 1.

Xu Guangqi and others did not know the difference, they thought this division was reasonable. And starting from the noon time zone, the 15th to 45th degree east longitude was set as the Wei time zone, including the two hour zones of East 2 and East 3. The 45th to 75th degree east longitude was set as the Shen time zone, including East 4 and East 5...

By analogy, 15 degrees to 45 degrees west longitude is the Si hour zone before noon, including the West 2nd Zone and the West 3rd Zone.

Each major time zone differs by one hour, and each hour zone differs by one hour, which makes calculation very convenient.

Then, based on this line, Zhu Youjian determined that 180 degrees east is the date line:
When crossing this line from east to west, one day is added to the date.

When crossing this line from west to east, the date is subtracted by one day.

Avoid date deviations when sailing around the world.

This line is easy to understand, because the 180 degrees corresponding to the prime meridian is the sub-time zone. When passing through the sub-time zone from east to west, if you only turn back the time according to the time zone without adjusting the date, you will arrive at the previous day. This is against the laws of nature, so of course it must be added manually.

The same is true in reverse. If you don't manually subtract one day from west to east, it will be one day more than the local time. This is also against the laws of nature, so you have to manually subtract it back.

This line is artificially set and is only tentative. The date line will be adjusted in the future according to usage needs.

The International Date Line of later generations is a winding route with many twists and turns. It is determined according to needs, including the countries and regions through which the 180-degree longitude passes.

The Ming Dynasty's sphere of influence has not yet expanded to such a large area, so there is no need to consider this issue for the time being.

(Time zone diagram in the book) (End of this chapter)