Below are the two sentences. Which one should be used and when?
The Earth is protected every second.
The Earth is protected every moment.
Below are the two sentences. Which one should be used and when?
The Earth is protected every second.
The Earth is protected every moment.
The pedantic answer is that a second is an SI unit of time with a precisely defined duration and that a moment is just a short period of time of arbitrary duration.
In common usage though, the two terms are effectively synonymous. In your example sentences both will be readily understood as meaning that the Earth is continuously protected.
Whether you use second or moment in writing such as this is a matter of style and preference.
A moment is an ambiguous unit of time, it could be anywhere between a few milliseconds and 3 seconds (not necessarily limited to 3 seconds).
A second is a precise unit of time, precisely equal to 9,192,631,770 cycles of the radiation that gets an atom of the element called cesium to vibrate between two energy states1.