While Christians celebrate Christmas for a particular birth almost 2000 years ago, scientists have two other anniversaries to honor: English physicist James Prescott Joule was born on Christmas Eve in 1818, and Isaac Newton (left) arrived in the world on 25 December 1642.
Legend has it that Joule, the son of a wealthy brewer, spent much of his honeymoon investigating a waterfall. By measuring the water temperature above and below, he demonstrated that the kinetic energy of the falling water was converted into heat. This sort of experiment lead to his discovery of the First Law of Thermodynamics, which states that a system's internal energy is determined by the heat absorbed and the work done. The mechanical unit of work, the joule, is named for him.
Sir Isaac Newton comes at or near the top of most any list of great scientists. A mathematician and physicist at Cambridge University, he discovered the Law of Universal Gravitation, began to develop calculus, and discovered that white light is composed of every color. In 1668, Newton also built the first reflecting telescope.