This is a good question that I can't find any solid answer on. The full, half, quarter, etc. moon phases happen because of the location of the earth between the moon and the sun. We have a small creek on our property that goes dry during the summer. I've noticed that when the moon is showing that the water comes to the surface and runs in the creek and when the moon is over the horizon that the creeks surface dries up again. It might be my imagination but I seem to get better depth with my detector when the moon is viewable during the day. The moons gravitational pull creates the oceans tides. Does it affect the ground moisture as well? Could it be that at certain times the moon slightly raises the ground moisture and also the halo surrounding metal objects such as coins? Food for thought. This subject was brought up on Findmall on the Metal Detecting Forum. To read the whole thread go to page 28 and scroll down to 3/18/2011 under the heading "Moon Phase".