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Why the Moon Looks Big at the Horizon and Smaller When Higher Up.

  1. Aug 6, 2004 #1

    Oculomotor micropsia/macropsia seems to be a truly fundamental angular size illusion. It shows up in many different kinds of visual spatial illusions (McCready, 1965, 1983, 1985, 1986, 1994a, 1994b, 1995). The present theory simply proposes that the moon illusion also is an example of the ubiquitous illusion of oculomotor micropsia/macropsia

    Indeed, two independent researchers, J. T. Enright (1975, 1987b, 1989a, 1989b) ) and Stanley N. Roscoe (1979 1984, 1985, 1989) have demonstrated that oculomotor micropsia occurs during viewing of the "zenith" moon, and oculomotor macropsia occurs during viewing of the "horizon" moon:

    In a nutshell, that can explain why the horizon moon looks angularly larger than the zenith moon.

    A demonstration of oculomotor micropsia can be conducted by most readers as follows.

    A Simple Demonstration.

    The next time you look at the horizon moon, deliberately create oculomotor micropsia by strongly converging ("crossing") your eyes, say by looking at the bridge of your nose, but pay attention to the moon. That over-convergence of the eyes will create double vision of the moon and some blurring, but notice that the moon's angular size momentarily looks smaller than it did. At the same time, the moon will look either farther away than it did, or its linear size will look smaller, or else both of those secondary illusions will occur. That illusion imitates what occurs during viewing of the zenith moon. However, in this demonstration the apparent decrease in angular size undoubtedly is greater than the decrease found during natural viewing of the zenith moon. [Indeed, this demonstration of micropsia even works for the zenith moon, which already looks angularly smaller than the horizon moon.]

    When you then return both eyes to being aimed straight ahead (their "far," divergence position) the moon will look single again and momentarily will look angularly larger than it just did (relative macropsia). Hence it also will look either closer than it just did, or its linear size will look larger, or else both of those secondary illusions will occur.

    Try the experiment.
  2. jcsd
  3. Aug 6, 2004 #2
    could it have something to do with light and our atmoshphere. The sky is blye because of this, and at sundown, the sky is red. Our atmosphere allows differenct frequency photons at different possitions in the sky. Im wondering if this could be the solution.
  4. Aug 6, 2004 #3
    The explanation in the link gives other reasons but this one seems to be proven by experimentation.

    Distance Cue Control
    The new theory points out that the reason the focus and convergence of the eyes change during natural viewing of the moon as it rises is because of changes in the visible patterns in the scene near the moon, which patterns are known to psychologists and artists as the many cues to distance, or distance-cues.

    The most likely relationship is as follows: While one is viewing the horizon moon over a typical landscape, the details one sees in the landscape form distance-cue patterns that signal "very far" for objects at the horizon. Those distance-cues for "far" make the eyes adjust for a great distance, and that induces macropsia (larger apparent angular size) for the horizon moon.

    On the other hand, the vista for the zenith moon typically offers relatively few distance-cue patterns, and when distance-cues are sparse, the eyes tend to adjust to a relatively near position, known as the resting focus position, about 1 or 2 meters from the face. That oculomotor adjustment to a relatively near point induces micropsia for the zenith moon

    Cue Conflicts, Again.
    The most common perceptions that the horizon moon either looks about the same distance away as the zenith moon or looks closer disagree with what is being 'signaled' by the distance-cue patterns responsible for the oculomotor micropsia/macropsia. As previously noted, other distance-cue factors are dominating the final percept. The report, "looks larger and closer" undoubtedly is due to relative perceived angular size cue. The report, "looks larger and about the same distance away," undoubtedly is the result of an intrinsic "equidistance tendency" (Gogel, 1965) or an "equal distance assumption" (McCready, 1965, 1985). The intermediate outcome illustrates a balance among the several conflicting distance cue patterns available.

    If this is the explanation then I would have a question. How come when I sit in my room fully lite up, looking out the window the full moon looks larger and my focus is a cue, of the window fram inside the room at about 2 meters. It seems the cue does not have to be at a long distance it can be a short distance also, only a black sky, that drowns out the stars at a full moon, is no cue at all.

    Go to Earth Science for the other answers, there is a few links there to blue skies and red sunsets. Could you pop over there and we could discuss this.
    Last edited: Aug 6, 2004
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