[neilep (OP)]

Loupe
From Wikipedia, the free encyclopedia


A loupe (pronounced loop), is a type of magnification device used to see things one is looking at more closely. In this respect, they are simply a form of a modified microscope, allowing the user to be able to better apply the phenomenon of microscopy to his or her trade.


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[rosalind dna]

M is for the childhood infection - Mumps



Which causes swelling of the throat glands and makes the person feel poorly.





More on mumps here and here

[neilep (OP)]

Niels Bohr
From Wikipedia, the free encyclopedia


Niels Henrik David Bohr [nels ˈb̥oɐ̯ˀ] (October 7, 1885 – November 18, 1962) was a Danish physicist who made fundamental contributions to understanding atomic structure and quantum mechanics, for which he received the Nobel Prize in Physics in 1922. He was also part of the team of physicists working on the Manhattan Project. Bohr married Margrethe Nørlund in 1912, and one of their sons, Aage Niels Bohr, grew up to be an important physicist, who like his father received the Nobel prize, in 1975. Bohr is widely considered to be the one of the greatest physicists of the 20th century.


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[Simulated]

O is for Ostrich


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The Ostrich (Struthio camelus) is a large flightless bird native to Africa (and formerly the Middle East). It is the only living species of its family, Struthionidae, and its genus, Struthio. Ostriches share the order Struthioniformes with emus, kiwis, and other ratites. It is distinctive in its appearance, with a long neck and legs and the ability to run at speeds of about 65 km/h (40 mph), the top land speed of any bird.[2] The Ostrich is the largest living species of bird and lays the largest egg of any bird species.

The diet of the Ostrich mainly consists of seeds and other plant matter, though it eats insects. It lives in nomadic groups which contain between five and 50 birds. When threatened, the Ostrich will either hide itself by lying flat against the ground, or will run away. If cornered, it can cause injury and death with a kick from its powerful legs. Mating patterns differ by geographical region, but territorial males fight for a harem of two to seven females.

The Ostrich is farmed around the world, particularly for its feathers, which are decorative and are also used for feather dusters. Its skin is used for leather and its meat marketed commercially.

[paul.fr]

P

W is for posts without text. My assumption is that posts here are to enlighten and inform the reader, so simply posting a title and link does neither. Please remember that members and nonmembers print these topics, a simple title and link does not let the person read at their leisure.

Please can we all post some information to accompany any links...even if it's a long, direct quot from said linked page.

[Simulated]

Q is for Queen (Band)

Science of Great Rock Music!

Queen are a British rock band formed in 1970 in London by guitarist Brian May, lead vocalist Freddie Mercury and drummer Roger Taylor, with bassist John Deacon joining the following year. Queen rose to prominence during the 1970s and are one of Britain's most successful bands of the past three decades.[1]

The band is noted for their musical diversity, multi-layered arrangements, vocal harmonies and incorporation of audience participation into their live performances.[2] Their 1985 Live Aid performance was voted the best live music performance of all time in a BBC poll.[3]

Queen had moderate success in the early 1970s, with the albums Queen and Queen II, but it was with the release of Sheer Heart Attack in 1974 and A Night at the Opera the following year that the band gained international success. All of the band's studio albums reached number one on numerous charts around the world. Since 1973, they have released fifteen studio albums, five live albums, and numerous compilation albums. They are estimated to have sold over 300 million albums worldwide ,[4] including more than 32.5 million in the United States alone,[5] making them one of the world's best-selling music artists.

Following Mercury's death in 1991 and Deacon's retirement later in the decade,[6] May and Taylor have performed infrequently under the Queen label. Since 2005, they have been collaborating with Paul Rodgers, under the moniker Queen + Paul Rodgers.

[neilep (OP)]

Roger Penrose

Sir Roger Penrose, OM, FRS (born 8 August 1931) is an English mathematical physicist and Emeritus Rouse Ball Professor of Mathematics at the Mathematical Institute, University of Oxford and Emeritus Fellow of Wadham College. He is renowned for his work in mathematical physics, in particular his contributions to general relativity and cosmology. He is also a recreational mathematician and philosopher. Roger Penrose is the son of scientist Lionel S. Penrose and Margaret Leathes, and the brother of mathematician Oliver Penrose and correspondence chess grandmaster Jonathan Penrose. He was born in Colchester, Essex, England.
Source: Wikipedia

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Credit:http://www.abc.net.au/quantum/stories/s108094.htm

The Road to Reality: A Complete Guide to the Laws of the Universe
From Wikipedia, the free encyclopedia


The Road to Reality is a book on modern physics by the British mathematical physicist Roger Penrose, published in 2004. It covers the basics of the standard model of modern physics, discussing general relativity and quantum mechanics and then expands on the possible unification of these two theories.

