[Sil_liS]

Originally Posted by murraypaul

You are confusing the definition of the unit with the use of the unit.

No, I'm not. The unit is used because the distances to nearby suns are measured by parallax.
How do astronomers determine the size and distances of stars?

To get distances, we use a variety of techniques. The most basic one is geometric parallax. By photographing the same star 6 months apart, the shift of the star relative to more distant background stars amounts to 1 second of arc at 1 parsec ( 3.26 light years), 1/2 arcsecond at 2 parsecs, 1/10 arcsecond at 10 parsecs etc.

Originally Posted by murraypaul

You don't calculate a distance in light-years by shining a light and then waiting to see how long it takes to arrive, do you?

The ongoing Lunar Laser Ranging Experiment measures the distance between the Earth and the Moon using laser ranging. Lasers on Earth are aimed at retroreflectors planted on the Moon during the Apollo program, and the time for the reflected light to return is determined.

This method relies on having a reflector on the surface of the celestial body. In the case of galaxies you don't need a reflector because they emit light themselves. You determine how far the light traveled by the redshift of the spectrum. The result is the distance between the Earth and the place where the galaxy was at the time when it emitted the light. The farther the star, the older the image.

Originally Posted by murraypaul

And as far as measuring parallax only for close stars:
"The parallax method is the fundamental calibration step for distance determination in astrophysics; however, the accuracy of ground-based telescope measurements of parallax angle is limited to about 0.01 arcseconds, and thus to stars no more than 100 pc distant.[5] This is because the Earth’s atmosphere limits the sharpness of a star's image.[6] Space-based telescopes are not limited by this effect and can accurately measure distances to objects beyond the limit of ground-based observations. Between 1989 and 1993, the Hipparcos satellite, launched by the European Space Agency (ESA), measured parallaxes for about 100,000 stars with an astrometric precision of about 0.97 milliarcseconds, and obtained accurate measurements for stellar distances of stars up to 1,000 pc away.[7][8] NASA's FAME satellite was to have been launched in 2004, to measure parallaxes for about 40 million stars with sufficient precision to measure stellar distances of up to 2,000 pc. However, the mission's funding was withdrawn by NASA in January 2002.[9] ESA's Gaia satellite, due to be launched in late 2012, is intended to measure one billion stellar distances to within 20 microarcseconds, producing errors of 10% in measurements as far as the Galactic Center, about 8,000 pc away in the constellation of Sagittarius.[10]"

You seem to have a problem with the concept of order of magnitude.
- Size of our galaxy roughly 30,000 parsecs across.
- Current maximum distances measured by parallax 1,000 parsecs.
- Best hope for the near future 8,000 parsecs.
- Closest galaxy 700,000 parsecs away.


I would also like to point out that if the lightyear cannot be a valid unit in the distant future because the year it is defined by the time it takes out planet to go around the Sun and there should be no units dependent on dimensions specific to our solar system, then the parsec should also be invalid since its definition is based on the distance between the Sun and the Earth.