what is meant by astronomical unit?

Answer 1

The astronomical unit (AU or au or a.u. or sometimes ua) is a unit of length nearly equal to the semi-major axis of Earth's orbit around the Sun. The currently accepted value of the AU is 149 597 870 691 ± 30 metres (about 150 million kilometres or 93 million miles).

or in a simpler language " the average astronomical distance between Sun and th Earth"

Answer 2

Official definition: the distance from the centre of the Sun at which a particle of negligible mass, in an unperturbed circular orbit, would have an orbital period of 365.2568983 days (one Gaussian year). More accurately, it is the distance at which the heliocentric gravitational constant (the product GM☉) is equal to (0.017 202 093 95)² AU³/d².

(International Astronomical Union)

This happens to be very close to the Mean distance of Earth, from the Sun, when measured over long periods of time.

Earth's orbit (more accurately, the orbit of the barycentre of the Earth-Moon system) varies over time because of the influence of other planets.

Sometimes the mean distance is a bit more, sometimes, a bit less.

At the start of 2007, the Mean distance is 0.999992 A.U. By the end of 2007, it will be closer to 0.999995 A.U.

(This is not the same as the difference between perihelion and aphelion, which is much greater.

perihelion 0.98326 AU on 3 Jan 2007
aphelion 1.01671 on 5 Jul 2007)


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OK: another interpretation of the question:

An astronomical unit is a unit used in astronomy (this is ALSO a definition found on the International Astronomical Union -- here, astronomical unit has no upper case A or U).

For example, in a paragraph titled 5.15 Astronomical units, they say that the symbol for astronomical unit (the distance) is au (not AU or A.U. as I have been using). The symbol for year is a (not yr); I guess a is from the Latin anno.

Unless otherwise noted, a day is 86,400 seconds and a year is the Julian year of 365.25 days. (Yes, they know that there are different types of year)

Therefore, when calculating a light year, begin with 365.25 days, multiply by 86,400 seconds per day, and multiply by the speed of light 299,792.458 km/s.

The light-year is one of the many astronomical units of distance (the parsec is a more official one); but it is not "the" distance that is called Astronomical Unit (au).

Answer 3

One Astronomical Unit is the distance between the Earth and the Sun.

Answer 4

The astronomical unit is the mean distance between the sun and the earth.

Answer 5

The astronomical unit (AU or au or a.u. or sometimes ua) is a unit of length nearly equal to the semi-major axis of Earth's orbit around the Sun.

Answer 6

The astronomical unit (AU or au or a.u. or sometimes ua) is a unit of length nearly equal to the semi-major axis of Earth's orbit around the Sun. The currently accepted value of the AU is 149 597 870 691 ± 30 metres (about 150 million kilometres or 93 million miles).

or in a simpler language " the average astronomical distance between Sun and th Earth"

Source(s):

Wikipedia

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jyAbout Me
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The astronomical unit (AU or au or a.u. or sometimes ua) is a unit of length nearly equal to the semi-major axis of Earth's orbit around the Sun.

Source(s):

http://en.wikipedia.org/wiki/astronomica...

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superfunk...About Me
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Bringing up a good point - in the time it takes to post a question on here asking for the definition of something, couldn't the person simply search the web (or use wikipedia) themselves?

I guess that's what having an entire generation being raised by a television set gives you.

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The astronomical unit is the mean distance between the sun and the earth.

Source(s):

http://neo.jpl.nasa.gov/glossary/au.html...

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Official definition: the distance from the centre of the Sun at which a particle of negligible mass, in an unperturbed circular orbit, would have an orbital period of 365.2568983 days (one Gaussian year). More accurately, it is the distance at which the heliocentric gravitational constant (the product GM☉) is equal to (0.017 202 093 95)² AU³/d².

(International Astronomical Union)

This happens to be very close to the Mean distance of Earth, from the Sun, when measured over long periods of time.

Earth's orbit (more accurately, the orbit of the barycentre of the Earth-Moon system) varies over time because of the influence of other planets.

Sometimes the mean distance is a bit more, sometimes, a bit less.

At the start of 2007, the Mean distance is 0.999992 A.U. By the end of 2007, it will be closer to 0.999995 A.U.

(This is not the same as the difference between perihelion and aphelion, which is much greater.

perihelion 0.98326 AU on 3 Jan 2007
aphelion 1.01671 on 5 Jul 2007)


-----

OK: another interpretation of the question:

An astronomical unit is a unit used in astronomy (this is ALSO a definition found on the International Astronomical Union -- here, astronomical unit has no upper case A or U).

For example, in a paragraph titled 5.15 Astronomical units, they say that the symbol for astronomical unit (the distance) is au (not AU or A.U. as I have been using). The symbol for year is a (not yr); I guess a is from the Latin anno.

Unless otherwise noted, a day is 86,400 seconds and a year is the Julian year of 365.25 days. (Yes, they know that there are different types of year)

Therefore, when calculating a light year, begin with 365.25 days, multiply by 86,400 seconds per day, and multiply by the speed of light 299,792.458 km/s.

The light-year is one of the many astronomical units of distance (the parsec is a more official one); but it is not "the" distance that is called Astronomical Unit (au).

Source(s):

http://www.iau.org/units.234.0.html...
Scroll down to 5.15

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One Astronomical Unit is the distance between the Earth and the Sun.

Answer 7

Bringing up a good point - in the time it takes to post a question on here asking for the definition of something, couldn't the person simply search the web (or use wikipedia) themselves?

I guess that's what having an entire generation being raised by a television set gives you.

Answer 8

A measure for distances in space, equal to the mean distance of the Earth from the Sun-that is, 93,000,000 miles (149,599,000 kilometers)