The old cat finished singing "Sailor" and sang "Stars and Lights", followed by "Blind Date and Love", "Friends" and "Heavenly Road", just like an old-fashioned song on demand, but Tang Yue did not have the right to choose songs, and could only tolerate the old cat's rough voice like a scrapped tape recorder with unstable voltage.

It drove all the way and howled loudly, and when the old cat sang, "That's the Qinghai-Tibet Plateau, oh-" The high-pitched donkey barked, Tang Yue didn't know how he suppressed the urge to jump out of the car and flee.

After the old cat finished singing "Heavenly Road", Tang Yue thought that it was finally going to stop.

But it started humming again.

Tang Yue was about to say something, but he was suddenly stunned.

The melody hummed by the old cat was very familiar to him, and Tang Yue almost hummed it along, it was the song "Tomorrow Will Be Better".

"The tune you sang traveled a distance of thirty astronomical units in two minutes, one hundred and fourteen times faster than the speed of light." Tang Yue said, "This must be the fastest thing in the galaxy." ”

The Martian stray dog drove for an hour and a half, and the old cat released the accelerator, stepped on the brakes, and turned off the power.

At this point, the rover's battery is still five percent charged.

"That's it for today, we've driven thirty kilometers and the next thing we need to do is to recharge the stray dogs." The old cat jumped out of the car, looked up at the sun, and went around to the cart to unload the panels.

Tang Yue followed and got out of the car, standing on the side of the wheel of the Martian stray dog and looking around, a large area of salt-alkali flat wasteland that could not be seen, no village in front of him, no shop in the back, not even a place to shade the sun, fortunately this is Mars, if it is on the earth, it will definitely be dehydrated and suffer from heat stroke under the sun.

Fortunately, they have stray dogs, and the means of transportation in the desert is lifeboats, otherwise it would be difficult to get out of this arid sea of death with manpower alone.

Standing here, he couldn't see Kunlun Station at all, Tang Yue knew which direction Kunlun was standing in, but he could only see the dark gray line of heaven and earth when he looked far away.

The old cat unfolded the solar panels one by one, erected on the ground, more than 40 solar panels all unfolded to occupy an area as large as half a football field, the next whole day is charging time, it is now a quarter of eight o'clock in the morning Mars time, until the sun sets in the evening, Tang Yue and the old cat have to stay here, waiting for the charging to be completed.

Tang Yue connected the transformer to the battery, took out the charging cable, and plugged in his Mingguang armor.

It's not just stray dogs that need to be recharged, but also old cats and extravehicular suits.

One man and one cat set up the solar panels, and finally the old cat tricked out two small Maza from the experimental cabin.

Tang Yue and the old cat sat side by side on a small stool, both dragging black cables behind them.

"When will I be able to leave again?"

"Tomorrow morning." The old cat replied, "It will take at least ten to twelve hours to recharge, and we expect to leave tomorrow morning at seven o'clock and go thirty kilometers northeast." ”

"The direction won't be off, right?"

"Deviations are inevitable, as long as we correct them in time." The old cat returned to the cab and pulled and pulled, took out a roll of forms and handed it to Tang Yue.

Tang Yue unfolded and glanced up and down, this table was densely packed with numbers, he couldn't understand what it was, the only thing he could understand was the English abbreviations on the table, "Mon", "Tues", "Wed", "Thur", "Fri", "Sat" and "Sun", Monday to Sunday.

"What is this?"

"Catalog." The old cat replied, "Those numbers are the coordinates of the celestial sphere, and they are the positions of the celestial bodies. ”

"Catalog?"

"A sextant can only determine the latitude we are in, not the longitude directly." "We need other means to determine the longitude of our location, and the most intuitive way is to look at the time, the concept of time zones and time differences on the earth, which time zone you are in, you can know your longitude." ”

"But that's too sketchy." Tang Yue said.

"Yes, it's too sketchy." The old cat nodded, "Judging longitude according to time is essentially observing the height and position of the sun - in areas with different longitudes at the same time, the position of the sun on the celestial sphere is different, but in order to ensure accuracy, it is best for us not to use the sun, but to use smaller and more accurate celestial bodies." ”

Tang Yue spread the table on his knees, and the hem of the paper was dragged to the ground, which was really an extremely long and complicated table, and each number was accurate to four or five decimal places, which reminded Tang Yue of the natural logarithmic table attached to the back of a high school math textbook.

"Smaller, more precise objects?"

"Yes." The old cat pointed to the top of his head, "We don't use the sun as a standard, we use Forbes and Demos. ”

Tang Yue recalled the meaning of these two terms, "Phobos and Deimos? ”

"That's right, we use the two satellites of Mars as a standard, there are very detailed orbital data in the Kunlun station, they are very good markers, and this star catalog in your hand is their celestial coordinates when observing the satellites at the position of the Kunlun station." The old cat pointed to the table in Tang Yue's hand again, "Now that we have left the Kunlun Station, when we observe Demos and Fobos at the same time, their positions will deviate from the records on the star catalog...... We will be able to know how far our own longitude is from the longitude of Kunlun Station. ”

Tang Yue pondered for a long time, the old cat's description was too brief, and he couldn't digest so much information for a while.

"You give you an example."

"Well, I'll give you an example, let's say at half past seven in the evening, at Kunlun station, the position of Demos is 1.00." "Then at the same time, at half past seven in the evening, we observed that the position of Demos was 1.01, and its coordinates were off—this is obvious, because the observer's position is different...... This deviation is equivalent to the time difference on Earth. ”

"Knowing the time difference, we can calculate the longitude."

Tang Yue nodded slowly, the old cat was really not joking, it said that they were Captain Cook on Mars, now it seems to be the truth, judging the position according to the celestial body and time is a very ancient method, hundreds of years ago, the ocean-going ships would even carry a sufficiently accurate nautical clock to do this.

Now all he has is a catalog in his hand, no cell phone, no radio, no GPS, no all sorts of navigation equipment, all he can rely on is time, stars and mathematics.

"Can it be that precise?" Tang Yue asked.

He was a little worried about the accuracy of navigation.

"Don't forget that this is Mars, and the planet beneath our feet is only half the diameter of the Earth, and the total surface area is only the size of the Earth's land area." The old cat said, "The distance of three hundred kilometers is negligible on Earth, but here, a sufficient deviation can be manifested...... As long as our calculations are accurate enough, we can determine the longitude to a very reliable value. ”

"But what does this thing do?"

"I'll teach you at night, this thing can't be used during the day."

The old cat took the form back, glanced up at the sun that was creeping overhead, and turned to take out the rudimentary sextant from the cab, "The next thing we're going to do is to measure our latitude." ”