Amazing Illusions

Showing posts with label Astronomy Facts. Show all posts

Moon Optical Illusion

The Moon Illusion is created by a conflict between the intuitive and rational parts of the brain. It can be depicted with an inverted Ponzo illusion as shown above. All four moons are the same size, yet on the left, the lower moon nearer to the horizon appears larger than the higher moon. Scroll the picture back and forth and see if you don’t agree.

-astronomy.org

Moon Illusion

Inflatable moon (an Early Illusion)
Our ancestors named one of the days of the week - Monday - after the Earth’s natural satellite. Despite its importance the Moon is the only celestial body not to retain its original mythological name but rather to have been given a decidedly more practical identity (Moon, from the Indo-European root *mens, "the star by which time is measured"). For the Greeks the Moon was Selene, twin sister of Apollo, goddess of love and hunting.

And there is yet another curiosity about the Moon: the Moon Illusion. Stated simply, the full Moon, when just above the horizon, appears twice as large as when it is overhead. Yet the Moon, a quarter of a million miles away from the Earth, always subtends the same angular magnitude (i. e. the angle the Moon subtends at the eye) wherever it is in the sky, roughly 1/2 degree (0.52°). The Moon Illusion has been studied for centuries, and first appeared in a cuneiform inscription on a clay tablet from the Royal Library at Nineveh (Babylon) dating back to the 7th century B.C.! A satisfactory explanation of the phenomenon is still being debated but most agree that the Moon Illusion exists. As explained above, the distance between the Earth and the Moon changes very little – not enough to cause the effects we see. Furthermore, there is an optical contradiction. When the Moon is located at the horizon, its apparent size should be theorically smaller and not larger… As in fact (as shown in fig. 1 above), the Moon at the horizon is a few thousands miles farther away than when it is overhead.

People have thought that the thicker atmosphere along the horizon could act as a magnifying glass enlarging the image of the full Moon when it is on the horizon. That could not be the case as there is not enough atmosphere around the Earth to cause a dramatic lens effect. Anyway, according to the laws of physics, if the atmosphere was really refracting the image of the Moon, it would appear smaller!

Some scientists have proposed that the Moon Illusion effect depends on our perception of the sky as a flat-topped dome the rim of which appears further away than the top of the dome. The effect of this error in perspective is for the Moon overhead to appear smaller than the horizon Moon. The diagram (fig. 2) opposite shows the apparent location of the Moon at various points as it travels across the sky. This is the diagram commonly seen in books promoting this hypothesis… But the diagram can be misleading! The hemispherical flat-topped dome in the picture is not of proven relevance to the effect and ought to be omitted as it falsely suggests a mental process of "projecting" the moon onto that dome.

Others have proposed that the Moon Illusion had to do with the fact that the eye-brain system is designed to work on the horizontal plane, not the vertical plane. On the horizon we process the Moon image in the optimal orientation giving us its true apparent size. Tipping our head back to view the high Moon, we see a non-optimal image. The illusion is not that the horizon Moon is larger, but that the overhead Moon is smaller in size than it "ought" to be. Others have argued that comparisons with buildings and other objects on the horizon are responsible for the differences between the Moon’s apparent size when looking horizontally and looking vertically (this explanation is contradicted by the fact that the Moon Illusion also occurs over open water).

Finally, here is an explanation that is sufficiently satisfactory. The effect of this illusion is due mainly to the fact that our brain interprets the sky as being

farther away near the horizon, and closer near the zenith (directly overhead, see fig. 3 opposite). This isn’t surprising; look at the sky on a cloudy day and the clouds overhead may be a few kilometers above you, but near the horizon they might be hundreds of kilometers away. The Moon, when it’s on the horizon, is interpreted by your brain as being farther away. Since it’s the same apparent size as when it’s high up, your brain figures it must be physically bigger (as illustrated in fig. 4 further below). Otherwise, the distance would make it look smaller. This effect is the well-known Ponzo Illusion (fig. 5.a). Actually, the Moon Illusion effect is the result of a mix of Ebbinghaus size illusion (fig. 5.b) plus Ponzo illusion (see resulting fig. 6).

