Why is 1 side of the moon always illuminated?

Answer 1

Roughly one half of the moon's surface is usually illuminated for the same reasons roughly one half of Earth's surface is usually illuminated. One half of the moon's surface is always visible from Earth, and it's always the same half, due to tidal lock.

The moon is usually roughly half illuminated. This is due to basic geometry: light travels in a straight line (approximately), and the sun is rather far from the moon. This will necessarily mean that roughly half of the moon will be illuminated for as long as the sun shines and nothing shades the moon.

You can verify this at home with a lamp and a ball. If the lamp is sufficiently far from the ball, you will see that it illuminates roughly half of the ball.

Now, there's also the chance that you meant something else entirely by your question. Perhaps you're referring to the "dark side of the moon". There exists the erroneous sentiment that one half of the moon's surface is perpetually illuminated, and the other half of the moon's surface is perpetually dark. That would require the moon to be in tidal lock with the sun, which it is not.

The source of this incorrect belief is likely that the moon is in tidal lock with Earth,

meaning that from Earth we always see the same portion of the moon's surface. That is, the moon's period of rotation is equal to it's period of revolution. The portion of the surface that faces us is not always illuminated, as can be seen from observing the different phases of the moon. The opposite portion of the surface will similarly go through phases, but this can not be observed from Earth.

Answer 2

One side of the Moon always faces the Earth due to a phenomenon called "synchronous rotation," where the Moon's rotational period is the same as its orbital period around the Earth. This means that as the Moon orbits the Earth, the same side is always facing us, leading to the perception that one side is always illuminated.