Isaac Newton is called Sir because he was knighted by Queen Anne of England. This honor was bestowed upon him in 1705. The title ‘Sir’ is a formal title given to men who have been knighted by the British monarch. Knighthood is a recognition of significant achievements or service to the country.
Newton was knighted for his contributions to science and his role as the Master of the Mint, where he helped reform the currency and combat counterfeiting. His work in physics, mathematics, and astronomy, including the formulation of the laws of motion and universal gravitation, had a profound impact on the scientific community and the world at large.
Thus, the title ‘Sir’ acknowledges his exceptional contributions and the high regard in which he was held by the British monarchy and society.