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.
Whoa! Luna's showing her full face tonight.