Geographic coordinate system

A geographic coordinate system is a coordinate system that enables every location on the Earth to be specified by a set of numbers or letters, or symbols. The coordinates are often chosen such that one of the numbers represents vertical position, and two or three of the numbers represent horizontal position. A common choice of coordinates is latitude, longitude and elevation.

To specify a location on a two-dimensional map requires a map projection.

History

The invention of a geographic coordinate system is generally credited to Eratosthenes of Cyrene, who composed his now-lost Geography at the Library of Alexandria in the 3rd century BC. A century later, Hipparchus of Nicaea improved upon his system by determining latitude from stellar measurements rather than solar altitude and determining longitude by using simultaneous timing of lunar eclipses, rather than dead reckoning. In the 1st or 2nd century, Marinus of Tyre compiled an extensive gazetteer and mathematically-plotted world map, using coordinates measured east from a Prime Meridian at the Fortunate Isles of western Africa and measured north or south of the island of Rhodes off Asia Minor. Ptolemy credited him with the full adoption of longitude and latitude, rather than measuring latitude in terms of the length of the midsummer day. Ptolemy's 2nd-century Geography used the same Prime Meridian but measured latitude from the equator instead. After their work was translated into Arabic in the 9th century, Al-Khwārizmī's Book of the Description of the Earth corrected Marinus and Ptolemy's errors regarding the length of the Mediterranean Sea, causing medieval Arabic cartography to use a Prime Meridian around 10° east of Ptolemy's line. Mathematical cartography resumed in Europe following Maximus Planudes's recovery of Ptolemy's text a little before 1300; the text was translated into Latin at Florence by Jacobus Angelus around 1407.

Coordinate system

In geometry, a coordinate system is a system which uses one or more numbers, or coordinates, to uniquely determine the position of a point or other geometric element on a manifold such as Euclidean space. The order of the coordinates is significant and they are sometimes identified by their position in an ordered tuple and sometimes by a letter, as in "the x-coordinate". The coordinates are taken to be real numbers in elementary mathematics, but may be complex numbers or elements of a more abstract system such as a commutative ring. The use of a coordinate system allows problems in geometry to be translated into problems about numbers and vice versa; this is the basis of analytic geometry.

Common coordinate systems

Number line

The simplest example of a coordinate system is the identification of points on a line with real numbers using the number line. In this system, an arbitrary point O (the origin) is chosen on a given line. The coordinate of a point P is defined as the signed distance from O to P, where the signed distance is the distance taken as positive or negative depending on which side of the line P lies. Each point is given a unique coordinate and each real number is the coordinate of a unique point.

Celestial navigation

Celestial navigation, also known as astronavigation, is the ancient art and science of position fixing that enables a navigator to transition through a space without having to rely on estimated calculations, or dead reckoning, to know their position. Celestial navigation uses "sights," or angular measurements taken between a celestial body (the sun, the moon, a planet or a star) and the visible horizon. The sun is most commonly used, but navigators can also use the moon, a planet or one of 57 navigational stars whose coordinates are tabulated in the Nautical Almanac and Air Almanacs.

Celestial navigation is the use of angular measurements (sights) between celestial bodies and the visible horizon to locate one's position on the globe, on land as well as at sea. At a given time, any celestial body is located directly over one point on the Earth's surface. The latitude and longitude of that point is known as the celestial body’s geographic position (GP), the location of which can be determined from tables in the Nautical or Air Almanac for that year.

Getting Started

Getting Started is a 1979 animated short by Richard Condie and produced in Winnipeg by the National Film Board of Canada.

The film is a comical look at procrastination, based partly on the filmmaker's own experiences, portraying the inability of a pianist to rehearse a Debussy composition.

Awards for Getting Started included the Genie Award for best animation film. The film also won awards at the Zagreb World Festival of Animated Films and the Tampere Film Festival, as well as a "Bijou" at the Canadian Short Film and Television Awards.

References

External links

The Nautical Almanac

The Nautical Almanac has been the familiar name for a series of official British almanacs published under various titles since the first issue of The Nautical Almanac and Astronomical Ephemeris, for 1767: this was the first nautical almanac ever to contain data dedicated to the convenient determination of longitude at sea. It was originally published from the Royal Greenwich Observatory in England. A detailed account of how the publication was produced in its earliest years has been published by the National Maritime Museum.

Since 1958 (with the issue for the year 1960), Her Majesty's Nautical Almanac Office and the US Naval Observatory have jointly published a unified Nautical Almanac, for use by the navies of both countries.

Publication history

The changing names and contents of related titles in the series are summarised as follows. (The issue years mentioned below are those for which the data in the relevant issue were calculated—and the issues were in practice published in advance of the year for which they were calculated, at different periods of history, anything from 1 to 5 years in advance).