Tonight’s sky holds a half-moon that looks somewhat like the picture on the right. Can you tell me, just by the picture, is the moon getting bigger or getting smaller?
Of course the moon isn’t really changing size. We use the shorthand “bigger” and “smaller” to refer to how much of the moon we see. Or more properly, how much of the moon is illuminated by the sun. Or even more properly, how much of the moon’s surface facing the Earth is illuminated by the sun.
You can see why we often just say it’s getting “bigger” or “smaller!” Or we can use the terms waxing (getting bigger) and waning (getting smaller).
So can you tell, from the picture, is the moon tonight waxing or waning?