[Sil_liS]

Originally Posted by murraypaul

I made three assertions:
a) Light-year was also not the best choice for measuring distance between galaxies
b) Light-year is used more in science fiction than in reality
c) Parsec is the preferred unit (for measuring distance between galaxies)

And for the latter, as you accept NASA as a source:
http://heasarc.nasa.gov/docs/cosmic/milkyway_info.html


International Astronomical Union:
http://www.iau.org/public/measuring/

It seems that you missed something in the quote:

Beyond the Solar System the distances in astronomy are so great that using the AU becomes too cumbersome. The IAU recognises several other distance units to be used on different scales. For studies of the structure of the Milky Way, our local galaxy, the parsec (pc) is the usual choice. This is equivalent to about 30.857×1012 km, or about 206,000 AUs, and is itself defined in terms of the AU – as the distance at which one Astronomical Unit subtends an angle of one arcsecond. Alternatively the light-year (ly) is sometimes used in scientific papers as a distance unit, although its use is mostly confined to popular publications and similar media. The light-year is roughly equivalent to 0.3 parsecs, and is equal to the distance traveled by light in one Julian year in a vacuum, according to the IAU. To think of it in easily accessible terms, the light-year is 9,460,730,472,580.8 km or 63,241 AU. While smaller than the parsec, it is still an incredibly large distance.

There is a reason why I'm pointing this out:
The name parsec is "an abbreviated form of 'a distance corresponding to a parallax of one second'".

Trigonometric parallax--the tiny, apparent back-and-forth shifts of nearby stars caused by our changing perspective as the earth orbits the sun--can indeed be used to measure distances only to comparatively nearby stars.

Notice the "only".

When it comes to galaxies:
In the 1920s Edwin Hubble used the period-luminosity relation for variable stars to establish the distances to various galaxies and proved that they lie far outside our Milky Way. In the course of that work, he discovered what we now call 'Hubble's law,' that galaxies display a linear relation between distance and redshift (the redshift is the shift in the positions of lines in the galaxies' spectra toward the red end of the rainbow). Hubble's law is the basis for the modern understanding that we live in an expanding universe. After measuring the redshift, which we can do by passing a galaxy's light through a spectrogram, we can deduce the distance using Hubble's law. This technique is the astronomer's basic tool for finding the distances to the farthest things in the universe.

The reason why using light-year makes sense is because when you say that a galaxy is 13.2 billion light years from Earth, it means that the image that we get is the image of how the galaxy looked like 13.2 billion light-years ago.


And can you tell me the basis for this assertion: b) Light-year is used more in science fiction than in reality.