The book is just over 1100 pages, of which the first 350 are dedicated to mathematics - Penrose's goal was to acquaint inquisitive readers with the mathematical tools needed to understand the remainder of the book in depth. On page 383 physics enters the discussion with the topic of spacetime. From there it moves on to fields in spacetime, deriving the classical electrical and magnetic forces from first principles; that is, if one lives in spacetime of a particular sort, these fields develop naturally as a consequence. Energy and conservation laws appear in the discussion of the Lagrangians and Hamiltonians, before moving onto a full discussion of quantum physics, particle theory and quantum field theory. A discussion of the measurement problem in quantum mechanics is given a full chapter; superstrings are given a chapter near the end of the book, so is loop gravity and twistor theory. The book ends with an exploration of other theories and possible ways forward.

The book discusses the physical world. Many fields that scientists in the 19th century believed were separate, electricity and magnetism for instance, are facets of a single property, electromagnetism. Some texts, both popular and university level, introduce these topics as separate concepts and then "force" the combination on them much later. In Road to Reality this process is reversed, by first demonstrating the mathematics that is needed to discuss the spacetime we appear to live in, then showing that electromagnetism simply falls out fully formed.

As Penrose admits, the final chapters reflect his personal perspective, as opposed to what he considers current fashion among theoretical physicists. He is skeptical about String Theory, optimistic about Twistor theory, and holds some controversial views about the role of consciousness in physics, as laid out in his earlier books (see Shadows of the Mind).

[Simulated]

S is for Sadness


Sadness is a mood characterized by feelings of disadvantage and loss. When sad, people often become quiet, less energetic and withdrawn. Sadness is considered to be the opposite of happiness, and is similar to the emotions of sorrow, grief, misery and melancholy. The philosopher Baruch Spinoza defined sadness as the “transfer of a person from a large perfection to a smaller one.”

Sadness is a temporary lowering of mood ('feeling blue'), whereas clinical depression is characterized by a persistent and intense lowered mood, as well as disruption to one's ability to function in day to day matters

[Simulated]

T is for Terminator





Terminator is from Latin and means "finisher."

The Terminator is a 1984 science fiction/action film directed and co-written by James Cameron. It features Arnold Schwarzenegger, Linda Hamilton and Michael Biehn.

The film takes place in 1984, introducing the concept of a "terminator", specifically the titular character (Arnold Schwarzenegger), a seemingly unstoppable cyborg assassin who has been sent back from the year 2029 by a race of artificially intelligent computer-controlled machines bent on the extermination of mankind. The Terminator's mission is to kill Sarah Connor (Linda Hamilton) whose future son founds a resistance against the machines. A human, Kyle Reese, is also sent back from the future to protect her.

[Simulated]

U is for Unibrow

A unibrow or monobrow, medically known as a synophrys, refers to a "confluence of eyebrows"; i.e. the presence of abundant hair between the eyebrows, so that they seem to converge to form one long eyebrow.

The words unibrow and monobrow are in the Oxford English Dictionary, and unibrow was added to the Merriam-Webster Dictionary in 2006. Medical dictionaries, such as Dorland's Medical Dictionary, contain the word synophrys.

[Karen W.]

Vortex


From Wikipedia, the free encyclopedia

"Vortex created by the passage of an aircraft wing, revealed by coloured smoke"




A vortex (pl. vortices) is a spinning, often turbulent, flow of fluid. Any spiral motion with closed streamlines is vortex flow. The motion of the fluid swirling rapidly around a center is called a vortex. The speed and rate of rotation of the fluid are greatest at the center, and decrease progressively with distance from the center.


Vortices display some special properties:

    * The fluid pressure in a vortex is lowest in the center where the speed is greatest, and rises progressively with distance from the center. This is in accordance with Bernoulli's Principle. The core of a vortex in air is sometimes visible because of a plume of water vapour caused by condensation in the low pressure of the core. The spout of a tornado is a classic and frightening example of the visible core of a vortex. A dust devil is also the core of a vortex, made visible by the dust drawn upwards by the turbulent flow of air from ground level into the low pressure core.

