Resembling a squat, wide fridge, the world's most accurate clock went on sale for US$3.3 million in Japan on Wednesday.

The "Aether clock OC 020" is so precise that it would take 10 billion years for it to deviate by one second, according to its Kyoto-based manufacturer Shimadzu Corporation.

Known as a "strontium optical lattice clock", it is 100 times more accurate than caesium atomic clocks, the current standard for defining seconds, the precision-equipment producer said in a statement.

The machine, a box around a metre tall, is small for its kind with a volume of around 250 litres. It can also be used in research fieldwork.

Shimadzu is aiming to sell 10 of its clocks over the next three years and hopes its customers will use them to advance scientific research in areas such as the observation of tectonic activity.

Optical lattice clocks have previously been installed in Tokyo's famous Skytree, the world's tallest tower, to test the general theory of relativity, which states that "time flows more slowly in places with strong gravity." (AFP)