Forum:Equipping a monster...

Okay, say you equip a face-up monster with an equip spell card, what happens to the equipped card when the monster is flipped face-down? With say, book of moon? Does the card stay on the field useless? Does it's effect come back into play when the monster is flipped back face-up? or, is the equip card "sent to the graveyard" or "destroyed"?

Now what if it were an equip trap card (not literally an equip, but a continuous, like call of the haunted, or birthright)?

I am a little baffled on this.