This is basically the problem I encountered with my time machine. Even traveling a few seconds either direction would leave me a few hundred kilometers away from where I started.<br><br>For longer trips I had to devise a system where it could reliably find the Earth, as a jump of, say, 6 months would leave it 300 million kilometers away (relative to the Sun, which, as by far the most massive thing near the Earth, was the most reliable point of reference I could use (though even then jumps of thousands of years would theoretically drop me into interstellar space, due to the Sun's orbit around the Mikly Way)).<br><br>The system I eventually had to come up with was to take advantage of the time machine's four-dimensional nature, and it would 'sample' gravitational fields from various points in time around my destination time, and, using predicting (or, in some cases, post-dicting) astronomy build a simple model of the solar system.<br><br>The trick then was to find out WHERE to land on Earth. It's especially tricky since the Earth's not a sphere, and even ignoring local elevations the equatorial areas are farther from the core (center of gravity) than the polar areas. Tidal forces from the moon don't help, and even though they're small in relation to the Earth, they're huge in relation to me. A fall of a dozen meters (or appearing a dozen meters underground) isn't pleasant. Then you have the fact that the Earth wobbles due to the Moon (that is, the barycenter isn't at the center of the Earth) by several thousand kilometers....<br><br>And THEN I'd have to calculate the momentum change needed to keep me aligned with the Earth's surface. For that, even going forward or backward 6 hours would result in me traveling several hundred meters per second perpendicular to the Earth's surface simply from it's rotation (and 30 km/s from it's orbit around the sun--jumping from night to dawn would be 30 km/s straight downward). And once again the wobble induced by the Moon. Tidal forces here weren't a problem, as they were only a few mm/s, which was about the error of my most accurate calculations anyway.<br><br>The maths were a nightmare. Even with computer assistance, each jump took several days to prepare, as even the smallest error would result in death. And in the end during one of my tests the machine just vanished. I think something faulted and it's now floating somewhere in deep space. :(