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A shadow is an area where direct light from a light source cannot reach due to obstruction by an object. It occupies all of the space behind an opaque object with light in front of it. The cross section of a shadow is a two-dimensional silhouette, or reverse projection of the object blocking the light.

An astronomical object casts human-visible shadows when its apparent magnitude is equal or lower than -4.[1] Currently the only astronomical objects able to produce visible shadows on Earth are the sun, the moon and, in the right conditions, the planets of Mercury and Venus.

Contents

Variation with time

Shadow lengths change dramatically throughout the day. The length of a shadow cast on the ground is proportional to the cotangent of the sun's elevation angle—its angle θ relative to the horizon. Near sunrise and sunset, when θ=0° and cot(θ) is infinite, shadows can be extremely long. If the sun passes directly overhead, then θ = 90°, cot(θ)=0, and shadows are cast directly underneath objects.

Non-point

For a non-point source of light, the shadow is divided into the umbra and penumbra. The wider the light source, the more blurred the shadow.

If there are multiple light sources there are multiple shadows, with overlapping parts darker, or a combination of colors. For a person or object touching the surface, like a person standing on the ground, or a pole in the ground, these converge at the point of touch.

Shadow propagation speed

Steam phase eruption of Castle Geyser in Yellowstone National Park casts a shadow on its own steam. Crepuscular rays are also seen.
Shadow cast by vapour trail of passing aircraft

The farther the distance from the object blocking the light to the surface of projection, the larger the silhouette (they are considered proportional). Also, if the object is moving, the shadow cast by the object will project an image with dimensions (length) expanding proportionally faster than the object's own length of movement. The increase of size and movement is also true if the distance between the object of interference and the light source are closer. This, however, does not mean the shadow may move faster than light, even when projected at vast distances, such as light years. The loss of light, which projects the shadow, will move towards the surface of projection at light speed.

The projected shadow may appear to have moved faster than the speed of light, but there is no actual physical manifestation moving upon the surface. The misconception is that the edge of a shadow "moves" along a wall, when in actuality the increase of a shadow's length is part of a new projection, which will propagate at the speed of light from the object of interference.

Fog Shadow of Sutro Tower

Since there is no actual communication between points in a shadow (except for reflection or interference of light, at the speed of light), a shadow that projects over a surface of large distances (light years) cannot give information between those distances with the shadow's edge.[2]

Fog Shadow of the South Tower of the Golden Gate Bridge

Fog shadows

Clouds and shadows over the Mediterranean

Fog shadows look odd since humans are not used to seeing shadows in three dimensions. The thin fog is just dense enough to be illuminated by the light that passes through the gaps in a structure or in a tree. As a result, the path of an object shadow through the "fog" appears darkened. In a sense, these shadow lanes are similar to crepuscular rays, which are caused by cloud shadows, but here, they are caused by the shadows of solid objects.


Other notes

A shadow cast by the Earth on the Moon is a lunar eclipse. Conversely, a shadow cast by the Moon on the Earth is a solar eclipse.

On satellite imagery and aerial photographs, taken vertically, tall buildings can be recognized as such by their long shadows (if the photographs are not taken in the tropics around noon), while these also show more of the shape of these buildings.

Inverted shadow of text

A shadow shows, apart from distortion, the same image as the silhouette when looking at the object from the sun-side, hence the mirror image of the silhouette seen from the other side (see picture).

Shadow as a term is often used for any occlusion, not just those with respect to light. For example, a rain shadow is a dry area, which, with respect to the prevailing wind direction, is beyond a mountain range; the range is "blocking" water from crossing the area. An acoustic shadow can be created by terrain as well that will leave spots that can't easily hear sounds from a distance.

Mythological connotations

An unattended shadow or shade was thought by some cultures to be similar to that of a ghost.

It is also believed as an alternative construct that shadows are in fact a representation of God's presence around an object; like a halo. Early eastern beliefs also play to this theory. For example, Vishnu (a prominent Hindu god) would appear to help followers by assisting with tasks by lending some of his extra arms to assist the burden of the person.

Heraldry

In heraldry, when a charge is supposedly shown in shadow (the appearance is of the charge merely being outlined in a neutral tint rather than being of one or more tinctures different from the field on which it is placed), it is called umbrated. Supposedly only a limited number of specific charges can be so depicted.

See also

References

External links


Source material

Up to date as of January 22, 2010

From Wikisource

Shadow
by Edgar Allan Poe


Yea, though I walk through the valley of the Shadow:
Psalm of David.

YE who read are still among the living; but I who write shall have long since gone my way into the region of shadows. For indeed strange things shall happen, and secret things be known, and many centuries shall pass away, ere these memorials be seen of men. And, when seen, there will be some to disbelieve, and some to doubt, and yet a few who will find much to ponder upon in the characters here graven with a stylus of iron.

