Question: why is time different in space?

Susan Cartwright answered on 20 Jun 2015:

The rate at which time flows depends on the strength of the gravitational field where you are: the stronger the gravity, the slower your clocks appear to tick as seen by an outside observer (you don’t notice any difference yourself).

Here on Earth, we are obviously in the Earth’s gravitational field. So time flows slightly quicker for objects in free space than it does for us. It’s not a big effect, but it is noticeable when comparing clocks on the Earth’s surface to clocks on GPS satellites, which are further from the Earth and therefore in less gravity. The atomic clocks on the GPS satellites are deliberately set to run slow to compensate for this effect: it’s very small (a difference of 38 microseconds per day), but it would be enough to mess up your satnav if it weren’t corrected for.