How are Astronomical Units different from Light-Years?

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belarafon eNotes educator| Certified Educator

Astronomical Units (AU) and Light-Years (ly) are both measurements of distance, generally used to show large amounts of distance between points. Light-years are better suited for calculating the vast distances between stars and galaxies, while AUs are better suited for calculating distance inside the (relatively) smaller space of a solar system.

One light-year is equal to about 6 trillion miles (9.5 trillion kilometers) and is the distance it takes light to travel in one year. Light, of course, travels at the speed of light -- 186,282 miles per second (299,792,458 meters per second) -- and so would travel about that distance in a perfect vacuum in one year. Because of the enormous distances involved on the galactic scale, light-years are well suited for measurement, although the astronomical society tends to prefer the Parsec, which is about 31 trillion km, to make calculations easier.

One astronomical unit is about 93 million miles (150 million kilometers) and is the approximate distance of the Earth to the Sun. This is a general unit of distance, as the Earth is closer to or farther from the Sun depending on season, but given a plus/minus of about 0.02 AU it is accurate. This unit, being much smaller than a light-year or a parsec, is very useful in calculating and graphing distance within a solar system or other body on the solar scale. Instead of working with fractions of light-years, AUs provide an easier base scale.

One light-year is about 63,281 AUs.

astrosonu | Student

Hello! AU (astronomical unit), is the unit --- 1 AU = to 150 million km.

that means the distance from earth to sun or from sun to earth. this is astronomcal unit. Light year = 9461 billion km. that means the amonut of distance covered by light in a year is a light year.



Hey Channing! R  U   a kid like me, its so simple. U  should know about it.  Yeah! I am studying at std 7.

Wiggin42 | Student

The Astronomical Unit is defined as the average distance from the Earth to the Sun. Its equal to approximately 150 million km and is abbreviated as AU.

A light year is defined as the distance light travels in a year. Light travels at 2.99 x 10^8 meters per second. Multiplying a rate by time gives you a distance so we multiply the speed of light by time ( years) and that gives us the light year. Its approximately 9461 billion km. Light year can be abbreviated as "  ly " or it can be simply denoted by the constant " c " .

astrosonuthird | Student

You are wrong my dear aneesh. 1 light year is not equal to 1 AU.


1 AU = 150 million km.


1 LY = 9461 billion km.


you better make yourself correct.

aneesh123 | Student

1 Astronomical Unit (AU) is the average distance between the earth and the sun. It is approximately 149,598,000 km. 1 light year

astrosonuthird | Student

I AU is 150 million km.


I LY is 9461 billion km.