 Analemmas (as they are known today) have been used in conjunction with sundials since the 18th century to convert between apparent and mean solar time.Before this, the term has a more generic meaning that refers to a graphical procedure of representing three-dimensional objects in two dimensions, now known as orthographic projection.Although the term "analemma" usually refers to Earth's solar analemma, it can be applied to other celestial bodies as well.