Some sci-fi time machines don't go anywhere. They just travel forward or backward in the same place they are occupying. But besides all the calculations of moving in time, the machine would also have to calculate place. I can have my time-machine onboard computer calculate where my living room will be tomorrow, and I can assume it will be at the same height above sea level and the building's foundation, but if I were to go backward or forward many years, how could I know I won't appear eighty feet off the ground? Or if the computer miscalculated I could end up in Earth's orbital path, but thousands of miles from the planet. Now if I saved my pennies and got one of those sorts of time machines that could take me to places beyond my living room, like ancient Greece, say, or Greece 10,000 years hence, the calculations become a lot trickier. Even if I could calculate the exact position of my destination in space, how could I know I won't appear 1000 feet above the ground, or even inside of a mountain?