    * The core of every vortex can be considered to contain a vortex line, and every particle in the vortex can be considered to be circulating around the vortex line. Vortex lines start and end at the boundary of the fluid, but they do not start or end in the fluid. (See Helmholtz's theorems.) Vortices readily deflect and attach themselves to a solid surface. For example, a vortex usually forms ahead of the propeller disk or jet engine of a slow-moving airplane. One end of the vortex line is attached to the propeller disk or jet engine, but when the airplane is taxiing the other end of the vortex line readily attaches itself to the ground rather than end in midair. The vortex can suck water and small stones into the core and then into the propeller disk or jet engine.

    * Two or more vortices that are approximately parallel and circulating in the same direction will quickly merge to form a single vortex. The circulation of the merged vortex will equal the sum of the circulations of the constituent vortices. For example, a sheet of small vortices flows from the trailing edge of the wing or propeller of an airplane when the wing is developing lift or the propeller is developing thrust. In less than one wing chord downstream of the trailing edge of the wing these small vortices merge to form a single vortex. If viewed from the tail of the airplane, looking forward in the direction of flight, there is one wingtip vortex trailing from the left-hand wing and circulating clockwise, and another wingtip vortex trailing from the right-hand wing and circulating anti-clockwise. The result is a region of downwash behind the wing, between the pair of wingtip vortices. These two wingtip vortices do not merge because they are circulating in opposite directions.

    * Vortices contain a lot of energy in the circular motion of the fluid. In an ideal fluid this energy can never be dissipated and the vortex would persist forever. However, real fluids exhibit viscosity and this dissipates energy very slowly from the core of the vortex. (See Rankine vortex). It is only through dissipation of a vortex due to viscosity that a vortex line can end in the fluid, rather than at the boundary of the fluid. For example, the wingtip vortices from an airplane dissipate slowly and linger in the atmosphere long after the airplane has passed. This is a hazard to other aircraft and is known as wake turbulence.

[edit] Dynamics

A vortex can be any circular or rotary flow that possesses vorticity. Vorticity is a mathematical concept used in fluid dynamics. It can be related to the amount of "circulation" or "rotation" in a fluid. In fluid dynamics, vorticity is the circulation per unit area at a point in the flow field. It is a vector quantity, whose direction is (roughly speaking) along the axis of the swirl. Also in fluid dynamics, the movement of a fluid can be said to be vortical if the fluid moves around in a circle, or in a helix, or if it tends to spin around some axis. Such motion can also be called solenoidal. In the atmospheric sciences, vorticity is a property that characterizes large-scale rotation of air masses. Since the atmospheric circulation is nearly horizontal, the (3 dimensional) vorticity is nearly vertical, and it is common to use the vertical component as a scalar vorticity. Mathematically, it is defined as,

        \vec \omega = \nabla \times \vec \mathit{u}

where \vec \mathit{u} is the fluid velocity.

[edit] Two types of vortex

In fluid mechanics, a distinction is often made between two limiting vortex cases. One is called the free (irrotational) vortex, and the other is the forced (rotational) vortex. These are considered as below:
Two autumn leaves in a counter-clockwise vortex (reference position).
Two autumn leaves in a counter-clockwise vortex (reference position).
      
Two autumn leaves in a rotational vortex rotate with the counter-clockwise flow.
Two autumn leaves in a rotational vortex rotate with the counter-clockwise flow.
      
Two autumn leaves in an irrotational vortex preserve their original orientation while moving counter-clockwise.
Two autumn leaves in an irrotational vortex preserve their original orientation while moving counter-clockwise.

[edit] Free (irrotational) vortex

When fluid is drawn down a plug-hole, one can observe the phenomenon of a free vortex. The tangential velocity v varies inversely as the distance r from the center of rotation, so the angular momentum, rv, is constant; the vorticity is zero everywhere (except for a singularity at the center-line) and the circulation about a contour containing r=0 has the same value everywhere. The free surface (if present) dips sharply (as r − 2 ) as the center line is approached.

The tangential velocity is given by:

    v_{\theta} = \frac{\Gamma}{2 \pi r}\,                 (2.1)

where Γ is the circulation and r is the radial distance from the center of the vortex.

In non-technical terms the circular streamlines toward the center can sweep out a given angle faster than the outer streamlines. The speed along the circular path of flow is held constant or decreases as you move out from the center. At the same time the inner streamlines have a shorter distance to travel to complete a ring. If you were running a race on a circular track would you rather be on the inside or outside, assuming the goal was to complete a circle? Imagine a leaf floating in a free vortex. The leaf's tip points to the center and the blade straddles multiple streamlines. The outer flow is slow in terms of angle traversed and it exerts a backwards tug on the base of the leaf while the faster inner flow pulls the tip forwards. The drag force opposes rotation of the leaf as it moves around the circle.

