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As viewed from a well chosen location at sunset, the lunar disk frames historic Lick Observatory perched on the mountain's 4,200 foot summit. Both observatory and Moon echo the warm color of sunlight (moonlight is reflected sunlight) filtered by a long path through the atmosphere. Substantial atmospheric refraction contributes the Moon's ragged, green rim. Of course, the March Full Moon is also known as the Full Worm Moon. In the telescopic photo, Lick's 40 inch Nickel Telescope dome is on the left. The large dome on the right houses Lick's Great 36 inch Refractor.
A popular belief, stretching back at least to Aristotle in the 4th century B.C., holds that the Moon appears larger near the horizon due to a real magnification effect caused by the Earth's atmosphere. This is not true: although the atmosphere does change the perceived color of the Moon, it does not magnify or enlarge it.
In fact, the Moon appears about 1.5% smaller when it is near the horizon than when it is high in the sky, because it is farther away by up to one Earth radius and also because of atmospheric refraction, which makes the image of the Moon slightly smaller in the vertical axis
A simple way of demonstrating that the effect is an illusion is to hold a small object (say, 1/4 inch wide) at arm's length (25 inches) with one eye closed, positioning it next to the seemingly large Moon. When the Moon is higher in the sky, positioning the same object near the Moon reveals that there is no change in size.
Historically, the best-known alternative to the "apparent distance" theory has been a "relative size" theory. This states that the perceived size of an object depends not only on its retinal size, but also on the size of objects in its immediate visual environment. In the case of the Moon illusion, objects in the vicinity of the horizon moon (that is, objects on or near the horizon) exhibit a fine detail that makes the Moon appear larger, while the zenith moon is surrounded by large expanses of empty sky that make it appear smaller
The effect is illustrated by the classic Ebbinghaus illusion shown above. The lower central circle surrounded by small circles might represent the horizon moon accompanied by objects of smaller visual extent, while the upper central circle represents the zenith moon surrounded by expanses of sky of larger visual extent. Although both central circles are actually the same size, many people think the lower one looks larger.
The "size" of an object in our view can be measured either as angular size (the angle that it subtends [is in opposition to] at the eye, corresponding to the proportion of the field of vision that it occupies) or physical size (its real size measured in, say, metres).
As far as human perception is concerned, these two concepts are quite distinct. For example, if two small, identical, and familiar objects are placed at distances of five and ten metres respectively, then the more distant object subtends approximately half the angle of the nearer object, but we do not normally perceive that it is half the size. Conversely, if the more distant object did subtend the same angle as the nearer object then we would normally perceive it to be twice as big.
A central question pertaining to the Moon illusion, therefore, is whether the horizon moon appears larger because its perceived angular size seems greater, or because its perceived physical size seems greater, or some combination of both. There is currently no firm consensus on this point.
The explanation is based on a pair of illusions known as oculomotor macropsia and oculomotor micropsia. Oculomotor macropsia causes objects to appear to have a larger angular size when we perceive them to be far away based on distance cues. Oculomotor micropsia makes objects seem to have a smaller angular size when they are close to us based on distance clues.
The moon overhead gives us few cues to its distance, so our eyes assume the object is a distance of one or two meters (even though we intellectually know it is very far away) and oculomotor micropsia sets in making it look smaller than we would normally perceive it. When the moon is at the horizon, objects on the horizon such as buildings and trees give us distance clues that the moon is very far away. This causes oculomotor macropsia and we perceive it as being larger than it would normally appear.
Why do oculomotor micropsia and macropsia exist? Apparently to make it easier and faster for us to turn our heads and find an object close to our faces. Because our eyes are on the front of our heads and our heads pivot near the back of our skull, there can be quite a difference between the visual angle we see an object to the left or right and the actual angle we need to turn our necks to put it square in front of our eyes. The closer the object, the bigger the difference. Micropsia helps us instinctively make the right amount of turn when it is important to find an object quickly because it is close and might be a threat to us. Because micropsia exists for close objects, macropsia, the opposite, occurs for far objects. The misperception of macropsia, seeing objects too large, is acceptable for far objects because they do not present an immediate threat to us and we can take our time turning our necks to find them.
Originally posted by bastardo
reply to post by nobody you know
That´s what I was thinking, so I don´t know why these pics are included in this thread, since the big size of the moon in those pics is the result of the lens, and not the moon illusion.