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An astronomical unit is defined as the measurement of distance between Earth and our Sun, my question is since distances between celestial objects beyond our solar system are vast and unimaginable astronomer adopted another familiar term "light-year" instead. My question is do people still use astronomical unit in the present days?

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    $\begingroup$ JPL uses the AU exclusively in calculating planetary ephemerides, and the astronomical unit is now defined as precisely 149597870.700km. Source: pages 7-8 of ilrs.gsfc.nasa.gov/docs/2014/196C.pdf $\endgroup$ – barrycarter Mar 26 '15 at 14:07
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Certainly. Astronomical Unit is probably one of the most used distance units used in astronomy. It is of course only used when discussing the distances within a stellar system, such as the distances between the Sun and its planets or other bodies in the solar system. It is also used to discuss distances in other stellar systems, e.g. the distances between stars and their (exo)planets.

It is easier to get a sense of the distances between bodies in the solar system if you use astronomical units. It doesn't say much when we say that the distance between Neptune and the Sun is $5\times10^{-4}$ light year. You would have to know the distance between Earth and the Sun ($1.5\times10^{-5}$ light year), and then calculate the difference (i.e. 30 times = 30AU) to get a sense of scale.

As a side note, in professional astronomy, the light year is hardly ever used. Professional astronomers use the parsec (=3.26 light year), kilo parsec (kpc), megaparsec (Mpc), and Gigaparsec (Gpc) to specify distances between stars, galaxies, and galaxy clusters.

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    $\begingroup$ +1 for mentioning that lightyears are never used. Also, AU is often used when referring to cell sizes in very-high-resolution simulations, e.g. hydrodynamical simulations of clouds collapsing to for a star. $\endgroup$ – pela Mar 26 '15 at 10:53
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I guess the use of one unit over the other (AUs, parsecs,lightyears, etc.) will depend mainly on the distances of the object under study. If you work with Solar System objects, it will be easier to use AUs, whereas if you work with Galaxies, AUs are not much use and you'd probably work better with lightyears.

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I want to expand on the use of an AU in comparing planetary systems a bit. When we look for example at this image of the (instantly famous) protoplanetary disc imaged last year in HL Tau: HL Tau: Planetary formation in action

In observational astronomy, when we look at an object at a distance $d$ and it has a certain angular size $a$, thanx to the use of the century old parsec we know the objects size $s$ in AU.

How does this work?

When you make the image of an object, you always know $a$. This is simply the apparent size on the celestial sphere in arcseconds. Nowadays we also know the distance to objects. So by use of the fundamental relation "angle = size / distance", we always can have $s(AU) = a(arcseconds) * d(pc)$.

This way any astronomer can quickly convert the apparent angular size into physical size and immediately tell about the dimensions of his object!

To check this method, we can know that the HL Tau is 450Lj away, which is 140pc and if you now look at the scale given in the image, this will be vagely familiar ;)

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