[edit] Forced (rotational) vortex

In a forced vortex the fluid essentially rotates as a solid body (there is no shear). The motion can be realised by placing a dish of fluid on a turntable rotating at T radians/sec; the fluid has vorticity of 2 T everywhere, and the free surface (if present) is a parabola.

The tangential velocity is given by:

    v_{\theta} = \omega r\,                 (2.2)

where ω is the angular velocity and r is the radial distance from the center of the vortex.

[edit] Observations

A vortex can be seen in the spiraling motion of air or liquid around a center of rotation. Circular current of water of conflicting tides form vortex shapes. Turbulent flow makes many vortices. A good example of a vortex is the atmospheric phenomenon of a whirlwind or a tornado or dust devil. This whirling air mass mostly takes the form of a helix, column, or spiral. Tornadoes develop from severe thunderstorms, usually spawned from squall lines and supercell thunderstorms, though they sometimes happen as a result of a hurricane.

In atmospheric physics, a mesovortex is on the scale of a few miles (smaller than a hurricane but larger than a tornado). [2] On a much smaller scale, a vortex is usually formed as water goes down a drain, as in a sink or a toilet. This occurs in water as the revolving mass forms a whirlpool. This whirlpool is caused by water flowing out of a small opening in the bottom of a basin or reservoir. This swirling flow structure within a region of fluid flow opens downward from the water surface.

[edit] Instances

    * In the hydrodynamic interpretation of the behaviour of electromagnetic fields, the acceleration of electric fluid in a particular direction creates a positive vortex of magnetic fluid. This in turn creates around itself a corresponding negative vortex of electric fluid.
    * Smoke ring : A ring of smoke which persists for a surprisingly long time, illustrating the slow rate at which viscosity dissipates the energy of a vortex.
    * Lift-induced drag of a wing on an aircraft.
    * The primary cause of drag in the sail of a sloop.
    * Whirlpool : a swirling body of water produced by ocean tides or by a hole underneath the vortex, where water drains out, as in a bathtub. A large, powerful whirlpool is known as a maelstrom. In popular imagination, but only rarely in reality, can they have the dangerous effect of destroying boats.[citation needed]
    * Tornado : a violent windstorm characterized by a twisting, funnel-shaped cloud. A less violent version of a tornado, over water, is called a waterspout.
    * Hurricane : a much larger, swirling body of clouds produced by evaporating warm ocean water and influenced by the Earth's rotation. Similar, but far greater, vortices are also seen on other planets, such as the permanent Great Red Spot on Jupiter and the intermittent Great Dark Spot on Neptune.
    * Polar vortex : a persistent, large-scale cyclone centered near the Earth's poles, in the middle and upper troposphere and the stratosphere.
    * Sunspot : dark region on the Sun's surface (photosphere) marked by a lower temperature than its surroundings, and intense magnetic activity.
    * The accretion disk of a black hole or other massive gravitational source.
    * Spiral galaxy : a type of galaxy in the Hubble sequence which is characterized by a thin, rotating disk. Earth's galaxy, the Milky Way is of this type.

[neilep (OP)]

Walton, Mary

Mary Walton (a girly ! and Woman inventor).....developed a method of deflecting smoke stack emissions.
In 1879, Mary Walton patented a method of deflecting smoke stack emissions (U.S. patent #221,880) through water tanks and later adapted the system for use on locomotives.

Mary Walton also invented a noise reduction system for elevated railroads in New York City. In the 1880s, many cities developed a mass transit system using noisy elevated trains. To reduce the noise, Mary Walton invented a sound-dampening system that cradled the track in a wooden box lined with cotton and then filled with sand.

She received U.S. patent #237,422 for the system on February 8, 1881, and later sold the rights to the Metropolitan Railroad of New York City.

Source:http://inventors.about.com/od/wstartinventors/a/Mary_Walton.htm

[Simulated]

X is for Xlyophone



The xylophone (from the Greek words ξύλο (wood) and φωνή (voice), meaning 'wooden sound') is a musical instrument in the percussion family which probably originated in Indonesia. [1] It consists of wooden bars of various lengths that are struck by plastic, wooden, or rubber mallets. Each bar is tuned to a specific pitch of the musical scale. Xylophone can refer to western style concert xylophones or to one of the many wooden mallet percussion instruments found around the world. Xylophones are tuned to different scale systems depending on their origin, including pentatonic, heptatonic, diatonic, or chromatic. The arrangement of the bars is generally from low (longer bars) to high (shorter bars).

