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Overview

Length describes the longest dimension of an object. For three-dimensional objects, it is usually measured horizontally.

Distance, on the other hand, refers to the measure of how far the objects are from each other.

Measuring Length and Distance

Units

The basic unit for length and distance in the International System of Units (SI) is a meter. Derivatives of the meter, such as kilometers and centimeters, are also used in the metric system. Units such as inch, foot, and mile are used where the metric system is not accepted, such as in the USA and the UK.

Distances in Science

Sciences like Biology and Physics work with very small distances, therefore additional units are used. A micrometer is 1×10⁻⁶ of a meter. It is commonly used in biology to measure microorganisms, as well as for measuring infrared radiation wavelengths. It is also known as micron and is represented by the sign µ. Nanometer (1×10⁻⁹ of a meter), picometer (1×10⁻¹² of a meter), femtometer (1×10⁻¹⁵ of a meter), and attometer (1×10⁻¹⁸ of a meter) are also used.

Distances in Navigation

Navigation uses nautical miles. One nautical mile equals 1852 meters. Historically it was defined as one minute of arc along the meridian or 1/(60×180) of a meridian. This allowed for simplified calculations of latitude because every 60 nautical miles were one degree of latitude. When calculating speed using nautical miles, often knots are used as units. One knot equals the speed of one nautical mile per hour.

Distances in Astronomy

In astronomy, because of the great distances under consideration, additional units are used for convenience.

An astronomical unit (AU, au, a.u., or ua) equals 149,597,870,700 meters. There is a constant, the unit distance, of the value of one astronomical unit. It is denoted as A. Earth is about 1.00 AU from the Sun.

A light-year (ly) equals 10,000,000,000,000 km, or 10¹³ km. It represents the distance that light travels in one Julian year in a vacuum. It is used more often in popular culture than in astronomical calculations.

A parsec (pc) is about 30,856,775,814,671,900 meters, or approximately 3.09 × 10¹³ km. One parsec represents the distance from the Sun to an astronomical object, such as a planet, a star, a moon, or an asteroid, which has a parallax angle equal to an arcsecond. One arcsecond is equal to 1/3600 of a degree, or about 4.8481368 µrad in radians. To calculate a parsec, the effect of parallax can be used, which is an apparent displacement of an object when it is viewed from two different points of view. Astronomers draw an imaginary line from the Earth (point E1) to the distant star or an astronomical object (point A2), line E1A2. Half a year later, when the Sun is on the opposite side of the Earth, they draw another imaginary line from the current position of the Earth (point E2) to the new apparent position of the distant star (point A1), line E2A1. They then also connect the two positions of the Earth, making line E1E2. The sun is in the middle of this line, at a point S. The distance of lines E1S and E2S each equal 1 AU. When one draws a line perpendicular to E1E2, going through S, it will also pass through the intersection of E1A2 and E2A1, point I. The distance from the sun to this point — that is, the line SI, is equal to 1 pc, if the angle formed by the lines A1I and A2I is two arcseconds. Please see the diagram below for better visualization. There, angle P is one arcsecond.

On this picture:

• A1, A2: apparent positions of a distant star
• E1, E2: positions of the Earth
• S: the position of the Sun
• I: the intersection point
• IS = 1 parsec
• ∠P or ∠XIA2: parallax angle
• ∠P = 1 arcsecond

Other Units

A league is an obsolete unit in most countries. It is still in use in some areas, such as Yucatan and rural Mexico. It is defined as a distance that a person can walk in one hour. A nautical league is defined as three nautical miles, which is about 5.6 km. A league was widely used in literature, such as in “Twenty Thousand Leagues Under the Sea” by Jules Verne.

A cubit is a length from the tip of the middle finger to the elbow. This unit was widely used from the Antiquity through to Early Modern Times.

A yard is used in the imperial system and it equals three feet or 0.9144 meters. In some countries like Canada, it is only used when measuring fabric, as well as sports grounds, such as swimming pools and cricket pitches.

Defining the Meter

The meter was originally defined to be 1/10,000,000 of the distance between the North Pole and the Equator. It was later redefined as a length of a prototype meter bar, created from platinum and iridium alloy. It was further redefined as equal to 1,650,763.73 wavelengths of the orange-red emission line in the electromagnetic spectrum of the krypton-86 atom in a vacuum. Later it was redefined once more using the speed of light. This definition is used today and states that one meter is equal to the length of the path traveled by light in a vacuum in 1/299,792,458 of a second.

Calculations

In geometry, the distance between two points A and B with the coordinates A(x₁, y₁) and B(x₂, y₂) is calculated using the formula:

In physics, distance is a scalar value and never negative. It can be measured by an odometer. Distance is measured along the object’s travel trajectory. It must not be confused with displacement, which is a vector that measures a straight line that is the shortest distance between the departure and the arrival points of an object.

Circular distance is a distance travelled by a circular object, such as a wheel. It can be calculated using frequency, or the radius of the wheel.

References

Artikel ini ditulis oleh Kateryna Yuri.

Unit Converter articles were edited and illustrated by Anatoly Zolotkov

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