Atomic clock. Atomic clocks use an 'atomic fountain' to measure the exact length of a second. The fountain contains atoms that oscillate between two energy levels at high speed. Counting these oscillations is the basis for the standard second. The current basis for the international definition of time is the caesium atomic clock, where one second is defined as 9,192,631,770 oscillations of caesium-133 atoms. This atomic clock is used at Hat Creek Radio Observatory, California, USA, to synchronise the Allen Telescope Array (ATA). The ATA is an array of 350 telescope antennas, which (when completed in 2008) will be one of the largest and most powerful telescopes in the world.