In trigonometry, the gradian – also known as the gon (from Ancient Greek γωνία (gōnía) 'angle'), grad, or grade^[1] – is a unit of measurement of an angle, defined as one-hundredth of the right angle; in other words, 100 gradians is equal to 90 degrees.^[2][3][4] It is equivalent to 1/400 of a turn,^[5] 9/10 of a degree, or π/200 of a radian. Measuring angles in gradians is said to employ the centesimal system of angular measurement, initiated as part of metrication and decimalisation efforts.^{[6][7][8][Note 1]}

In continental Europe, the French word centigrade, also known as centesimal minute of arc, was in use for one hundredth of a grade; similarly, the centesimal second of arc was defined as one hundredth of a centesimal arc-minute, analogous to decimal time and the sexagesimal minutes and seconds of arc.^[12] The chance of confusion was one reason for the adoption of the term Celsius to replace centigrade as the name of the temperature scale.^[13][14]

Gradians are principally used in surveying (especially in Europe),^[15][7][16] and to a lesser extent in mining^[17] and geology.^[18][19]

As of May 2020,^[update] the gon is officially a legal unit of measurement in the European Union^[20]: 9 and in Switzerland.^[21]

The gradian is not part of the International System of Units (SI).^{[22][20]: 9–10}

The unit originated in France in connection with the French Revolution as the grade, along with the metric system, hence it is occasionally referred to as a metric degree. Due to confusion with the existing term grad(e) in some northern European countries (meaning a standard degree, 1/360 of a turn), the name gon was later adopted, first in those regions, and later as the international standard. In France, it was also called grade nouveau. In German, the unit was formerly also called Neugrad (new degree) (whereas the standard degree was referred to as Altgrad (old degree)), likewise nygrad in Danish, Swedish and Norwegian (also gradian), and nýgráða in Icelandic.

Although attempts at a general introduction were made, the unit was only adopted in some countries, and for specialised areas such as surveying,^[15][7][16] mining^[17] and geology.^[18][19] The French armed forces' artillery units have used the gon for decades.^{[citation needed]} Today, the degree, 1/360 of a turn, or the mathematically more convenient radian, 1/2π of a turn (used in the SI system of units) is generally used instead.

In the 1970s –1990s, most scientific calculators offered the gon, as well as radians and degrees, for their trigonometric functions.^[23] In the 2010s, some scientific calculators lack support for gradians.^[24]

Each quadrant is assigned a range of 100 gon, which eases recognition of the four quadrants, as well as arithmetic involving perpendicular or opposite angles.

0°	=	0 gradians
90°	=	100 gradians
180°	=	200 gradians
270°	=	300 gradians
360°	=	400 gradians

One advantage of this unit is that right angles to a given angle are easily determined. If one is sighting down a compass course of 117 gon, the direction to one's left is 17 gon, to one's right 217 gon, and behind one 317 gon. A disadvantage is that the common angles of 30° and 60° in geometry must be expressed in fractions (as 33+1/3 gon and 66+2/3 gon respectively).

In the 18th century, the metre was defined as the 10-millionth part of a quarter meridian. Thus, 1 gon corresponds to an arc length along the Earth's surface of approximately 100 kilometres; 1 centigon to 1 kilometre; 10 microgons to 1 metre.^[25] (The metre has been redefined with increasing precision since then).

The gradian is not part of the International System of Units (SI). The EU directive on the units of measurement^{[20]: 9–10} notes that the gradian "does not appear in the lists drawn up by the CGPM, CIPM or BIPM." The most recent, 9th edition of the SI Brochure does not mention the gradian at all.^[22] The previous edition mentioned it only in a footnote, which said the following:^[26]

The gon (or grad, where grad is an alternative name for the gon) is an alternative unit of plane angle to the degree, defined as (π/200) rad. Thus there are 100 gon in a right angle. The potential value of the gon in navigation is that because the distance from the pole to the equator of the Earth is approximately 10000 km, 1 km on the surface of the Earth subtends an angle of one centigon at the centre of the Earth. However the gon is rarely used.

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