Phases and Reflections
From the Earth, it can be seen throughout the month that the Moon and some of the planets in the solar system experience regular changes. It is these changes that provide us with information about why they shine, and how they orbit the Sun.
The Moon orbits the Earth. Although the Moon appears to give off a substantial amount of light, it is non-luminous, i.e. it does not produce its own light. In actual fact, it is the Sun's light reflecting off the Moon's surface that is illuminating the Moon as the Moon goes through its phases.
Phases of the Moon
The phases of the Moon are caused by the fact that it orbits Earth while being illuminated by and reflecting light from the Sun.
It begins as a new Moon which, because it is in direct line of sight with the Sun, cannot be seen from Earth.
Next comes a thin crescent, setting just after the Sun in the western twilight.
A few days later, the Moon is roughly due south in the Northern Hemisphere, or due north in the Southern Hemisphere, at sunset. It shows half a disc. This is called first quarter, because at this point of time, the Moon is already a quarter of its way through its orbit around Earth.
Seven days after the first quarter, the Moon rises as a full Moon in the eastern sky opposite the Sun at sunset. It has now orbited halfway around the Earth. Before and after Full Moon, it is called gibbous, which means hump-shaped.
Seven days after the Full Moon, the Moon again shows a half-disc, but this time it faces the other direction. It is now at the last quarter.
A few days after that, it is a thin crescent again, rising before the Sun in the east.
The Moon takes 27.3 Earth days to orbit the Earth. During this period, any observer on the Moon would experience 2 weeks of sunlight and 2 weeks of darkness. This would happen no matter where on the Moon they were located, since the Moon also takes this time to spin once on its axis.
Illumination on the dark segment of the crescent Moon is caused by earthshine – sunlight reflected off Earth. The brightness of the earthshine on the Moon depends on the amount of cloud cover on Earth. Clouds are more reflective than the Earth's surface so the more cloud cover on the Earth, the brighter the earthshine.
However, the interval between successive new Moons as seen from Earth, is actually 29.5 days. This is because the Earth is orbiting the Sun and the changing angle of sunshine on the Moon, as seen from Earth, slows down the phases.
The Other planets
Both Mercury and Venus exhibit phases like those of the Moon. They orbit closer to the Sun than to the Earth and thus appear in full phase only when they are on the opposite side of the Sun from Earth. At other times during their orbits, they show a range of phases including half phase and even thin crescents.
Due to the fact that Mercury is further away from the Earth than Venus, Mercury's phases can only be seen through a small telescope. But those of Venus can be seen with binoculars. Mars is the only other planet to show any noticeable phases to observers from Earth, but it is never less than 89% illuminated. Viewing of the phases would thus be difficult.