[Alandriel]

Y - YODA



Yoda is a fictional character from the Star Wars universe. Yoda first appears in the saga in The Empire Strikes Back as a Jedi Master, and trains Luke Skywalker in the ways of the Jedi. In the prequel trilogy, he serves as Grand Master of the Jedi Order. Yoda

"Luminous beings are we, not this crude matter"

[Simulated]

Z is for Zebra

[rosalind dna]

A is Automobile/car.
From Wiki.
Automobile
From Wikipedia, the free encyclopedia
Jump to: navigation, search
"Car" redirects here. For other uses, see Car (disambiguation).
 
Karl Benz's "Velo" model (1894) - entered into an early automobile race
Passenger cars in 2000
World map of passenger cars per 1000 people.An automobile (via French from Greek auto, self and Latin mobilis moving, a vehicle that moves itself rather than being moved by another vehicle or animal) or motor car (usually shortened to just car) is a wheeled passenger vehicle that carries its own motor. Most definitions of the term specify that automobiles are designed to run primarily on roads, to have seating for one to eight people, to typically have four wheels, and to be constructed principally for the transport of people rather than goods.[1] However, the term is far from precise because there are many types of vehicles that do similar tasks.

There were 590 million passenger cars worldwide (roughly one car for every eleven people) as of 2002.[2]
http://en.wikipedia.org/wiki/Automobile

[Simulated]

B is for Bananna



Banana is the common name for a fruit and also the herbaceous plants of the genus Musa which produce the commonly eaten fruit. They are native to the tropical region of Southeast Asia and Australia. Today, they are cultivated throughout the tropics. [1]

Banana plants are of the family Musaceae. They are cultivated primarily for their fruit, and to a lesser extent for the production of fibre and as ornamental plants. As the bananas are mainly tall, upright, and fairly sturdy, they are often mistaken for trees, when the truth is the main or upright stem is called a pseudostem, literally meaning "fake stem", which for some species can obtain a height of up to 2–8 m, with leaves of up to 3.5 m in length. Each pseudostem would produce a bunch of yellow, green, or even red bananas before dying and being replaced by another pseudostem.

The banana fruit grow in hanging clusters, with up to 20 fruit to a tier (called a hand), and 3-20 tiers to a bunch. The total of the hanging clusters is known as a bunch, or commercially as a "banana stem", and can weigh from 30–50 kg. The fruit averages 125 g, of which approximately 75% is water and 25% dry matter content. Each individual fruit (known as a banana or 'finger') has a protective outer layer (a peel or skin) with a fleshy edible inner portion. Typically, the fruit has numerous strings (called 'phloem bundles') which run between the skin and the edible portion of the banana, and which are commonly removed individually after the skin is removed. Bananas are a valuable source of vitamin B6, vitamin C, and potassium.

Bananas are grown in at least 107 countries.[1] In popular culture and commerce, "banana" usually refers to soft, sweet "dessert" bananas that are usually eaten raw. The bananas from a group of cultivars with firmer, starchier fruit are called plantains, and are generally used in cooking rather than eaten raw. Bananas may also be dried and eaten as a snack food. Dried bananas are also ground into banana flour.

Although the wild species have fruits with numerous large, hard seeds, virtually all culinary bananas have seedless fruits. Bananas are classified either as dessert bananas (meaning they are yellow and fully ripe when eaten) or as green cooking bananas. Almost all export bananas are of the dessert types; however, only about 10-15% of all production is for export, with the U.S. and EU being the dominant buyers.

[DoctorBeaver]

Castor Fiber

The Eurasian beaver. Up to 50% larger than the American/Canadian Castor Canadensis, and far more intelligent  [^]

[Simulated]

We just talkeda bout Castor Canadensis in Envirothon.

