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Sundial
A SUNDIAL is a device that tells the time of day by the apparent position of the Sun
Sun
in the sky . In the narrowest sense of the word, it consists of a flat plate (the dial) and a gnomon , which casts a shadow onto the dial. As the Sun
Sun
appears to move across the sky, the shadow aligns with different hour-lines, which are marked on the dial to indicate the time of day. The style is the time-telling edge of the gnomon, though a single point or nodus may be used. The gnomon casts a broad shadow; the shadow of the style shows the time. The gnomon may be a rod, wire, or elaborately decorated metal casting. The style must be parallel to the axis of the Earth\'s rotation for the sundial to be accurate throughout the year. The style's angle from horizontal is equal to the sundial's geographical latitude . In a broader sense, a sundial is any device that uses the Sun's altitude or azimuth (or both) to show the time
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Armillary Sphere
An ARMILLARY SPHERE (variations are known as SPHERICAL ASTROLABE, ARMILLA, or ARMIL) is a model of objects in the sky (in the celestial sphere ), consisting of a spherical framework of rings, centred on Earth
Earth
or the Sun, that represent lines of celestial longitude and latitude and other astronomically important features such as the ecliptic . As such, it differs from a celestial globe , which is a smooth sphere whose principal purpose is to map the constellations. It was invented separately in ancient Greece and ancient China
China
, with later use in the Islamic world
Islamic world
and Medieval Europe
Medieval Europe
. With the Earth
Earth
as center, an armillary sphere is known as Ptolemaic . With the sun as center, it is known as Copernican . The flag of Portugal
Portugal
features an armillary sphere
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Singapore
SINGAPORE (/ˈsɪŋəpɔːr/ ( listen ), /ˈsɪŋɡə-/ ), officially the REPUBLIC OF SINGAPORE, is a sovereign city-state and island country in Southeast Asia
Southeast Asia
. It lies one degree (137 kilometres or 85 miles) north of the equator , at the southern tip of the Malay Peninsula , with Indonesia
Indonesia
's Riau Islands to the south and Peninsular Malaysia
Malaysia
to the north. Singapore's territory consists of one main island along with 62 other islets . Since independence, extensive land reclamation has increased its total size by 23% (130 square kilometres or 50 square miles)
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Greenwich
GREENWICH (/ˈɡrɛnɪtʃ/ ( listen ), /ˈɡrɪnɪdʒ/ , /ˈɡrɪnɪtʃ/ or /ˈɡrɛnɪdʒ/ ) is an early-established district of south east London
London
, England
England
, centred 5.5 miles (8.9 km) east south-east of Charing Cross . The town lends its name to the Royal Borough of Greenwich . The town gives its name to one of the five boroughs of the London Docklands , and is connected to areas with docks of London historically by river and today to Canary Wharf
Canary Wharf
and other districts by the Docklands Light Railway
Docklands Light Railway

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Conic Section
In mathematics , a CONIC SECTION (or simply CONIC) is a curve obtained as the intersection of the surface of a cone with a plane . The three types of conic section are the hyperbola , the parabola , and the ellipse . The circle is a special case of the ellipse, and is of sufficient interest in its own right that it was sometimes called a fourth type of conic section. The conic sections have been studied by the ancient Greek mathematicians with this work culminating around 200 BC, when Apollonius of Perga
Apollonius of Perga
undertook a systematic study of their properties. The conic sections of the Euclidean plane have various distinguishing properties. Many of these have been used as the basis for a definition of the conic sections
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Ellipse
In mathematics , an ELLIPSE is a curve in a plane surrounding two focal points such that the sum of the distances to the two focal points is constant for every point on the curve. As such, it is a generalization of a circle, which is a special type of an ellipse having both focal points at the same location. The shape of an ellipse (how "elongated" it is) is represented by its eccentricity , which for an ellipse can be any number from 0 (the limiting case of a circle ) to arbitrarily close to but less than 1. Ellipses are the closed type of conic section : a plane curve resulting from the intersection of a cone by a plane (see figure to the right). Ellipses have many similarities with the other two forms of conic sections: parabolas and hyperbolas , both of which are open and unbounded . The cross section of a cylinder is an ellipse, unless the section is parallel to the axis of the cylinder
