Magnifying glasses operate on a pretty simple concept: when an object is within the focal length of a convex lens, a magnified ‘virtual’ image is produced, as demonstrated by simple ray optics:

… with just the slight problem that the magnified image is always further away. An object at the focal point produces an image that is literally infinitely big – but it’s also at an infinite distance. And that really doesn’t help; the moon is quite sizeable itself, but that won’t show you the dust on its surface.

Still, magnifying glasses certainly seem to have an effect – but …