The year had been a year of terror, and of feelings more intense than terror for which there is no name upon the earth. For many prodigies and signs had taken place, and far and wide, over sea and land, the black wings of the Pestilence were spread abroad. To those, nevertheless, cunning in the stars, it was not unknown that the heavens wore an aspect of ill; and to me, the Greek Oinos, among others, it was evident that now had arrived the alternation of that seven hundred and ninety-fourth year when, at the entrance of Aries, the planet Jupiter is conjoined with the red ring of the terrible Saturnus. The peculiar spirit of the skies, if I mistake not greatly, made itself manifest, not only in the physical orb of the earth, but in the souls, imaginations, and meditations of mankind.

Over some flasks of the red Chian wine, within the walls of a noble hall, in a dim city called Ptolemais, we sat, at night, a company of seven. And to our chamber there was no entrance save by a lofty door of brass: and the door was fashioned by the artisan Corinnos, and, being of rare workmanship, was fastened from within. Black draperies, likewise, in the gloomy room, shut out from our view the moon, the lurid stars, and the peopleless streets- but the boding and the memory of Evil they would not be so excluded. There were things around us and about of which I can render no distinct account- things material and spiritual- heaviness in the atmosphere- a sense of suffocation- anxiety- and, above all, that terrible state of existence which the nervous experience when the senses are keenly living and awake, and meanwhile the powers of thought lie dormant. A dead weight hung upon us. It hung upon our limbs- upon the household furniture- upon the goblets from which we drank; and all things were depressed, and borne down thereby- all things save only the flames of the seven lamps which illumined our revel. Uprearing themselves in tall slender lines of light, they thus remained burning all pallid and motionless; and in the mirror which their lustre formed upon the round table of ebony at which we sat, each of us there assembled beheld the pallor of his own countenance, and the unquiet glare in the downcast eyes of his companions. Yet we laughed and were merry in our proper way- which was hysterical; and sang the songs of Anacreon- which are madness; and drank deeply- although the purple wine reminded us of blood. For there was yet another tenant of our chamber in the person of young Zoilus. Dead, and at full length he lay, enshrouded; the genius and the demon of the scene. Alas! he bore no portion in our mirth, save that his countenance, distorted with the plague, and his eyes, in which Death had but half extinguished the fire of the pestilence, seemed to take such interest in our merriment as the dead may haply take in the merriment of those who are to die. But although I, Oinos, felt that the eyes of the departed were upon me, still I forced myself not to perceive the bitterness of their expression, and gazing down steadily into the depths of the ebony mirror, sang with a loud and sonorous voice the songs of the son of Teios. But gradually my songs they ceased, and their echoes, rolling afar off among the sable draperies of the chamber, became weak, and undistinguishable, and so faded away. And lo! from among those sable draperies where the sounds of the song departed, there came forth a dark and undefined shadow- a shadow such as the moon, when low in heaven, might fashion from the figure of a man: but it was the shadow neither of man nor of God, nor of any familiar thing. And quivering awhile among the draperies of the room, it at length rested in full view upon the surface of the door of brass. But the shadow was vague, and formless, and indefinite, and was the shadow neither of man nor of God- neither God of Greece, nor God of Chaldaea, nor any Egyptian God. And the shadow rested upon the brazen doorway, and under the arch of the entablature of the door, and moved not, nor spoke any word, but there became stationary and remained. And the door whereupon the shadow rested was, if I remember aright, over against the feet of the young Zoilus enshrouded. But we, the seven there assembled, having seen the shadow as it came out from among the draperies, dared not steadily behold it, but cast down our eyes, and gazed continually into the depths of the mirror of ebony. And at length I, Oinos, speaking some low words, demanded of the shadow its dwelling and its appellation. And the shadow answered, "I am SHADOW, and my dwelling is near to the Catacombs of Ptolemais, and hard by those dim plains of Helusion which border upon the foul Charonian canal." And then did we, the seven, start from our seats in horror, and stand trembling, and shuddering, and aghast, for the tones in the voice of the shadow were not the tones of any one being, but of a multitude of beings, and, varying in their cadences from syllable to syllable fell duskly upon our ears in the well-remembered and familiar accents of many thousand departed friends.


1911 encyclopedia

Up to date as of January 14, 2010
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From LoveToKnow 1911

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Bible wiki

Up to date as of January 23, 2010

From BibleWiki


used in Col. 2:17; Heb. 8:5; 10:1 to denote the typical relation of the Jewish to the Christian dispensation.

This entry includes text from Easton's Bible Dictionary, 1897.

what mentions this? (please help by turning references to this page into wiki links)


Simple English

When a beam of light shines onto something, that thing will cast a dark image of itself onto the ground or another surface. This image is usually called shadow. A place where the sun does not reach when it is shining can also be called shade.

To make a shadow, there must be a light source, and an object to shine the light at. As light travels in straight lines, it cannot reach the shadow area without bending around the object. This makes the shadow. A shadow is not completely black though, as some light reflects off other objects to reach the shadow area.








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