Why does the moon and clouds seem to follow you?
Have you ever noticed, especially when you are in a moving car, that the moon seems to follow you?
The moon is so distant compared to earthly objects around you. It appears to follow along on your journey.
When we move in a car, our brains automatically compare the distant moon and stars to the part of the view that is nearest to us, sweeping past. Meanwhile, because the moon and stars are so far away, they seem to stay in one spot. And it can seem like these objects in space are moving right along with you.
When the car moves 200 feet forward, the relative angle between the direction of motion and the moon remains the same,
The distance between you and the moon is about a quarter million miles. Any distance you travel by car on Earth is small in comparison. So, when you move in a car, no matter which angle you view the moon from, it appears to remain in the same place, following you!
This illusion is due to:
- The moon is so far away from Earth. It orbits at an average distance of 384,400 km, 30 times the diameter of the Earth.
- When we move, the distance is small compared to the distance between the Moon and Earth. Therefore, it does not affect the angle from which we observe the Moon.
- This indicates that the Moon appears in the same angular position and seems to move with us.
Do you want to simplify the answer again? Please have a look at this picture.
Let the distance between A and B be S. The interior angle is ∆θ (the angle between the dotted line and the partially thick, blue in color):
Tan ∆θ =Opposite side /adjacent side = S/ Distance from the moon (d).
∆θ = arctan(S/d).
Suppose that you are driving at about 31 m/s. In one second, the car will have moved 31 meters. S=31 meters
If the distance to the moon is 4 x 108 meters (384400000 meters, rounded to d= 4 x 108 meters).
Therefore, ∆θ = arctan(S/d) = arctan(31/4 x 108 ) radian= 4 x 10-6 degrees.
The angular position seen from the car changes by 4 x 10-6 degrees.
The distance between the moon and you is significant, but your displacement (distance between your initial and final position on the ground) is almost negligible compared to it.
Assume an isosceles triangle with two equally large sides representing the lines of sight. The third side is your displacement, which is negligible.
The far side of the moon
Therefore, the angle between those two lines of view (parallax) will also become negligible. That is why the moon appears to be moving with you when you move in a car.
- If the viewer and objects are far away (like the Earth and moon), the angle between the view lines (parallax) is negligible. In this case, it seems that objects follow us.
- If the object is close to the viewer, parallax will be considerably higher, and we perceive that we are leaving the object behind us as we move forward.
Cloud movement:
The same concept applies to the clouds moving with us when we travel in a car. We can summarize it simply.
It is an optical illusion, a parlor trick. Clouds only move when you blink or look away. Imagine you are moving in a car, and you see clouds following you. It can be a coincidence that you are driving in the direction of the wind, and the clouds are moving with you.
It is because of parallax, the apparent displacement of an object when seen from two different lines of view. The angle between the two lines of sight quantifies the parallax.
The occupants of the cars in front of you will see the clouds moving from a different perspective.
If the object is close to you, the angle will be high, and the parallax will have some appreciable value. Therefore, you leave those objects behind you while moving in your car. Held down by gravity, objects tend to stay in their expected positions relative to each other. It confuses our perception when they do not.
Influence of other factors:
Clouds move as the rapid movement of air currents builds into winds, trying to equalize air pressure, churning up air currents, and forcing them upward as air masses collide.
They usually move horizontally but, in certain conditions, can move vertically for many miles.
Like everything on the planet, clouds are subject to the Coriolis effect. The Coriolis effect is a visual effect of the rotation of the earth.
The Coriolis effect is responsible for our global weather. The planet’s orbit and tilt give us daily and seasonal temperature and pressure changes, which drive the winds.
It is not a physical force, and the path taken only looks as if it is bending.
Its impact can be seen in water and air as currents deflect away from the equator, moving into either the northern or southern hemisphere, depending on which side of the equator they form.
For instance:
When you sit on a train and wonder if it is moving as the train on the other line pulls out.
It is a common experience caused by an object’s relative motion.
Therefore, the brain fixates on something else, and the actual cloud movement becomes misperceived.
