An Easy-to-Understand Explanation of the Moon Terminator Illusion

There’s a strange optical illusion that we all experience, without even thinking about it, relating to the way we see the moon. The Moon Terminator Illusion is that weird phenomenon when the side of the moon that is lit by the sun doesn’t look like it actually lines up with the sun at all. Vsauce host Michael Stevens explores the science behind this illusion and explains perspective, angles, and zoom effects involved:

Our vision is influenced by visual clues and past experiences, making us susceptible to optical illusions. From our perspective from the ground, it often doesn’t look like the sunlight is hitting the moon is a straight line. Instead, the line appears curved. That’s the Moon Terminator Illusion. So, what’s going on with our eyes and brain that we see it this way?

First things first, the terminator. A terminator is the term used to describe the line that separates the illuminated and dark side of the moon (or any object).

illuminated side of moon photo

When we see the illuminated side not aligned with the direction of light creating it, it’s because of visual angles, foreshortening and horizon lines. The same thing happens to our perspective with up-close objects, but the difference is in the clues. The brain is able to use our past experiences and knowledge of the world around us to recognize shapes that are distorted by perspective.

Steven uses a great example of this by simply opening a door. When the door is closed, it is a perfect rectangle. But when opened, the edge closest to us appears bigger, the shape distorted, but we still know the door is rectangular.

angle perspective photo

A door’s shape appears distorted from an angle.

The Moon Terminator Illusion and Dolly Zoom Effect

When it comes to capturing a scene on camera, our brains can be manipulated because the camera crops out the visual cues we normally rely on.

To better portray this better, Stevens explains the difference between moving and zooming in on a scene and why each produces a different effect or perspective. When moving a camera forward—like when doing a tracking shot—closer objects are more distorted than objects further away from view. But, when zooming in, the entire scene is affected in equal measure.

The Dolly Zoom Effect uses both zooming and moving simultaneously to deliver a trippier effect, which was popularized by Hitchcock in his films. The effect keeps the subject the same size while altering the distance and focal length.

camera zoom move

Understanding both the Moon Terminator Illusion and the Dolly Zoom Effect helps us make sense of the strange optical illusions that we see so often. This video does a great job explaining everything. Plus, there are a few fun little experiments you can do to see for yourself how the brain and eyes process what we see.

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3 responses to “An Easy-to-Understand Explanation of the Moon Terminator Illusion”

  1. Robert says:

    I’m a design and drawing teacher and a professional lighting and model photographer.
    I looked at the film.
    I know based on teaching my drawing students that
    Steven doesn’t know how to draw images with a light source in perspective.
    He has a hard time understanding the moon as a light source.
    The moonlight can be measured with a photography lux meter.

    https://mrcotton.files.wordpress.com/2010/04/back-to-basics-perspective1.jpg

  2. G says:

    The viewer, the moon, and the sun form a flat triangle. Any straight line in that triangle must always appear as straight to the viewer, even for example holding up a pencil at arms length.

    The explanation for the terminator angle is not provided above by Jennifer. It remains mysterious.

  3. Bill says:

    This explanation serves as a classic misapplication of deductive reasoning. Deductive reasoning begins with a premise and is thereafter utterly dependent upon that premise for it’s conclusions, whereas inductive reasoning begins with an observation that then results in a premise or conclusion. Though both kinds of reasoning have their proper applications, each of them are also subject to misapplication and error if improperly applied. Jennifer Berube’s obvious premise, that the moon’s terminator is due to the sun’s light striking it, is what leads her to the conclusion that the terminator’s angular disparity can therefore ONLY be the result of an optical illusion. In this she ignores her explanation’s failure to geometrically explain the angular disparity of the moon’s terminator, apparently thinking instead that any time anything (in this case the moon’s terminator) doesn’t look as it should, it’s somehow due to an optical “illusion”, since optical illusion explains lots of other things that don’t look as they should. A far better use of deductive reasoning here might begin with the premise that a sphere’s terminator line is always perpendicular to it’s light source. Sherlock Holms – the literary champion of deductive reasoning – might then be heard saying to Mr. Watson, “Deduction, my dear Watson. The moon’s terminator line isn’t caused by the sun’s light. It’s angle is all wrong”

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