At night, in the imperial palace and in the imperial study, several exquisitely crafted clocks are lined up on the cabinet. Each clock represents the time in the same place. From left to right, it is Dunhuang time, Chengdu time, Chang'an time, Luoyang time, Yecheng time, and Guangling time.

There is also Danluo time and Hakata time.

Eight wall clocks show the eight longitudes from west to east. Yuwen Wen looked at these eight wall clocks and remembered the international time wall clocks that many small hotels in later generations would have.

In order to show the "class" cheaply, small hotels will hang several clocks on the front desk wall. Each clock has different time, and then the words such as "London", "Paris", "New York", "Sydney" are marked below.

A few small wall clocks that are basically inaccurate when walking are used to express the international style, which is full of a strong copycat flavor, but for Yu Wen Wen, the row of clocks in front of him are not used to make up for the numbers.

A few days ago, he walked to the Star Gazing Courtyard and saw the clock wall recording time in various places. He found it very interesting, so he asked someone to take a few clocks to the right and take them back as ornaments.

Now, he spread out a "Sketch of the Great Zhou Territory" and placed it on the desk, then took out a candle and a whip... a tape ruler, explaining to Yuchi Chifan what it means to "the time at the same longitude must be the same."

The entry point is Yongzhou that Yuchi Chifan suddenly asked about when the couple was chatting.

According to the observation of the Taishishi Department, the longitude of Xuanhua in Yongzhou Prefecture is very similar to that of Chang'an, which means that Xuanhua in Yongzhou is located directly south of Chang'an. The time between the two places is basically no different. According to the different latitudes, it is converted into a straight-line distance, which is about 2,700 to 2,800 miles.

Similarly, Suiyuan in the Hetao region, Jinyang in Bingzhou Prefecture, Luozhou Prefecture in Luoyang, Xiangzhou Prefecture in Xiangyang, and Jiangling on the north bank of the Yangtze River, because the longitudes are close, it means that the five places are probably on a straight line running north and south, and the time is the same in various places.

The same longitude means the same time, because the determination of time (longitude) is related to the sun, and the determination of latitude is also related to the sun.

Yuwen Wen used candles and a tape measure to explain in an easy-to-understand manner what is "longitude" and "latitude".

After ten minutes of explanation, Yuchi Chifan understood what the longitude and longitude are, but she actually wanted to find out whether Yuwen Wen had the idea of ​​enfeoffing his son to sit in the west road of Lingnan.

Yuchi Chifan did not want his second son Yuwen Weihan to be conferred on the west road of Lingnan by Yuwen Wen, and he has been there for generations since then, but there is no news at all at this time, so there is no way to start the topic.

Yuwen Weiqian was about to return to Chang'an and submitted an "observation report" to Yuwen Wen. Yuchi Chifan wanted to prepare for the future, but he could only indirectly attack. Unexpectedly, he aroused Yuwen Wen's interest in preaching and began to explain to her how the "Brief Picture of the Great Zhou Territory" was drawn.

The principle is simple. First, measure the latitude of an integer from south to north, measure the longitude of an integer from west to east, and then draw a grid diagram using this.

Then, measure the latitude and longitude in each general administration and mark it on the grid diagram.

Then, the latitude and longitude of other major cities and the latitude and longitude of cities along major transportation roads (water routes, land routes) are measured and marked on the grid diagram.

Finally, the trends of the main mountains, rivers and rivers are divided into several surveying and mapping points. After measuring the latitude and longitude, mark them on the grid map, and then connect the surveying and mapping points, and the trends of the mountains, rivers and rivers are created.

This kind of drawing method sounds simple, but it is very complicated to do and has a huge workload. The "Simplified Picture of the Great Zhou Territory" in Yuwen Wen's hand is just a simple picture, for reference only.

If you specifically go to canal construction or marching and fighting, the required maps require high accuracy requirements.

Therefore, more detailed territory maps are still being drawn, and it may take more than ten years or even decades, because it requires countless surveyors to travel all over the mountains and rivers.

One of the important responsibilities of the observatory built by the imperial court at key points in various parts of the world is to measure the latitude and longitude, determine the most accurate "local time" for surveying and mapping personnel, and then compare it with "Chang'an time" to obtain the longitude.

