How long was Isaac Newton's first reflecting telescope?

Isaac Newton’s first reflecting telescope was about 6 inches long, more precisely approximately 6.25 inches. He completed the prototype in late 1668 while investigating how mirrors could overcome the color distortion of lenses.

The instrument was remarkably compact. Its primary mirror was roughly 2 inches across and made from speculum metal, an alloy of copper and tin. Newton ground and polished the mirror himself, then added a small diagonal mirror to send the image sideways to an eyepiece. That side-mounted viewing arrangement became the defining layout of the Newtonian reflector.

The telescope magnified roughly 35 times despite its tiny size. Newton reported using it to observe the four Galilean moons of Jupiter and the crescent phase of Venus. Its short length made it a striking demonstration that a mirror telescope could be practical, not merely theoretical.

The answer is sometimes confused with the length of later Newtonian instruments or with the mirror’s focal length. Historical accounts also distinguish Newton’s small 1668 prototype from a somewhat later telescope presented to the Royal Society. For a quiz asking about the first instrument, “6 inches” is the accepted rounded answer.