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# What is the length?

We explain what is the length in physics and geography, what they have in common in both disciplines and their relationship with other concepts.

1. ### What is the length?

The length can designate different notions, depending on the specific scope, but always in the sense of a flat and linear distance. In fact, the Dictionary of the Spanish Language defines it in its first two entries as a “physical magnitude” and as the “linear dimension of a flat surface”.

In both cases, it is a metric concept, that is, measurable, with which you can realize the distances . However, it is not a synonym or equivalent, but it is a fundamental measure of dimension.

It is used in physics and geography, being able to have a different meaning in the latter, in what refers to the Geographic Coordinate System.

In general, thus, the term length is used to measure the distance or length of things or phenomena, such as “wavelength”, “long jump”, etc.

1. ### Physical length

In the field of physics, length is considered one of the fundamental magnitudes of the universe , from which different ones derive, but which cannot be explained by them. It serves as a reference frame for distances, and from it come the length or dimensional length of objects, real or imaginary.

Initially it was considered as a distance of straight segments, then proceed to measure the length of the curves or circumferences. As long as it is linear, one can speak of length , since other magnitudes such as area or volume imply two and up to three simultaneous dimensions.

However, these notions about length were questioned by Albert Einstein in the twentieth century because, according to the Theory of Relativity, length depends on the measurements made by its observers , that is, it is not intrinsic to objects. and spaces.

1. ### Lenght measures

Length is commonly measured in meters (m) and its multiples and submultiples , according to the International System of Units  (SI). The most commonly used are kilometers, centimeters and millimeters.

However, to measure particularly large lengths, such as those in outer space, units such as the light year (the distance that light travels in a year, that is, about 9,460,730,472,580.8 km) are used.

The Astronomical Unit (UA, equivalent to the average distance between the Earth and the Sun , that is, 149,597,870,700 meters) or the parsec (parallax of a second arc, equivalent to 206,265 UA).

1. ### Geographical length

In the geographical and cartographic field , on the other hand, length is understood as the angular distance between any point on the globe and the zero meridian (or Greenwich meridian ), which divides the planet into two hemispheres: western (west) and eastern ( East). This distance is measured with respect to the closest parallel and having the center of the Earth as its center .

It is, therefore, an imaginary construction that allows you to locate any point on the horizontal axis of the globe, and which together with latitude forms a coordinate system used in geolocation.

The length is measured in sexagesimal degrees, ranging from 0 ° to 360 ° along the earth’s circumference, and indicating the hemisphere in which it is: West (O, sometimes W) or East (E).

1. ### Longitude and latitude

Together with latitude , longitude forms a system of angular geographic coordinates . From them, the planetary reference of any point on the earth’s surface is established. This system is used by technologies such as GPS ( Global Positioning System ). It is expressed in sexagesimal degrees, both for latitude and longitude, in brackets.

For example: the latitude and longitude of the city of Hong Kong, China is 22 degrees, 15 minutes and 00 seconds north latitude (N) and 114 degrees, 10 minutes and 00 seconds east longitude (E). Or what is the same: (22 ° 15 ’00’ ‘N; 114 ° 10’ 00 ” E).