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Astronomical measurement relies on the earth's rotation, and the unevenness of the earth's rotation makes the accuracy of time (universal time) obtained by astronomical methods reach only 10-9, which cannot meet the needs of social and economic aspects in the mid-20th century. A more accurate and stable standard of time has been developed, known as the atomic clock. All countries in the world use atomic clocks to produce and maintain standard time, which is called "time reference". Then, time signals are sent to users through various means and media, including short wave, long wave, telephone network, Internet, satellite and so on. This whole process is called "timing system".