Science Fiction constantly imagines scenarios where a character time-travels on earth using a time machine. In most cases, the concept seems simple–a character uses a time machine and ends up in the same place on earth at a different time. The difficulty of time travel aside, how difficult would it be to end up at the same location on earth at a different time? And I’m even ignoring geological changes to the earth’s surface.
Let’s say you have a 3D coordinate system for the entire universe, and have compensated for universal expansion. At a given moment, you are on the surface of earth at a specific coordinate. But the earth is rotating every 24 hours. Its axis wobbles. It orbits the sun once a year. The sun is orbiting the center of the milky way galaxy. And our galaxy is hurtling through space at 630 km per second. Because of this, the earth travels 51.84 million km per day. (And we’re worried about an asteroid hitting *us*?)
Just by staying completely still on the surface of earth, you are traveling in a squiggly, rotating, wobbling, looping, swirling path hurtling through space generally pointed away from the center of the universe at a tremendous speed. Just once I would like to see a character finally invent a time machine, use it to go back just an hour, and end up thousands of miles from earth in space because they didn’t bother to compensate for that. As for people who accidentally time travel or cannot control the amount of time they travel…God help ‘em.