When you can see the dark sphere of the moon and the edge of the illuminated side it's referred to as 'the old moon in the new moon's arms.' Since the moon doesn't have an atmosphere to scatter the sunlight around, you might wonder why you can make out the dark portion. It's called 'earthshine' (a subset of 'planetshine'): the dim illumination of a satellite by sunlight reflected off the planet it orbits. The amount of earthshine varies. Oceans reflect the least amount of light (10%), land reflects more (up to 25%), and cloud cover reflects up to 50%. Depending on the conditions of the sunlit portion of the earth determine how much light is reflected and how bright the dark portion of the moon will appear. The brightness of earthshine is being studied as part of climate change research. While all clouds increase the Earth's albedo, the amount of heat trapped in a cloud affects the amount of sunlight reflected.
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