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Cylinder (geometry)
In its simplest form, a CYLINDER (from Greek κύλινδρος – kulindros, "roller, tumbler" ) is the surface formed by the points at a fixed distance from a given straight line called the AXIS of the cylinder. It is one of the most basic curvilinear geometric shapes. CONTENTS* 1 Common use * 1.1 Volume * 1.2 Surface area * 1.3 Cylindric sections * 2 Other types of cylinders * 2.1 Right circular hollow cylinder * 3 About an arbitrary axis * 4 Projective geometry * 5 Related polyhedra * 6 See also * 7 References * 8 External links COMMON USECommonly the word cylinder is understood to refer to a finite section of a RIGHT CIRCULAR CYLINDER having a finite height with circular ends perpendicular to the axis as shown in the figure. If the ends are open, it is called an open cylinder. If the ends are closed by flat surfaces it is called a solid cylinder
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Cone (geometry)
A CONE is a three-dimensional geometric shape that tapers smoothly from a flat base (frequently, though not necessarily, circular) to a point called the apex or vertex . A cone is formed by a set of line segments , half-lines , or lines connecting a common point, the apex, to all of the points on a base that is in a plane that does not contain the apex. Depending on the author, the base may be restricted to be a circle , any one-dimensional quadratic form in the plane, any closed one-dimensional figure , or any of the above plus all the enclosed points. If the enclosed points are included in the base, the cone is a solid object ; otherwise it is a two-dimensional object in three-dimensional space. In the case of a solid object, the boundary formed by these lines or partial lines is called the lateral surface; if the lateral surface is unbounded, it is a conical surface
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True North
TRUE NORTH (geodetic north) is the direction along the earth's surface towards the geographic North Pole
North Pole
. True geodetic north differs from magnetic north (the direction a compass points toward the magnetic north pole), and from grid north (the direction northwards along the grid lines of a map projection ). Geodetic true north also differs very slightly from astronomical true north (typically by a few arcseconds ) because the local gravity may not point at the exact rotational axis of the earth. The direction of astronomical true north is marked in the skies by the north celestial pole . This is within about 1 degree of the position of Polaris
Polaris
, so that the star appears to trace a tiny circle in the sky each day. Due to the axial precession of the Earth, true north rotates in an arc with respect to the stars that takes approximately 25,000 years to complete
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Aberdeen, Scotland
ABERDEEN (/æbərˈdiːn/ ( listen ); Scots : Aiberdeen, listen (help ·info ); Scottish Gaelic : Obar Dheathain ; Latin
Latin
: Aberdonia) is Scotland
Scotland
's third most populous city , one of Scotland's 32 local government council areas and the United Kingdom
United Kingdom
's 37th most populous built-up area , with an official population estimate of 196,670 for the city of Aberdeen
Aberdeen
and 229,800 for the local authority area. Nicknames include the Granite
Granite
City, Aberdoom, the Grey City and the Silver City with the Golden Sands. During the mid-18th to mid-20th centuries, Aberdeen's buildings incorporated locally quarried grey granite, which can sparkle like silver because of its high mica content. Since the discovery of North Sea oil
North Sea oil
in the 1970s, Aberdeen has been known as the off-shore oil capital of Europe
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Sitka, Alaska
The CITY AND BOROUGH OF SITKA (Tlingit : Sheetʼká), formerly NOVO-ARKHANGELSK , or NEW ARCHANGEL under Russian rule (Russian : Ново-Архангельск or Новоaрхангельск, t Novoarkhangelsk), is a unified city-borough located on Baranof Island and the southern half of Chichagof Island in the Alexander Archipelago of the Pacific Ocean
Pacific Ocean
(part of the Alaska
Alaska
Panhandle ), in the U.S. state of Alaska
Alaska
. As of the 2010 census , the population was 8,881. In terms of land area, it is the largest city-borough in the U.S., with a land area of 2,870.3 square miles (7,434 square kilometres ) and a total area (including water area) of 4,811.4 square miles (12,461 square kilometres); however, it is the smallest of Alaska's boroughs. Urban Sitka, the part that is usually thought of as the "city" of Sitka, is on the west side of Baranof Island
