Eclipses

Solar eclipse happens when our Moon blocks the light coming to us from the Sun.

If we apply simple logic that can only happen when the Moon is exactly in line, between the Sun and Earth. That in turn means that the day side of the Moon is completely on the far side of the Moon since all three bodies are in line.

Day side of the Moon, completely on the far side of the Moon, is exactly what we call “the new moon” on Earth; i.e. Earth can only see the night side (dark side) of the Moon.

Here's a picture explaining what's described above, not to scale obviously.