Earth and Luna
Luna is the proper name given to Earth’s own moon, hence the word Lunar. ((These terms are used interchangeably in this chapter, and Appendix B only, as this rest of this book refers to other moons). Luna is tidally locked to the Earth so that one side always faces us, but due to a slight rocking motion called libration, we can actually see about 59 % of the total surface area of the moon (not all at once, of course). Many other moons exhibit tidal locking with their own planet. Still, the far side (often incorrectly referred to as the dark side, as it is spends as much time lit as the close side) remained mostly hidden until 1959. In 1959, the Soviet Luna 3 probe photographed this mysterious side (Figs. 2.1, 2.2, 2.3, 2.4, and 2.5).