So I go out every night with a homemade sextant and sight Deneb. It’s kind of silly if you think about it. I’m in my space suit on Mars and I’m navigating with sixteenth-century tools. But hey, they work.

Longitude is a different matter. On Earth, the earliest way to work out longitude required them to know the exact time, then compare it to the sun’s position in the sky. The hard part for them back then was inventing a clock that would work on a boat (pendulums don’t work on boats). All the top scientific minds of the age worked on the problem.

Fortunately, I have accurate clocks. There are four computers in my immediate line of sight right now. And I have Phobos.

Because Phobos is ridiculously close to Mars, it orbits the planet in less than one Martian day. It travels west to east (unlike the sun and Deimos) and sets every eleven hours. And naturally, it moves in a very predictable pattern.

I spend thirteen hours every sol just sitting around while the solar panels charge the batteries. Phobos is guaranteed to set at least once during that time. I note the time when it does. Then I plug it into a nasty formula I worked out and I know my longitude.

So working out longitude requires Phobos to set, and working out latitude requires it to be night so I can sight Deneb. It’s not a very fast system. But I only need it once a day. I work out my location when I’m parked, and account for it in the next day’s travel. It’s kind of a successive approximation thing. So far, I think it’s been working. But who knows? I can see it now: me holding a map, scratching my head, trying to figure out how I ended up on Venus.

• • •

MINDY PARK zoomed in on the latest satellite photo with practiced ease. Watney’s encampment was visible in the center, the solar cells laid out in a circular pattern as was his habit.

The workshop was inflated. Checking the time stamp on the image, she saw it was from noon local time. She quickly found the status report; Watney always placed it close to the rover when rocks were in abundance, usually to the north.

To save time, Mindy had taught herself Morse code, so she wouldn’t have to look each letter up every morning. She opened an e-mail and addressed it to the ever-growing list of people who wanted Watney’s daily status message.

“ON TRACK FOR SOL 494 ARRIVAL.”

She frowned and added “Note: five sols until dust storm entry.”

LOG ENTRY: SOL 466

Mawrth Vallis was fun while it lasted. I’m in Arabia Terra now.

I just entered the edge of it, if my latitude and longitude calculations are correct. But even without the math, it’s pretty obvious the terrain is changing.

For the last two sols, I’ve spent almost all my time on an incline, working my way up the back wall of Mawrth Vallis. It was a gentle rise, but a constant one. I’m at a much higher altitude now. Acidalia Planitia (where the lonely Hab is hanging out) is 3000 meters below elevation zero, and Arabia Terra is 500 meters below. So I’ve gone up two and a half kilometers.