A microlensing event seen from three positions in space

The path of a light beam will be bent by the presence of mass, an effect explained by General Relativity, and a massive body can therefore act like a lens – a so called “gravitational lens” – to distort the image of an object seen behind it. Scientists first confirmed this prediction quantitatively during the now famous total eclipse of 29 May 1919 by observing starlight bent by the mass of the sun. Microlensing is the name given to a related phenomenon: the short flash of light produced when a cosmic body, acting as a gravitational lens, changes the intensity of visible light from a more distant, background star as the body’s motion fortuitously moves in front of it.