D is for Damn (i mean dam ha)

A dam is a barrier that divides waters. Dams generally serve the primary purpose of retaining water, while other structures such as levees and dikes are used to prevent water flow into specific land regions. The tallest dam in the world is the 300 meter high Nurek Dam in Tajikistan.[1]


The word dam can be traced back to Middle English,[2] and before that, from Middle Dutch, as seen in the names of many old cities.[3]

Most of the first Dams were built in Mesopotamia up to 7,000 years ago. These were used to control the water level, for Mesopotamia's weather affected the Tigris and Euphrates rivers, and could be quite unpredictable. The earliest recorded dam is believed to have been on the Sadd Al-Kafara at Wadi Al-Garawi, which is located about 25 kilometers south of Cairo, and built around 2600 B.C.[4] It was destroyed by heavy rain shortly afterwards.[4]

The oldest surviving and standing dam in the world is believed to be the Grand Anicut, also known as the Kallanai, an ancient dam built on the Kaveri River in the state of Tamil Nadu located in southern India. It was built by the Chola king Karikalan, and dates back to the 2nd century AD.[5] Du Jiang Yan in China is the oldest surviving irrigation system included a dam that directed waterflow. It was finished in 251 B.C.

The Kallanai is a massive dam of unhewn stone, over 300 meters long, 4.5 meters high and 20 meters (60 feet) wide,[5] across the main stream of the Kaveri. The purpose of the dam was to divert the waters of the Cauvery across the fertile Delta region for irrigation via canals. The dam is still in excellent repair, and served as a model for later engineers, including the Sir Arthur Cotton's 19th-century dam across the Kollidam, the major tributary of the Cauvery. The land area irrigated by the ancient irrigation network, of which the dam was the centerpiece, was 69,000 acres (280 square kilometers). By the early 20th century the irrigated area had been increased to about 1,000,000 acres (4,000 square kilometers).

In ancient China, the Prime Minister of Chu (state), Sunshu Ao, is the first known hydraulic engineer of China. He served Duke Zhuang of Chu during the reign of King Ding of Zhou (606 BC-586 BC), ruler of the Eastern Zhou Dynasty. His large earthen dam flooded a valley in modern-day northern Anhui province that created an enormous irrigation reservoir (62 miles in circumference), a reservoir that is still present today.[6]

In the Netherlands, a low-lying country, dams were often applied to block rivers in order to regulate the water level and to prevent the sea from entering the marsh lands. Such dams often marked the beginning of a town or city because it was easy to cross the river at such a place, and often gave rise to the respective place's names in Dutch. For instance the Dutch capital Amsterdam (old name Amstelredam) started with a dam through the river Amstel in the late 12th century, and Rotterdam started with a dam through the river Rotte, a minor tributary of the Nieuwe Maas. The central square of Amsterdam, believed to be the original place of the 800 year old dam, still carries the name Dam Square or simply the Dam.

[neilep (OP)]

EQUINOX

In astronomy, equinox can have two meanings:

    * The moment when the Sun is positioned directly over the Earth's equator and, by extension, the apparent position of the Sun at that moment—see below.

    * A moment in time at which the vernal point, celestial equator, and other such elements are taken to be used in the definition of a celestial coordinate system—see Equinox (celestial coordinates)

An equinox in astronomy is that moment in time (not a whole day) when the center of the Sun can be observed to be directly above the Earth's equator, occurring around March 20 and September 23 each year.

More technically, at an equinox, the Sun is at one of two opposite points on the celestial sphere where the celestial equator (i.e. declination 0) and ecliptic intersect. These points of intersection are called equinoctial points—the vernal point and the autumnal point. By extension, the term equinox may be used to denote an equinoctial point.

There is either an equinox (autumn and spring) or a solstice (summer and winter) on approximately the 21st day of the last month of every quarter of the calendar year. On a day which has an equinox, the center of the Sun will spend a nearly equal amount of time above and below the horizon at every location on Earth and night and day will be of nearly the same length. The word equinox derives from the Latin words aequus (equal) and nox (night).

In reality, the day is longer than the night at an equinox. Commonly the day is defined as the period that sunlight reaches the ground in the absence of local obstacles. From Earth, the Sun appears as a disc and not a single point of light; so, when the center of the Sun is below the horizon, the upper edge is visible. Furthermore, the atmosphere refracts light; so, even when the upper limb of the Sun is below the horizon, its rays reach over the horizon to the ground. In sunrise/sunset tables, the assumed semi-diameter (apparent radius) of the sun is 16 minutes of arc and the atmospheric refraction is assumed to be 34 minutes of arc. Their combination means that when the upper limb of Sun is on the visible horizon its center is 50 minutes of arc below the geometric horizon, which is the intersection with the celestial sphere of a horizontal plane through the eye of the observer. These effects together make the day about 14 minutes longer than the night at the equator, and longer still at sites toward the poles. The real equality of day and night only happens at places far enough from the equator to have at least a seasonal difference in daylength of 7 minutes, and occurs a few days towards the winter side of each equinox.

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SOURCE:http://en.wikipedia.org/wiki/Equinox