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Celestial Equator
The CELESTIAL EQUATOR is a great circle on the imaginary celestial sphere , in the same plane as the Earth
Earth
's equator . In other words, it is a projection of the terrestrial equator out into space. As a result of the Earth's axial tilt , the celestial equator is inclined by 23.4° with respect to the ecliptic plane . An observer standing on the Earth's equator visualizes the celestial equator as a semicircle passing directly overhead through the zenith . As the observer moves north (or south), the celestial equator tilts towards the opposite horizon. The celestial equator is defined to be infinitely distant (since it is on the celestial sphere); thus the observer always sees the ends of the semicircle disappear over the horizon exactly due east and due west, regardless of the observer's position on Earth
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Celestial Longitude
In astronomy , a CELESTIAL COORDINATE SYSTEM is a system for specifying positions of celestial objects: satellites, planets, stars, galaxies, and so on. Coordinate systems can specify a position in 3-dimensional space , or merely the direction of the object on the celestial sphere , if its distance is not known or not important. The coordinate systems are implemented in either spherical coordinates or rectangular coordinates . Spherical coordinates, projected on the celestial sphere , are analogous to the geographic coordinate system used on the surface of Earth
Earth
. These differ in their choice of fundamental plane , which divides the celestial sphere into two equal hemispheres along a great circle . Rectangular coordinates, in appropriate units , are simply the cartesian equivalent of the spherical coordinates, with the same fundamental (x, y) plane and primary (x-axis) direction
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Ecliptic
The ECLIPTIC is the circular path on the celestial sphere that the Sun
Sun
appears to follow over the course of a year ; it is the basis of the ecliptic coordinate system . The term also refers to the plane of this path, which is coplanar with Earth\'s orbit around the Sun
Sun
(and hence the Sun's apparent orbit around Earth
Earth
). The ecliptic is not normally noticeable from Earth's surface because Earth
Earth
rotates , carrying the observer through the cycles of sunrise and sunset , which obscure the Sun's apparent motion against the background of fixed stars
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Declination
In astronomy , DECLINATION (abbreviated DEC; symbol δ) is one of the two angles that locate a point on the celestial sphere in the equatorial coordinate system , the other being hour angle . Declination's angle is measured north or south of the celestial equator , along the hour circle passing through the point in question. Right ascension and DECLINATION as seen on the inside of the celestial sphere . The primary direction of the system is the vernal equinox , the ascending node of the ecliptic (red) on the celestial equator (blue). Declination
Declination
is measured northward or southward from the celestial equator, along the hour circle passing through the point in question. The root of the word declination (Latin, declinatio) means "a bending away" or "a bending down". It comes from the same root as the words incline ("bend toward") and recline ("bend backward")
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Fixed Stars
The FIXED STARS (Latin : stellae fixae) comprise the background of astronomical objects that appear to not move relative to each other in the night sky compared to the foreground of Solar System objects that do. Generally, the fixed stars are taken to include all stars other than the Sun
Sun
. Nebulae and other deep-sky objects may also be counted among the fixed stars. Exact delimitation of the term is complicated by the fact that no celestial objects are in fact fixed with respect to each other. Nonetheless, extrasolar objects move so slowly in the sky that the change in their relative positions is nearly imperceptible on typical human timescales, except to careful examination, and thus can be considered "fixed" for many purposes. Furthermore, distant stars and galaxies move even slower in the sky than comparatively closer ones. People in many cultures have imagined that the stars form pictures in the sky called constellations
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