Clouds move because of the wind, which can force them to change speed and direction, something we can observe from ground level.
The Earth’s rotation, manifesting as the Coriolis effect, organizes winds into weather systems traveling west-to-east with the rotation.
However, the deceiving factors are:
- The angle of observation
- The distance from the cloud
These provide a false frame of reference for the observer’s brain attempting to work out.
Finding a suitable point of reference is not always easy. For instance, you may be in a fast-moving car looking at the clouds on the horizon. To your brain, you are stationary in the car.
The relative motion of an object is by comparing it to another that can either be moving or fixed.
One example of relative motion is two girls on a train, with another girl observing them from the platform. The two girls sit together and are at rest, stationary. When considering the girl standing on the platform, understand that the girls on the train are traveling very fast.
Another optical illusion is the moon illusion.
Do you know what it is?
Moon illusion
The Moon illusion is an optical illusion that causes the Moon to appear larger near the horizon than similarly it does higher up in the sky. It has been known since ancient times and recorded by various cultures. [1][2] The explanation of this illusion is still in debate. [2][3][4]
Possible explanations
The Moon looks like larger, near-distant buildings than nearby ones in this simulated skyline.
The size of a viewed object can be measured objectively, either as an angular size (the visual angle that it subtends at the eye, corresponding to the proportion of the visual field that it occupies) or as physical size (its original size measured in, say, meters). Perceived size is only loosely related to these concepts, however. For example- if we place two identical, familiar objects at distances of five and ten meters, respectively. Then, the more distant object subtends approximately half the visual angle of the nearer object. It is perceived to be the same size (a phenomenon of size constancy), not as half the size. Conversely, if the more distant object subtended the same angle as the nearer object, it is perceived to be twice as big. [6]
Why does the Moon appear large on the horizon?
Moon appears large because of any of the following:
- Its perceived angular size seems large.
- Its perceived physical size seems large.
- Some combination of both above.
There is currently no consensus on this point.
Psychologists (specializing) in human perception researched the Moon illusion (The 2013 book). The Moon Illusion, edited by Hershenson, offers 19 chapters written by various illusion researchers reaching different conclusions. [7] After reviewing the many explanations in their 2002 book The Mystery of the Moon Illusion, Ross and Plug conclude- no single theory has emerged victorious.[8] They argue that the size of the illusion is variable but is usually an apparent increase in diameter of about 50 percent. The most significant factor is the sight of the terrain.
Proof of illusion
The angle that the diameter of the full Moon subtends at an observer's eye can be measured directly with a theodolite to show that it remains constant as the Moon rises or sinks in the sky. Photographs of the Moon at different elevations also show that the size remains the same. A simple way of demonstrating that the effect is an illusion is to hold a smooth round stone (say, 8.4 millimeters wide) at arm's length (64 centimeters) with one eye closed, positioning the pebble so that it covers (eclipses) the full Moon when high in the night sky. Then, when the seemingly large Moon is on the horizon, the same pebble will also cover it, revealing that there has been no change in the size of the Moon.
Across different full moons, the Moon's angular diameter can vary from 29.43 arcminutes at apogee to 33.5 arcminutes at perigee—a variation of around 14% in apparent diameter or 30% in apparent area. [5][6]
It happens because of the eccentricity of the Moon's orbit.
A diagram of the Moon seen against a cloud of the same size, at different heights in the sky. When the Moon is high, the clouds it is against are closer to the viewer and appear larger. When the Moon is low in the sky, the same clouds are further away and appear smaller, giving the illusion of a larger Moon. [6]
Reference:
1 Wade, Nicholas J (1998). A natural history of vision. A Bradford Book. Cambridge MA, London, UK: The MIT Press. p. 377 ff. ISBN 978-0-262-23194-7.
2 ^ goog_508213617Jump up to a b c d e Ross, Helen E.; Plug, Cornelis (2002). The mystery of the moon illusion. Oxford, New York: Oxford University Press. ISBN 019-850862-X
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