In the E-Reader Era, a Book a Year Is Slacking

[QuantumIguana]

You saw parsec in some old science fiction. Star Wars used parsec: "You've never heard of the Millennium Falcon?... It's the ship that made the Kessel Run in less than twelve parsecs." It appears to be a misunderstanding of the term, as he appears to be talking about how little time it took his ship, but it has been retconned into making the run in a short distance, rather in a short time.

[Sil_liS]

Originally Posted by murraypaul

I'm not sure I've ever seen parsec used in science-fiction, and if it is used at all, its used is overwhelmed by the use of light-years. In reality, both light-years and parsec are used.

So what point were you trying to make here:

Originally Posted by murraypaul

Nor are lightyears, which is why they are used more in science fiction than in reality. The parsec is the preferred unit.

Were you trying to say that since parsec it used in reality, although far less than lightyear, it is almost never used in fiction so that makes it better?

Originally Posted by murraypaul

The parsec was defined based on a calculation of a parallex of 1 arc second at the distance of the sun, but that has nothing to do with the measurement of the distance between two objects, and you don't measure parallex to calculate that distance. In the same way that the kilo was defined as the mass of 1 litre of water, but you don't measure people's weight by comparing them to volumes of water.
A parsec is a distance of 30.857×10^12 km.

You do measure parallex, I quoted the text that explained that. Maybe you would be more comfortable with an explanation that involved drawings: http://www.astronomy.ohio-state.edu/...distances.html

[murraypaul]

Originally Posted by Sil_liS

You do measure parallex, I quoted the text that explained that. Maybe you would be more comfortable with an explanation that involved drawings: http://www.astronomy.ohio-state.edu/...distances.html

You are confusing the definition of the unit with the use of the unit.
From Wiki:
"The parsec is equal to the length of the adjacent side of an imaginary right triangle in space. The two dimensions on which this triangle is based are the angle (which is defined as 1 arcsecond), and the opposite side (which is defined as 1 astronomical unit, which is the distance from the Earth to the Sun). Using these two measurements, along with the rules of trigonometry, the length of the adjacent side (the parsec) can be found."

The only parallax measurement required to calculate this is to the closest star of all, the sun.
That gives a fixed definion of the parsec as a distance.
You can then describe any distance in parsecs, just as you could in feet or metres or miles.
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?

You can also use parallax as a measuring tools, but any errors involved in that measurement have nothing to do with the units you choose to express the answer in. There would still be the same error regardless of whether you quoted the results in parsecs or light-years.

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]"

And from the site you posted:
"The light year is used primarily by writers of popular science books and science fiction writers. It is rarely used in research astronomy."

That sounds quite a lot like saying that light-years are used more often in science-fiction than they are in reality.

[QuantumIguana]

Originally Posted by murraypaul

And from the site you posted:
"The light year is used primarily by writers of popular science books and science fiction writers. It is rarely used in research astronomy."

That sounds quite a lot like saying that light-years are used more often in science-fiction than they are in reality.

It really doesn't. Pick up any book on astronomy, at least any book on astronomy intended for the general public, and it is going to talk about light-years rather than parsecs. Virtually every release from NASA uses light-years instead of parsecs. Maybe internally they use parsecs. But, even leaving out science fiction, the overwhelming use is light-years.

[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.

[Fluribus]

My questions are...

Is it the same book that slacks every year or do they trade off?
How many books used to slack every year?
Is the slacking in terms of sales or quality?

I hope I don't have any slacker books in my library. If I do, I'll have to start beating them.