"i didn't notice if they ever landed but if they do i'll understand the reason" Of course alluding to the law of gravity, but also still (purposefully or not) tying in with the other apparent 'scientific' details in this song such as the sun's dwindling illumination and the moon's decorative origins.
actually, i was just trying to promote unconditional understanding. "if ____ happens, i'll understand." more of a spiritual vibe here