Physics and measurement of time
Gravitational redshift
Also: gravitationele roodverschuiving
The gravitational redshift is a consequence of general relativity: the deeper a clock sits in a gravitational field, the slower it runs. A clock at sea level therefore ticks slightly slower than a clock on a mountain.
Modern optical atomic clocks are so accurate that they measure this difference over a height difference of just a few centimetres, opening applications in geodesy (relativistic height measurement).