-archimedes-lab.org

Smiling Moon : Planets Align in a Frown

Monday night (December 1, 2008) there was a bright cosmic frown in the sky. The planets Jupiter and Venus briefly aligned forming two eyes and a moon forming a smiling mouth.

Jupiter is much larger than Venus, it appeared dimmer, because the latter planet is so much closer to Earth.Plus, Jupiter is much farther away from the sun than Venus, so the light bouncing off it is much less intense than the light bouncing off Venus, which hasn't had to travel so far.

It is called a planetary conjunction, the two planets —the brightest in the night sky — appeared extremely close. They won't be this close together and well-placed for viewing again until May 2013.

How Rainbows Are Made ?

There are two raindrops illustrated on the image.

The droplet with the light rays depicted with colored, solid lines has the long wavelength (red) ray going into the observer's eye. The blue ray depicted by the solid line does not enter the eye. The rain drop with the light rays depicted with colored, dashed lines has the short (blue) wavelength ray going into the observer's eye. The long wavelength (red) depicted by the red dashed line does not go into the eye.

Only one wavelength from a rain drop makes it into the eye. But there are many rain drops, in many different positions and because light will interact with each rain drop in the same way, there is steady progression from long wavelengths (red) near the top of the rainbow to short wavelengths (blue) near the bottom of the rainbow with the yellow and green colors in between.

When we see a rainbow be aware that the entire sky is loaded with raindrops. Yet, we see only a narrow band of color we call a rainbow. That is because the eye receives only a narrow band of rays from a narrow band of raindrops. None of the other light rays reflected from the other raindrops make it into our eyes from the particular position we are in at any one time.

What are sun dogs?

Sun dogs are bright spots that appear on either side of the sun when there are thin layers of high ice crystal clouds otherwise known as Cirrus clouds. This can also happen at night. These are called moon dogs.

Brown Sun Bubbling

Our Sun may look like all soft and fluffy, but its not. Our Sun is an extremely large ball of bubbling hot gas, mostly hydrogen gas. The above picture was taken in a specific color of light emitted by hydrogen gas called Hydrogen-alpha. Granules cover the solar photosphere surface like shag carpet, interrupted by bright regions containing dark sunspots. Visible at the left edge is a solar prominence. Our Sun glows because it is hot, but it is not on fire. Fire is the rapid acquisition of oxygen, and there is very little oxygen on the Sun. The energy source of our Sun is the nuclear fusion of hydrogen into helium deep within its core. Astronomers are still working to understand, however, why so few neutrinos are measured from the Sun's core.

-Robert Gendler

What is a Blue Moon?

A real blue moon, of course, is not actually blue, but rather simply the third full moon in a season which has four.
What is a Blue Moon?

This one is surprisingly tough to answer, because of a recent redefinition of when there is a blue moon. The word month originally stems from "moonth" or one rotation of the moon. However, the lunar cycle is shorter than the modern solar calendar of months, so occasionally a year will have thirteen full moons in it instead of twelve. The extra moon is the "Blue Moon".

So which one is the spare? For the last 50 years we have taken a blue moon to mean the second full moon in the same month. However, in the May 1999 edition of Sky and Telescope Magazine, this definition was identified to have been a mistake, made by them in 1946.

The original Farmer's Almanac definition of a blue moon was when a season had four full moons instead of three. Secondly (and this is the really strange part) the third full moon of the four was the one considered the extra one and not the fourth. Finally, the year itself was counted from Yule to Yule (December 25th to December 25th) and not from January 1st to January 1st.

So, after all this, the Farmer's Almanac definition of a blue moon is: the third full moon in a season which has four when counting from December 25th to December 25th (whew!).