 the length of the earth's day can vary (which is why the metrologists were specific about the mean solar day). But even that day slows down and speeds up, only to a miniscule degree, but basing the second on such a varying phenomenon is too inaccurate for some purposes. After some time, because metrologists knew the year was more stable than the day, they changed the definition of the second to one based on a fraction of a year (1/31,556,925.9747). Of course, it is no surprise that the year is not really a stable measure of time either, as the earth's orbit around the sun is slowly changing and, anyway, observations of the sun can be somewhat difficult and inaccur- ate. These days, universal time is determined by observations of distant celestial bodies (technically, there are three forms of UT known as UTO, UT1, and UT2, but that is beyond the scope of what is needed here to explain time). By the 1950s, scientists developed atomic clocks, which were more stable than the earth's rotation (or orbit), and SO in 1967 the metrologists changed the definition of the second to "the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom." Since 1967, cesium atomic clocks have been the standard for defining the second, although there are new generations of extremely accurate optical clocks which might take over at some point. Overseeing and managing the definitions of these units is an organization based in Paris, the Bur- eau International des Poids et Mesures (BIPM), known in English as the International Bureau of Weights and Measures. The BIPM website (https://www.bipm.org) is full of very detailed informa- tion on metrology and International System of Units (SI) units, including the second, SO that is an excellent source should you desire to better understand the definitions of SI units. Effective May 2019, the BIPM changed the definition of the second. It is now described as follows: The second, symbol S, is the S