December 24

1818 – Physicist James Prescott Joule is born. He established that the various forms of energy (mechanical, electrical, and heat) are basically the same, and can be changed into each other. He thus formed the basis of the law of conservation of energy, the first law of thermodynamics. The SI unit of energy or work , the joule (symbol J), is named after him.
1906 – Reginald Fessenden makes the first radio broadcast, consisting of a poetry reading, a violin solo, and a speech.
1968 – The crew of Apollo 8 enter into orbit around the Moon, becoming the first humans to do so. They performed 10 lunar orbits, and broadcast live TV pictures that became the famous "Christmas Eve Broadcast," one of the most watched programs in history.