Half Moon – why does the shadow look straight?

Posted on 19th September 2010 in half moon/ jaffa cakes/ shadow of the moon/ the moon

When my 10 year old was about 5 he asked me “when it’s half moon, why does the shadow look straight?”

I asked a friend at the time who told me to tell him this:

Half moon is still curved, but only looks straight. Optical illusion.

When the moon is reflecting more light when the earth first starts to cause a shadow, the dark area is more pronounced, plus it’s only covering part of the curvature of the moon, so the shadow looks smaller and more defined.

By the time of half moon, a larger part of the circumference of the earth is casting the shadow and the curve is less pronounced. Plus, with less light reflecting back, the sharpness of the shadow image on the moon is reduced.

Added to this, the moon is also curved, which means the shadow disappears over the moon horizon. Together they make it look straight to the naked eye.

This satisfied the 5 year old (although many other questions followed) but even now I look up at the half moon and see that oh so straight line, I still frown and think how?!

I also can’t think about it without the Jaffa Cakes advert running repeatedly through my head – do you remember it?

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  • Reply @MinaCab 20th September 2010 at 10:18 am

    Well of course *now* I’ll always think of that advert when I see the moon. And thank you so much for putting that in my head!

  • Reply Whimsical Wife 20th September 2010 at 8:41 pm

    Obviously I want Jaffa cakes right now….

    • Reply mrsljhall 21st September 2010 at 11:31 am

      I’m quite lucky as I can’t stand them, so it’s just the half moon/full moon/half moon/full moon that I’m left with rather than a hankering for jaffa cakes!

  • Reply Him Up North 20th September 2010 at 8:46 pm

    My fall-back answer for such questions: “It just is.”

    • Reply mrsljhall 21st September 2010 at 11:32 am

      That is also my favourite answer, sadly my boys don’t accept it anymore!

  • Reply @Twundit 20th September 2010 at 9:05 pm

    The shadow of the earth has nothing to do with it. The moon is a sphere, and if the sun shines on it from one side, the other side is in the shade. We call it half moon when that line is nice and straight! .. and as a there are quite a lot of days in a month, there is always one day when that line is at very straight.

    When the shade of the earth falls on the full moon, it is called a lunar eclipse.

    • Reply mrsljhall 21st September 2010 at 11:52 am

      oooh thank you for your answer – interesting to have a different opinion :)

  • Reply Kate 21st September 2010 at 6:50 pm

    My dad was a proponent of the “just BECAUSE” school of answering. But I know I was a pain in the ass child that kept asking why.

  • Reply carol redmond 8th February 2012 at 4:54 pm

    hi! got to your page while trying to find out when ‘that jaffa cake ad’ was aired! but cant find anything on it! found the youtube clip of course but thats all. no date on it. do you know?

    also, the ‘its just because’ answer is my favourite! carol, wicklow ireland

    • Reply CoffeeCurls 8th February 2012 at 6:25 pm

      Hi Carol – sorry I’m going to be no help what so ever as have no idea! Actually, I’ll ask the good people of twitter if they know and I’ll get back to you :)

  • Reply SpaceBunker Needs YOU! | CoffeeCurls - A Family and Lifestyle Blog 24th March 2015 at 9:34 pm

    […] from a very young age (approx 5) I’ve had to try to explain things to him like why there is a straight line on the moon when it’s a half-moon and why the sky is […]

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