Moon

Observations of the Moon from Earth, reveal different phases of the Moon with respect to the percentage of lunar surface reflecting sunlight back to Earth. As the Moon orbits Earth, it comes between Earth and the Sun once a month at the time of a new Moon, and orbits Earth in very nearly the same plane that the Earth orbits the Sun. If the Moon's orbital plane had zero tilt off the Earth's orbital plane, a total lunar eclipse would be visible every month.

There is actually about a 20° tilt, so total eclipses over selected area occur only about seven times a year, when the Earth is directly between the Sun and the Moon. The Earth's shadow falls on the full Moon, and it slowly becomes dark and then bright again as it moves out of the Earth's shadow.

A close-up of the back side of the moon taken from Apollo 13. The large mare area is Mare Moscoviense. The large crater on the horizon is IAU number 221. U.S. National Aeronautics and Space Administration (NASA).