1020

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political events

Norway's Olaf II Haraldsson gains recognition as king from Faroes, Orkney, and Shetland islanders.

Pisa annexes the Adriatic island of Corfu.

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Communication

Alhazen (Ibn al-Haytham) describes the camera obscura: a real image is projected into a darkened box. The image can be viewed from inside the box or can be projected onto ground glass to be seen from outside the box. Alhazen reports that he did not discover this device. See also 1553 Communication.

In Mizan al-hikma ("on the balance of wisdom") Alhazen discusses how the atmosphere refracts light and uses the onset of twilight as a means to make an estimate of the height of the atmosphere.

Alhazen solves several problems from what is now called number theory involving remainders (equivalent to congruence for numbers) and perfect numbers (a number is perfect if the sum of its factors is the number). He also writes on using lunes for squaring the circle, but may not have realized that this method will not work. See also 100 Mathematics; 1801 Mathematics.

Alhazen determines that light travels from objects to the eye, emanating from some bodies and being reflected from others, and determines laws of reflection in curved mirrors. These laws enable him to explain correctly how lenses work and to describe focusing by parabolic mirrors, such as those used in today's reflecting telescopes. His optical work Kitab al-manazir ("treasury of optics") is influential in Europe after translation into Latin as Opticae thesaurus Alhazeni in 1572. See also 140 ce Physics; 1270 Physics.

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