Quote from answers.com:
“Why can’t you see the back side of the moon?”
You cannot see the “back side of the Moon” because that is the part of the Moon that faces away from the earth. When you look at the Moon, you always see the “front side,” the side that faces Earth.
This is because the Moon is in synchronous rotation, which means it rotates on its axis in about the same time it takes to orbit the Earth. This results in it keeping nearly the same face turned towards the Earth at all times. This is caused by gravity, and the Moon is said to be “tidally locked” with the Earth.
What is considered front side and back side in space?
I would consider the front side being a position of anything I observe opposite my field of vision provided it is illuminated. The front of a car is opposite my field of vision; the front of a aircraft is opposite the pilot and passengers field of vision; when I knock on your front door, it is opposite my field of vision.
In our solar system, we consider the front side as being the side of an object directly opposite the sun (facing the sun). When I view the moon, am I looking at the front side or the backside? From my perspective, I am viewing the front side. From the sun’s perspective, I am facing neither the front side nor the backside. I am facing that portion of the moon that is illuminated by the sun. If earth is the moon’s source of illumination from my perspective then again, I am neither facing the front side nor the backside. I now know that I am not the point of measurement for determining what is the front side and what is the backside.
The front side of anything is that area opposite the source of illumination otherwise I wouldn’t see it. Yet, if I take out the “human visable spectrum“, the front side is the side of an object opposite and facing whatever it is attracting or pulling on it. On earth the front side is the side facing the sun. At night, I would be on the backside. Yet, if I take gravity into account, the front side of two objects would be the area of gravitational flux between the two objects opposite one another.
The front side in space is the gravitational field itself between two objects. (pulling or attraction)
Based on the above, it would seem there is no back side in space. Yet, if one were to examine opposites, the back side in space is the source (or object) facing and pushing all other objects outwards. (pushing or expansion)