# Astronomical unit

Unit of length, roughly the distance from Earth to the Sun and equal to 150 e6km or 8.3 light minutes.

- Astronomical unit

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## Parsec

Unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to 1 pc or 1 pc (au), i.e. 30.9 e12km.

The two dimensions on which this triangle is based are its shorter leg, of length one astronomical unit (the average Earth-Sun distance), and the subtended angle of the vertex opposite that leg, measuring one arcsecond.

## Apsis

Farthest or nearest point in the orbit of a planetary body about its primary body.

At perihelion, the Earth's center is about 0.983 astronomical units (AU) or 147098070 km from the Sun's center.

## Solar mass

Standard unit of mass in astronomy, equal to approximately 2 kg. It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxies and black holes.

Based on the length of the year, the distance from Earth to the Sun (an astronomical unit or AU), and the gravitational constant (

Succession of methods by which astronomers determine the distances to celestial objects.

Direct distance measurements are based upon the astronomical unit (AU), which is defined as the mean distance between the Earth and the Sun.

## Light-year

Large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers , or 5.88 trillion miles (5.88 mi).

The unit most commonly used in professional astronomy is the parsec (symbol: pc, about 3.26 light-years) which derives from astrometry; it is the distance at which one astronomical unit subtends an angle of one second of arc.

## Parallax

Displacement or difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines.

The diurnal parallax has been used by John Flamsteed in 1672 to measure the distance to Mars at its opposition and through that to estimate the astronomical unit and the size of the Solar System.

## Lunar distance (astronomy)

Moon's average distance to Earth.

A little less than 400 lunar distances make up an astronomical unit.

## Minute and second of arc

Unit of angular measurement equal to 1⁄60 of one degree.

an object of diameter 725.27 km at a distance of one astronomical unit,

## Unit of length

A unit of length refers to any arbitrarily chosen and accepted reference standard for measurement of length.

astronomical unit au. Defined as 149,597,870,700 m. Approximately the distance between the Earth and Sun.

## Earth's orbit

Earth orbits the Sun at an average distance of 149.60 million km (92.96 million mi) in a counterclockwise direction as viewed from above the Northern Hemisphere.

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