The Earth rotates once per day (no duh!), and is approx. 24,900 miles around at the equator. It’s turning counter-clockwise from the perspective of someone hovering over the North pole. At noon in Quito (on the equator), you step into your time machine. You go back 6 hours. They opened the door at 6am that morning and found your body crushed to a pulp. Bone fragments too tiny to piece back together, and a really big dent in the floor and one wall.

What happened? At noon, you were on the equator in Quito, which was traveling at 24900 ÷ 24 ( = 1,037 mph), currently on the side of the Earth facing the Sun (that’s why it’s called ‘noon’) in a direction at right angles to a line between Earth and Sun. Send yourself back 6 hours, and you’re still traveling at that speed in that direction, but Quito is a quarter of a revolution away moving at about 1000 mph directly toward the Sun while your velocity (speed and direction) haven’t changed. Your velocity is now directly into the center of the Earth. Result: splat.

… At least, that’s what would happen if you showed up in the same place relative to Earth, with the Earth only spinning in place.

But the Earth is in an elliptical orbit around the Sun, more easily thought of as a circle with a radius of about 93 million miles. That means it is traveling around the Sun at 2*π*93MM miles (circumference of a circle) per year, or about 67,000 mph. So it’s all right, the good news is that you won’t be smashed to a pulp by going backwards 6 hours. Of course, the bad news is that you’ll be in space, and will therefore die by exploding in the sudden zero-pressure of outer space. Alone – a little over 400,000 miles from Quito and the surface of the Earth.

… Or you would, if it weren’t for that fact that the solar system is also rotating around the Milky Way galaxy at just under 500,000 mph. So we may as well forget about the trivial orbital velocity from the Solar System – you’re now approximately 3 million miles away. But still alone and exploded.

Of course, we haven’t taken into account the fact that the Milky Way itself is moving through space at approximately 3 times that speed.

So don’t take up any offers of time travel. You won’t like it (for a very, very short time).

On the other hand, if we could send a rocket backwards or forwards in time – even 5 minutes either way – then we would have found a way to defeat the Earth’s gravity well. The rocket would be out in space instantly – no vast expenditure of fuel. In fact, if we did it right (i.e., the right time and place on Earth, as well as the right amount of time forwards or backwards), the rocket could be placed directly into the gravitational attraction of the planet we want to visit – it would save enormous amounts of fuel. Supplies could go at the same time and bombed onto the planet.

Up the ante by sending the (unmanned) rocket back a thousand years and you may be able to reach another planet on the far side of the galaxy. (Maybe that’s how the Ancients planted the Stargates before they had any to travel with.) Send out really powerful radio pulses in Morse code and drive SETI nuts for the past 10 years, trying to find out how anyone invented an English-based Morse code 20,000 light years away. On a more serious note, you could play this game to look for inhabitable planets. It would be a one-way trip for the colonists, though.

Or again, for the Stephen Hawkins-type astrophysicists among us: how about finding at least one component of our absolute movement through the universe? Drop a nuclear bomb into space, timed to go off 5 minutes after the time-switch (don’t send me a ticket to the launch, thank you). 5 minutes will leave it close enough that the detonation will be visible and you can plot the distance and position. Drop 2 or 3 more at 1-second intervals and you can plot absolute speed, taking into account the angular momentum, the orbit of the Earth around the Sun, that of the solar system around the galaxy, the galaxy around … something bigger?

Just remember – these ideas: copyright – me!