At the same time, the wind speed and weather conditions must be measured every day, and the "rainfall" must be measured when it rains to provide necessary observation data for agricultural production, flood control and disaster relief.

It is precisely because of this that although the officials of the Zhengshitang complained about the spending too much money on the Taishi Secretariat, they had a deep understanding of the responsibilities they shouldered. Therefore, when allocating money and grain to the Taishi Secretariat every year, the decree drafted by the Secretariat will always be passed.

Yuwen Wen became more and more excited as he spoke. Yuchi Chifan saw the topic and became farther and farther away, and he felt a little anxious: "Then... Is there an observatory on the west road of Lingnan?"

"Yes, Guizhou Prefecture has the capital of Guilin. Didn't Aju say it in his letter?"

Aju is Yuwen Weiqian's nickname, but Yuchi Chifan knew that Guizhou Prefecture was "Lingui", so she was very surprised: "Uh... Where is "Guilin"?"

"Hmm? Guilin...I remembered it badly."

Yuwen Wen waved his hand and continued: "It is very difficult to manage the place in Lingnan West Road. Whether it is necessary to restrain the prefectures and counties for a long time, let the local wealthy chiefs serve as local officials, or to send migrant officials to take charge of the affairs of prefectures and counties, it remains to be studied..."

Yuchi Chifan saw Yu Wen's "entering the trap", and suddenly became energetic. Unexpectedly, the other party changed the topic and went elsewhere: "There are many mountains in the west of Lingnan, and there are several main water systems, with many reference objects and obvious reference objects, so the demand for surveying and mapping is far less than that of the grassland..."

"The vast grassland has no references. When people from other places first enter the grassland, they are like ships sailing on the vast sea, and they don't know where the southeast, southwest and northwest are."

"Does Li Guang, the flying general of the former Han Dynasty, know? He is brave and good at fighting, but he always gets lost every time he attacks the grassland, and misses the opportunity for the fight. However, he is rarely named a marquis for many years."

"The Turkic troops lived in the grasslands and came and went without a trace on the grasslands. The same was true for their khan's tent. In the future, when the two countries fought, the official cavalry wanted to find the main force of the enemy on the vast grasslands. If they were not careful, they would easily get lost and even be lured into the encirclement by the other party."

"Just just as sea ships need effective means to observe the longitude and latitude on the sea and determine their own position, the official army attacks the grassland and also needs to measure the longitude and latitude and know their location."

"Then, a precise latitude and longitude map of grasslands, as well as fortresses built at key water sources, became the key to winning the war."

Yuwen Wen took out another sketch, spread it out, and introduced it with great enthusiasm. Yuchi Chifan sighed in his heart and could only make an interesting statement in order to cooperate with Yuwen Wen's explanation.

"To put it bluntly, it is to grid the grassland by measuring the longitude and latitude!"

"The main water sources, main rivers, and the main grasslands of each season are all marked on gridded maps, and then, according to actual conditions, build forts at the main locations and equip them with artillery!"

"These fortresses control water sources and main grasslands. The garrisons rely on artillery and sufficient food reserves to hold on for a year without reinforcements. When the cavalry of the official army attacks the grasslands, they will rely on these fortresses to sweep away the grasslands and water sources."

"At that time, only the herdsmen hired by the Central Plains Trading Company can grazle these grasslands. Some of the grazing cattle and sheep will be supplied to local fortress garrisons. When encountering a white disaster, people and animals can also shelter from the fortress."

"The area controlled by the fortress on the outer edge is that nomadic tribes can grazing, so they pay taxes first. If they don't pay taxes, they don't want to eat grass quietly!"

"This is grid-based. It relies on the steamship ships of the Yellow River to support the Yinshan defense line on the outer edge of the Hetao. Then the imperial court used Wuchuan at the northern end of the Baidao as the support point to grid-based control of the grassland."

"Relying on firearms, the fortresses advance, gradually encroach on and control the grasslands, this is the expansion of Tsarist Russia eastward..."

The voice of his speech came to an abrupt end because Yuwen Wen found that he had lost his words again. This situation is rare because Yuwen Wen is very alert, and only when he is alone with his most trusted wife, his alertness will be minimized.

"Uh..." Yuchi Chifan saw that Yuwen Wen suddenly stopped explaining, and was a little puzzled: "Erlang, what is 'killing goose'."
Chapter completed!