Luna

Though usually referred to merely as the moon, Luna is Earth's only natural satellite. For the purposes of referring to Luna indirectly, it is proper to use feminine pronouns, such as "she" and "her." Due to the orbital characteristics of the Earth-Luna system, only one face of Luna is visible at any given time from Earth. This is because Luna's orbital period and rotational period are the same, and her orbit is retrograde to her rotation. Due to this inconvenient orbit, it was not until 1959, with the Soviets' launch of Luna 3, that humans even had photographs of the far side of Luna. Since then, her entire surface has been mapped. As seen from Earth, Luna appears to undergo a series of phases over a period of approximately a month. These phases are not, as is often thought, the result of Earth casting any shadow on Luna. Luna's phases are the result of the angle between Sol (the sun), Luna, and Earth. Because Luna orbits Earth, this angle constantly changes, and consequently, so does our viewpoint of Luna. At any given time, only one side of Luna is ever in any light, cast from Sol. The other side of her is in darkness. (This is not strictly true, since Earth does in fact reflect some of Sol's light onto Luna's dark side.) As our viewpoint of Luna changes, so does our ability to see this lighted side of her. The result is shrinking and growing crescents of light across her face over the course of a month while she moves around Earth. As of April 10, 2004, when this was written, Luna is the only extraterrestrial world visited by humans. Hopefully this will change.