It's called Earthshine. Like Moonshine, if you were on the Moon, but a whole lot brighter, because the Earth is much bigger. A New Moon in our sky means a Full Earth in the Lunar sky.

So when the conditions are right, it's possible to see the disc of the Moon faintly visible, lit up by the Earthshine. Not with the light pollution nwe have in most places these days, but they didn't have that in the time of Sir Patrick Spens.