• Question: When there is a half moon, you are seeing a circle cut in half. Shouldn't there be a smooth black to white (silver) gradient rather than this instant "white-one-half black-the-other" effect?

    Asked by conn207 to Antonia, Douglas, Hugh, Matt, Tom on 14 Jun 2010 in Categories: .
    • Photo: Matthew Hurley

      Matthew Hurley answered on 13 Jun 2010:

      Good question – isn’t it the way the light makes a shadow? The earth and moon are massive and so the curves are so slight it’s just like a straight line – so you get a straight distinction between light and dark.

    • Photo: Douglas Blane

      Douglas Blane answered on 14 Jun 2010:

      You’d get that black to white gradient you talk about if the dark part was caused by the Earth’s shadow, which is what happens in an eclipse of the Moon. I think you might also get it if the Moon had an atmosphere.

      What’s happening in a half Moon, though, is that the Sun is shining on the Moon from off to the side, seen from where we are. Imagine looking straight at a football in a dark room, and someone to the side of you shines a torch on it. You’d see the same thing, a circle cut in half.

      The Sun is like the torch and the Moon’s like the football.

    • Photo: Tom Hardy

      Tom Hardy answered on 14 Jun 2010:

      This is a little outside of my subject area I’m afraid. But I can offer an opinion. The “white-one-half black-the-other” margin that you mention is created due to the shadow the Earth creates on the moon. Obviously the whole moons remains there but the shadow hides some of the moon from us, making it dark.

      If you make a shadow puppet on the wall with your hand and use a lamp as the light source (sun) you will see that you can change from a smooth to a sharp edge depending on the position of the lamp behind your hand. This is the same for the moon and the sun. Some times a sharp edge will appear sometimes it will appear softer and more of a gradient and this depends on the suns position in relation to the Earth.

      Also, don’t forget the moon is a long way away and looks very small to us here on Earth. If you were closer or had a telescope you would see that the instant colour change you see isn’t all that sharp really.

    • Photo: Hugh Roderick

      Hugh Roderick answered on 14 Jun 2010:

      I don’t know, but I would guess that because the shadow is being cast by the earth, which has an atmosphere that gets thinner the further out from the planet it gets, that there would be a gradient, but that as we are so far away all we can see is an instant change.

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