How do you measure longitude? Hong Tao can do and use a sextant, with this thing, the latitude and longitude of the ship can be calculated through the angle of the sun, and the longitude of the ship can be calculated through the angle between the moon and the stars, and it is relatively accurate, and the accuracy can be within five nautical miles. Even in the 21st century, many ships sailing on the seas still carry this ancient gadget, why? Because modern equipment is always likely to have all kinds of problems, such as running out of power, human error, sunspot explosion, etc. At this time, as long as you have a sextant in your hand, some geometric formulas in your head, and know the lunar distance table, then your ship will at least not deviate too far from the course. This is also the reason why you need to learn so much ancient navigation knowledge to get a sailing class A license, and you must take the exam. It's dangerous to sail a sailboat, and one more thing is one more life!

But at present, Hong Tao can't make a sextant, he can find all the raw materials, but there is only one thing that has nowhere to find, that is, the mirror! You need two small mirrors and two lenses on the sextant, so if you want to make a sextant, you need to be able to make a mirror.

In fact, it doesn't have to be a mirror, and you can make do with a silver plate behind the crystal, but unfortunately Hong Tao can't even get the crystal at present. However, he himself was not in a hurry to sail, the purpose of sailing was to open up new routes, the purpose of opening new routes was to draw charts and train sailors, and the purpose of ocean trade was to make money. He didn't even understand what the Great Song City was like, and he didn't have a sailboat suitable for long-distance sailing, so why was he in a hurry to get a sextant? Even if Hong Tao is given a global GPS positioning system now, he doesn't dare to sail to the ocean with the small wooden boat of Bo Xiaoer, let alone drive Luo Youfu's green eyebrow bird ship, they are too slow, and the performance of wind and wave resistance is too poor, the only advantage is that there are more loads.

Hong Tao and the people of this era are not the same concept at all, he does not seek to carry too much cargo, he only seeks seaworthiness and speed, the profit point of ocean voyage is far away, not how much cargo. Of course, it would be happier to have a large sailing ship with a lot of cargo, strong seaworthiness and high speed, such as the kind of flying shear sailing ship that only appeared in the 19th century. But that thing Hong Tao really can't get it at the moment, he has a belly of nautical knowledge, but for how to make a wooden sailboat is a bottle less than half a bottle, it is not up to the level of being able to design and supervise large sailboats by himself, at most he is giving the shipbuilder a few ideas on the side.

"Uncle Cai, what do you think of this Hong Tao?" Song people go to bed very early, at least the Tan family lies down at 8 o'clock every day, Hong Tao is embarrassed to stay up with Luo Youde, you have to ask people if they are sleepy, people must say that it is not in the way to talk about it again, so Hong Tao does not plan to attract annoyance, or take the initiative to apply to sleep. When a maid took Hong Tao out of the living room on the second floor of the stern, Luo Youde stood in front of the window, looked at Hong Tao, who had already entered the cockpit door, and asked the Ah Cai next to him softly.

"It's hard to say, I've never seen anything like this guy, and his origin is too unbelievable. I've been running in Nanfan for 15 years, and I've been to countries and islands of all sizes, but I've never heard of any Australia! But his conversation, habits, and dress are indeed different from everyone I've ever seen, and it's not something that can be pretended, it's really unbelievable, incredible! At this time, the butler named Ah Cai changed his original respectful expression, sat down on the chair in a big way, and the emphasis of his speech became different from just now.

"He said that he can burn glass, I think he is also proficient in navigation, not only does he know nautical charts, but the conversation with Luo Tian just now is also substantial, can we keep him in our fleet, maybe we can reverse our decline." Luo Youde was still standing in front of the window, looking at the dark sea outside, thoughtfully.

"It should be hard to look at his attitude...... Although this son is very casual, he has a ravine in his heart, unlike a person who is willing to condescend to others. He was just curious about all the furnishings and utensils here, and he didn't have the slightest envy, and he had no interest in our prostitutes, and he didn't have the mood to take a few more looks. It's the first time I've seen such a young and stylish person, old man, it's hard to figure it out! Youde, although you have been troubled since the death of your father-in-law, you are not in a hurry at this time, I think it is better to take a look again. Ah Cai told the details of his observation of Hong Tao again, and the advice he gave was not positive.

"It's okay, then let's take a look, after I arrive in Zhenzhou, I will talk to him more, get along for a few days, and touch his roots. Oops, glass, glass...... He actually said that he could make glass, and that glass was as cheap as porcelain in his hometown, so why hadn't he seen a merchant ship from Australia for so many years? Luo Youde took Ah Cai's suggestion, but his mouth was still chattering, and it seemed that he still hadn't let go of that heart.

Zhenzhou, a small port in Luo Youde's mouth, made Hong Tao fascinated. When the ships are still a few nautical miles away from the port, the sea is different from other places, and the boats, big and small, come and go! It is not at all less than the three Yagang ships of later generations. It's just that the tonnage of these ships is obviously smaller, and more of them are wooden ships with a displacement of tens of tons like the Tanjia row of ships, which are parked in the ocean near the port in the east and west. There are also big ships, some are bigger than Luo Youde's ship, and the appearance is not the same, some ships actually have five masts, and the sail is three layers of upper, middle and lower, just like a flower blooming on the sea, but it is not very helpful for the speed.

"These big ships went to Guangzhou and Quanzhou by way here, and then continued north to Jiangyin, Xiuzhou Huating, Xiuzhou Lupu, Hangzhou, Mingzhou, and Wenzhou. Some of them continued eastward to Goryeo and Japan. Luo Youde stood next to Hong Tao, his attention was not on the boats at sea, but observing Hong Tao's reaction with his spare eyes.

“…… These eight ports all have a city shipping company? Hong Tao was surprised, this Nima Song Dynasty was too open, almost all sea areas were open, and they were all foreign trade ports.

"There are also Zhenzhou and Qiongzhou, but these two places are built by the military towns themselves, mainly to exchange rice from Nanfan, just by transporting rice here by land, the distance is far and expensive, it is better to bring rice back from Nanfan by the sea." Luo Youde is very willing to explain these rules of the Song Dynasty to Hong Tao, not only to talk about it, but also to understand it.

"What if the maritime merchants are not willing to transport rice? I don't think there's any money to be made by shipping rice as much spice. Hong Tao is also willing to discuss related issues with Luo Youde, the Great Song Dynasty is still very mysterious to him, and it is completely different from what he thought before, it is necessary to understand it carefully.