Apollo 16 Commander John Young leaps in the moon’s low gravity and salutes Old Glory. The bright morning Sun is to the left (east) of the field of view of this image, causing the Lunar Module Orion in the background to cast a westward-pointing shadow. All Apollo landings took place during lunar morning. Apollo 16 Lunar Module Pilot Charles Duke/NASA

One of the questions members of the public most often ask space educators is, “why does the moon change shape?” The answer is, of course, that our planet’s natural satellite does not change shape; it is always a sphere. What changes is the way light from the Sun strikes the side of the moon we can see.

Earth’s moon, like most other Solar System moons, is a synchronous rotator; that is, the period of time it needs to rotate on its axis once is equal to the period of time it needs to orbit its primary once. For our moon, the time required for both one rotation and one revolution about the Earth is 28 days.

This is why humans on Earth see only the moon’s Nearside hemisphere. The Farside hemisphere, turned always away from Earth, remained mysterious until 1959, when the Soviet Union’s Luna III spacecraft imaged it for the first time.

The day/night cycle for the Nearside traditionally begins with new moon. The moon is not visible when it is new; this is because it is at the point in its orbit where it is situated between Earth and the Sun. The Nearside is turned away from the Sun and the moon is lost in the Sun’s glare. Occasionally the moon crosses over the Sun: new moon is the time of partial, total, and annular solar eclipses.

As the moon orbits Earth, the areas the Sun’s light can reach change. Three or four days after new moon, people on Earth who look west in evening twilight can glimpse a slender crescent moon. The horns of the crescent point toward the east, away from the setting Sun. If one looks carefully, one might see that the part of the Nearside not yet lit by the Sun is just barely visible.

This is a good place to mention that the Earth changes shape as viewed from the Nearside. When the moon is new, the Earth is full. The full Earth is about four times larger and reflects about 75 times as much light as the full moon. When the moon is a crescent, the Earth is mostly full. This means that sunlight reflected off the Earth can light the part of the Nearside that direct sunlight does not reach.

As on Earth, the Sun on the moon rises in the east. The line of dawn – the dawn terminator – advances westward a little faster than a typical human can comfortably jog. High mountains and crater rims catch the morning Sun’s bright rays first. Viewed through even a modest telescope, they appear as isolated islands of light amid dark lowlands. As the Sun climbs higher, light fills in the lowlands and crater floors.

Seven days past new, the Nearside is half lit for observers on Earth. This shape, or “phase,” is called first quarter. The first-quarter moon rises in the east as the Sun stands at noon, reaches its highest point at sunset, and sets in the west at midnight.

Fourteen days past new, the Nearside is fully lit by the Sun as viewed from Earth. The full moon is visible all night; it rises in the east as the Sun sets in the west, stands highest at midnight, and sets in the west as the Sun rises in the east. When the Nearside is full, the Earth is new as viewed from the moon. The moon is at the point in its orbit where Earth stands between it and the Sun. For this reason, full moon is when lunar eclipses – during which the shadow of the Earth falls on the moon – can occur.

http://www.wired.com/2014/06/high-noon-on-the-moon-1991/