[pvg]

Relativistic effects are measurable with accurate enough clocks in earth satellites (or even just taking a clock to a mountain) and correction is necessary in systems with finicky time requirements like GPS.

Incidentally, the relative speed you can napkin math estimate - an object in orbit flying that close to the planet is going to have escape-velocity-ish speed. It's around 60km/sec for Jupiter.

[dmd]

> taking a clock to a mountain

http://www.leapsecond.com/great2005/tour/

[belter]

They also did the First Atomic Clock Wristwatch...

http://leapsecond.com/pages/atomic-bill/

[cmroanirgo]

Fantastic! Has this been shared on HN before?

> We would come back about 20 ns older compared to her [the wife who stayed behind].

≥ Or, the other way to look at it (since this is relativity after all), is that she would become 20 ns younger than us upon our return. Note to husbands: this could be a useful gift idea for your wife.

[pvg]

It has, in the HN equivalent of the Archean eon

https://news.ycombinator.com/item?id=598090

[atoav]

By now I think they managed to measure relativistic effects of clocks that are a few milimeters apart in terms of distance from earths gravitational center.

[pvg]

Sufficiently clever experimentalists also pulled this off in about 20 meters in 1959, the clever bit being ditching the clocks entirely:

https://en.wikipedia.org/wiki/Pound%E2%80%93Rebka_experiment