After Old Cat finished singing "Sailor," it sang "Star Lights Up," followed by "Dear Love," "Friends," and "Heavenly Road." It was like an oldies player, but Tang Yue did not have the right to choose the songs. He could only tolerate Old Cat's rough voice that sounded like a broken tape recorder with unstable voltage.
As it drove, it sang loudly. When Tomcat sang, "That's the Tibetan Plateau! Ooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo —"
After Old Cat finished singing "Heavenly Road," Tang Yue thought it was finally going to stop.
But it started humming again.
Tang Yue was about to say something when he suddenly froze.
He was very familiar with the tune Old Cat was humming. Tang Yue almost hummed along with it. It was the song "Tomorrow Will Be Better."
"The tune you sang to covered a distance of thirty astronomical units in two minutes. It's 114 times faster than the speed of light," Tang Yue said. "That's definitely the fastest thing in the Milky Way."
The Mars Wanderer drove for an hour and a half. Old Cat released the accelerator, stepped on the brake, and turned off the power.
At this time, the Mars Wanderer's battery was left with 5% charge.
"That's it for today. We've driven 30 kilometers. Next, we need to charge the Wanderer." Old Cat jumped out of the car, looked up at the sun, and went around to the handcart to unload the solar panels.
Tang Yue got out of the car and stood beside the Wanderer's wheels to look around. It was a vast expanse of saline-alkali wasteland. There was no village ahead, no shop behind, not even a place to shelter from the sun. Fortunately, this was Mars. If they were on Earth, they would definitely suffer from dehydration and heatstroke under the sun.
Fortunately, they had the Wanderer. The only means of transportation in the desert was the lifeboat. Otherwise, it would be difficult to get out of this dry sea of death by relying on manpower alone.
From where he stood, he could no longer see Kunlun Station. Tang Yue knew which direction Kunlun Station was in, but he could only see a line of dark gray sky.
Old Cat unfurled the solar panels one by one and set them up on the ground. More than 40 solar panels were fully unfurled, occupying an area as large as half a football field. The rest of the day was spent charging. It was now a quarter past eight in the morning, Mars time. Tang Yue and Old Cat had to stay there until the sun set, waiting for the charging to be completed.
Tang Yue connected the battery to the transformer, took out the charging cable, and plugged it into his Bright Light Armour.
Stray Dog was not the only one who needed to be recharged. Old Cat and the EVA suit also needed to be recharged.
The man and the cat finished setting up the solar panels. Finally, the Old Cat took out two small stools from the experiment cabin.
Tang Yue and Old Cat each took one and sat side by side on a small stool, dragging a black cable behind them.
"When can we leave again?"
"Tomorrow morning."
Old Cat replied, "It will take at least ten to twelve hours to recharge. We are expected to leave at seven tomorrow morning and head 30 kilometers northeast."
"The direction won't deviate, right?"
"Deviations are inevitable. As long as we correct them in time, it will be fine." Old Cat returned to the cockpit and pulled out a roll of form and handed it to Tang Yue.
Tang Yue opened it and glanced up and down. The form was densely filled with numbers. He could not understand what they were. The only thing he could understand were the English abbreviations at the top of the form: "Mon," "Tues," "Wed," "Thur," "Fri," "Sat," and "Sun," Monday to Sunday.
"What is this?"
"Star chart," Old Cat replied. "Those numbers are celestial coordinates, marking the positions of celestial bodies."
"Star chart?"
"The sextant can only determine our latitude, not our longitude."
Old Cat explained, "We need other means to determine the longitude of our location. The most intuitive way is to look at the time. On Earth, there are time zones and time differences. You can know your longitude based on which time zone you are in."
"But this is too rough," Tang Yue said.
"Yes, it is too rough."
Old Cat nodded. "Determining the longitude based on the time is actually observing the height and position of the sun. The sun's position on the celestial sphere is different in different areas with different longitude at the same time. But to ensure accuracy, we better not use the sun. We have to use a smaller and more accurate celestial body."
Tang Yue spread the form on his knees. The bottom of the paper dragged on the ground. It was a very long and complicated form. Each number was accurate to four or five decimal places. It reminded Tang Yue of the natural logarithm table in the appendix of his high school math textbook.
"A smaller and more accurate celestial body?"
"Yes."
Old Cat pointed above his head. "We don't use the sun as a standard. We use Phobos and Deimos."
"I remember these two names are …" Tang Yue recalled the meaning of these two nouns. "Mars I and Mars II?"
"Yes, we use the two satellites of Mars as a standard. Kunlun Station has very detailed orbital data. They are very good markers. The star chart in your hand is the celestial coordinates of the satellites when they are observed from Kunlun Station."
Old Cat pointed to the form in Tang Yue's hand. "Now that we have left Kunlun Station, when we observe Deimos and Phobos at the same time, their positions will deviate from the records on the star chart … We can know how far our longitude is from the longitude of Kunlun Station."
Tang Yue pondered for a while. Old Cat's description was too brief. He couldn't digest so much information in a short time.
"Give me an example."
"Well, I'll give you an example. For example, at 7: 30 p.m., we observe from Kunlun Station. Deimos' position is 1.00."
Old Cat further explained, "Then at the same time, at 7: 30 p.m., we observe from here. Deimos' position is 1.01. Its coordinates deviate. 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. Old Cat wasn't joking. It said that they were the Captain Cook of Mars, and now it seemed that it was true. Determining one's position based on celestial bodies and time was a very ancient method. Hundreds of years ago, ocean-going ships even carried nautical clocks that were accurate enough to do this.
Now, he only had this star chart in his hand. He had no cell phone, no radio, no GPS, and no other navigation devices. He could only rely on time, stars, and mathematics.
"Can it be so accurate?" Tang Yue asked.
He was a little worried about the accuracy of navigation.
"Don't forget that this is Mars. The diameter of the planet under our feet is only half that of Earth. The total surface area is only equivalent to the land area of Earth."
Old Cat said, "A distance of three hundred kilometers is negligible on Earth, but here, it can show enough deviation … As long as our calculations are accurate enough, we can determine the longitude to a very reliable value."
"But how do we use this thing?"
"I'll teach you at night. This thing can't be used during the day."
Old Cat took back the chart and looked up at the sun that was gradually climbing above their heads. It turned around and took out the simple sextant from the cockpit. "What we have to do next is to determine our latitude."