28 April

1900: On 28 April 1900, Dutch astronomer Jan Oort was born.

By the time of his death, at the age of 92, Professor Oort was recognised as one of the greatest astronomers of the 20th century.

He is best known for his suggestion that the Solar System is surrounded by a vast cloud of perhaps 100 000 million comets. By studying the orbits of long-period comets, Oort noticed that many of them seem to originate in a zone far beyond the orbit of Pluto.

Although he was not the first astronomer to propose such a comet swarm, the as-yet unconfirmed cloud is usually known as the ‘Oort Cloud’.