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Astronomical Unit

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Define Astronomical Unit

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How to define astronomical unit? Astronomers use astronomical units – or AU – to represent the distance of the solar system. We can say that one astronomical unit (AU) outlines the mean distance between the Earth and our Sun for general reference. An AU is nearly 93 million miles (150 million km). It’s approximately eight light minutes. The definition of AU also means distances in astronomical units to pre-eminent solar system objects. The astronomical unit is applied primarily for measuring distances around other stars or within the Solar System. 

It is also an essential element in the definition of another unit of astronomical length, the parsec. The astronomical unit is quite instrumental in formulating and understanding the distance between stellar objects and is crucial in the calculations and computation involving astronomical problems. One astronomical unit is equal to 92955807 miles.

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Astronomical Unit Definition 

According to the prevailing astronomical convention, 1 astronomical unit is equal to 149,597,870.7 kilometres (or 92,955,807 miles). As the earth orbits the sun with a varying orbital distance, we need to consider the average distance, therefore, one astronomical unit is the average distance between the Earth and the Sun. In other words, the Earth and the Sun mid-distance varies in a single year. The varying distance between the earth and the sun is maximum at the aphelion ( 152,100,000 km or 94,500,000 miles or 1.016 AUs) and the minimum at the perihelion (147,095,000 km or 91,401,000 miles or 0,983 AUs).

1 astronomical unit value = 149597870700 metres (exactly)

  ≈ 92955807 miles

  ≈ 499.00478384 light-seconds

  ≈ 4.8481368×10−6 parsecs

  ≈ 1.5812507×10−5 light-years

The speed of light can be represented in terms of astronomical units. As we know  299792458 m/s is the speed of light which is equal to precisely 299792458 × 86400 ÷ 149597870700 or about 173.144632674240 AU/d, some 60 parts per trillion less than the 2009 estimate.

Development of Astronomical Unit

The earliest documented example of astronomers calculating the distance between the Earth and the Sun dates back to Classical Antiquity. In the 3rd century BCE, Greek mathematician Aristarchus of Samos imputed that the distance was estimated to be between 18 and 20 times the distance between the Earth and the Moon.

According to the oldest Chinese mathematical writing, Zhoubi Suanjing the 1st century BCE treatise also estimates the distance between the Earth and Sun. According to the anonymous exposition, the distance is calculated by conducting geometric measures of the length of noontime shadows formed by objects aligned at specific distances. However, the predictions were based on the belief that the Earth was flat.

By the 19th century, ascertainments of the speed of light and the constant deviation of light resulted in the first direct measurement of the Earth-Sun distance in kilometres. By 1903, for the first time, the term “astronomical unit” emerged. Developments in precision have always been a key to developing astronomical understanding. Throughout the twentieth century, measurements became more precise and ever more reliant on accurate observation of the effects described by Einstein’s theory of relativity and upon the analytical tools it used. 

Astronomical Unit Modern Usage

The astronomical unit finds great applicability in the measurement of the stellar distance of extraterrestrial objects. It can be used to calculate the heliocentric distance of an asteroid or measure the distance of a planet’s orbiting moon. In the solar system, it finds usage in the development of mathematical and numerical methods for computational purposes. Also, it can be used in general to measure the distance between planetary systems and understand the extent of gaseous clouds around planets. However, an astronomical unit is an inept way of measuring distance when it comes to interstellar objects. For interstellar purposes, it is best to use measures such as light-years and parsec. While fabricating a numerical model of the Solar System, the astronomical unit proffers a relevant scale that minimizes floating-point calculations errors.

FAQ (Frequently Asked Questions)

Q1. Define One Astronomical Unit.

Answer: An astronomical unit is a unit of distance in astronomy, it is applied primarily for measuring distances around other stars or within the Solar System. During the General Assembly of the International Astronomical Union (IAU) in August 2012, the precise length of an astronomical unit in terms of meters was redefined, which was equal to 149,597,870,700 meters. The astronomical unit (au) is a convenient unit of measure for distance in the Solar System, being approximately equal to the average Sun-Earth distance. 


One of the most common units of distance in astronomy that is useful in the measurement of vast distances in the cosmos is the Astronomical Unit (Au). Based on the distance between the Earth and the Sun, the astronomical unit allows astronomers to distinguish the vast distances between the Solar planets and the Sun and between extrasolar planets and their stars.

Q2. What is the Use of AU?

Answer: Astronomical units have vast applicability mostly in stellar distance measurement, for example, they can be used for measuring the heliocentric distance of an asteroid or protostellar disk’s size. It is also used to create numerical models and measure distances for the Solar System. It is used while measuring extra-solar systems and the extent of protoplanetary clouds or the distance between extrasolar planets and their parent star. When measuring interstellar distances, AUs are too scanty to extend convenient measurements such as the parsec and the light-year – are relied upon. While fabricating a numerical model of the Solar System, the astronomical unit proffers a relevant scale that minimizes floating-point calculations errors.