The fact that the Moon is always moving in respect to the Earth and the Sun is the most important indication as to why it always seems different when you gaze up at the sky. Because it circles the Earth, it appears in various areas and at different times. If we looked down on the Moon's surface with the right equipment, we would see that there are many changes even over small distances. For example, between one side of the Moon and the other there is a difference in elevation which can be as great as 4,430 feet (1,417 meters). This is because the far side of the Moon is exposed to the Earth's atmosphere which fills up the low spots with clouds more than 500 miles (800 kilometers) away.
The Moon is constantly changing shape due to erosion caused by the wind and water. Some parts of the Moon are very old while others are very young. The oldest objects we know about on the Moon are called "craters" and they date back to the early days of solar system development more than 4.5 billion years ago. As craters get older, they gradually fill up with dust and rocks that were once part of their host planet or comet. So the Moon has the appearance it does today because over time it has been shaped by these events.
The Moon's revolution around the Earth causes it to seem differently to us each night. The Moon, you know, rotates around the Earth in the same manner that the Earth revolves around the sun. The Moon completes one revolution around the Earth every 29.53 days. As the Moon passes over a given point on Earth, the angle at which it is seen from that spot changes relative to the direction it is pointing. During a full moon, when the entire lunar surface is visible, the angle of elevation remains constant throughout the month because the center of mass of the Moon is offset from its geometric center.
So the answer is simple: the position of the Sun, the Earth's shadow, and the Moon itself all affect how the Moon appears from our planet. As the Moon moves across the sky each day, these factors cause it to appear to change size, shape, and color.
The Earth's atmosphere also affects how we see the Moon. Because clouds and other atmospheric particles reflect light from the Sun back to our eyes, they make objects on the ground look smaller than they actually are. So if there are clouds overhead when the Moon comes up, it will seem like half of it is always hidden by the Earth.
Another reason for the variation in appearance is the type of camera used. A digital camera captures images by recording the changing voltage levels of an array of pixels attached to the chip that makes up its sensor.
The Moon's fluctuating locations as it revolves our planet cause the Sun to illuminate different sections, generating the appearance that the Moon's form is changing over time. The easiest approach to learn about the lunar phases is to go outside on a clear night when the Moon is visible in the sky and study it. You will see that there are exactly two types of markings on the face of the Moon: maria (moonquakes) and craters. Maria are low-lying regions that seem to burst into life whenever the Moon is close to Earth's surface.
Cratesers can only be seen from certain angles because they appear to be inside out jars stuck on top of each other. When viewed from directly above or below, they look incomplete with a hole in the middle. This is because they are missing their far side! All crater surfaces look alike when they are first formed due to all the energy being absorbed by the rock at that moment. As the Moon ages, its surface crust continues to break up and collapse into existing craters or is thrown away as volcanic eruptions, so new surfaces are created every day.
You may have heard that the Moon is actually getting bigger over time because its gravity is pulling on Earth's mantle plume, causing it to bulge outwards. This is called "lunar tectonics" and it really does happen but it is not significant enough to account for all the changes we see in the Moon's appearance over time.