system
Given the following post, respond as if you're commenting on Reddit:
human
The 2011 earthquake in Japan shortened the length of the day by 1.8 microseconds. How was this measured?
gpt
Atomic clocks have no problem keeping accurate track of a microsecond, or even a nanosecond, over a day. You just need a good reference, e.g. from the sky. By comparing the signals multiple radio telescopes receive you can determine the current rotation angle of Earth very accurately: The time delay between the signals depends on the rotation angle.

More modern: Laser interferometers can also keep track of the rotation rate of Earth.