Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Stupid question looking for stupid answer

  1. May 18, 2004 #1
    Dozen of science site, offical or not, are saying that moon is always the same apparent size. Most of these site are explaining the perception of size as a simple legendary optical illusion. Some personnal experience lead me to believe that it was not!

    After about 6 month waiting, I finally had a chance to have one of these very large moon that are said to be "illusion".

    Making a simple measurement device that will measure the apparent size of the moon and compare it with an everyday moon about 3 months after, it appears that the large moon was in fact more than 2 time the size of the regular moon.

    Who said first that this was an illusion and why? Any explanation? Any one with similar experiment?
  2. jcsd
  3. May 18, 2004 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    If you got a difference that large than there was a flaw in either your equipment or methodology.

    The maximum actual change in the moon's apparent size is about ten percent as per this site:


    This has nothing to do with the illusion that the Moon is larger when near the horizon.
  4. May 18, 2004 #3

    My instrument is very simple and do not use any optic, the methodology is always the same. I did the verification 3 times in the last 7 years always with an impressive difference far from 10%, in fact the largest difference was close to 2.3 times bigger.

    The only optical illusion that I though to was the possibility that the moon size was amplified by the different atmosphere sheet when close to the horizon.(measure size, not apparent size)

  5. May 19, 2004 #4


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    How about you explain the device & methodology used?
  6. May 20, 2004 #5


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    When I first noticed the apparent difference in the size of the Moon near the horizon and high in the sky - when I was a kid - I didn't believe what my mother told me (that it is an 'illusion'). Later, when I was in high school, I tested it for myself, by holding a ruler out in my hand, with my arm stretched out, and noted how big the Moon was. I remember being quite amazed that the ruler gave such a different answer from what my eyes were telling me. :surprise:
  7. May 20, 2004 #6


    User Avatar
    Gold Member

    a similar view

    I grew up in a river valley in north central Maine, and in the late fall and early winter months, I noticed that people who didn't spend much time outside would make outrageous comments about the size of the moon when they noticed it grazing over the local mountains and hills, outlining trees, etc.

    Yes, the moon does look bigger near the horizon, but if you are in a northern latitude where the moon sweeps across the back of a local range of mountains or ridges for a long period, it can seem absolutely HUGE around moonrise. I had a really crappy Newtonian as a child (and I am eternally grateful to my parents, who probably had to scrape and sacrifice to buy it!) and I learned early that the moon was always the same size in my telescope, it always looked a lot better when it was high in the sky on a clear cold night.
  8. May 21, 2004 #7
    Moreover when all kinds of mirages distort the actual form of the moon when near the horizon, it increases the illusion of increased size. I've seen it in the Arctic a couple of times

    Found no pics of it but something similar is happening to the Sun http://virtual.finland.fi/finfo/english/mirage2.html [Broken].
    Last edited by a moderator: May 1, 2017
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Stupid question looking for stupid answer
  1. A stupid question (Replies: 11)