Re: Re: McCoy's grasp of error analysis is totally absent.

On 11 Apr 2006 14:44:15 -0700, mccoy@xxxxxxxxxx enriched this group
when s/he wrote:


r norman wrote:
On 10 Apr 2006 13:55:16 -0700, mccoy@xxxxxxxxxx wrote:



It's interesting to not the reason surveyers know as much as they do
regarding accuracy and triagulation is because they can actually drive
their car to the site that they are measuring. Hence the methods have
been verified. This is not so with astronomy.

Yes it is. There are several other ways of measuring distance.

Let's continue.






Astronomers have a very different problem.

The biggest problem is that they do not know how far stars are.

Yes they do. That has been worked out.

They
have constructed scenarios as to how they could achieve rough
estimates, but those estimates are based on empty assumptions.

No, the only empty thing is your head.


They can easily measure
angles to a fraction of an arc second and they don't need 0.01%
accuracy.

Interestingly enough nobody around here wants to put forth the distance
of a light year. Compare that to the pitiful distance of the Earth's
orbit from one side to the other and you have error seemingly
compounded.

you hve been told, several times that a Parsec stands for "PARallax of
one arc SECond". Now, remember that the parallax of a star is half the
angular distance a star appears to move against the celestial sphere
due to the orbit of the Earth around the Sun. In other words it is the
angle subtended at a star by the radius of the Earth's orbit. One
parsec is defined to be the distance from the Earth of a star that has
a parallax of 1 arcsecond. It is, therefore, approximately:
360×60×60/(2?) = 206,264.8062 AU or 19,173,511,600,000 miles or
3.26156378 light years

Now, as pointed out by several people, an arcsecond is a large amount
by modern standards. Parallax works out to sever hundred light years
from ground based - any much further from space based telescopes.



In fact an estimate of distance that is only 1% accurate,
even 10% accurate is quite sufficient for most purposes. Hence, the
rules for astronomers are very different. If you measure angles to
0.05 arc second and need 1% accuracy, than you can measure the
distance of a star up to 5 arc second parallax . That is a distance
better know as 5 Parsec which corresponds to about 16 light years.
If you only need 10% accuracy, you can measure distances out to 160
light years. Frankly I don't know how accurate the angular measurement
is, other people have told you and you could look it up in the back
posts here. But that is what the analysis tells you.

The problem is you can't even estimate what is 10% accuracy. Say that
astronomers have been assuming that a star was 4.4 light years away
when in fact it was 10 light years away. How could this happened?

It can't.


Well, the supposed triangulation attempt was made (and one wonders as
to why it was attempted in the first place), and based on the 10% rate
of acceptance the figure of 4.4 light years is generated. The fact is,
you can't determine accuracy rates unless you can actually measure the
actual rate.

Of course you can.

At least that's how accuracy rates are calculated based
on surveying techniques! What's the difference between some astronomers
and surveyers? Surveyers prove their work and some astronomers keep on
guessing.

Both measure using their instriments - usually several times just to
make sure.


That is, it tells you if you had the slightest grasp of error
analysis.

Incidentally, as so many other people have told you, astronomers do
NOT measure parallax by measuring the angle at which their telescopes
are pointing but by measuring the apparent shift in position of nearby
stars with respect to much more distant "fixed" stars.

You haven't figured it out yet.

True - you haven't.

The fact be told is that these are
complete guesses.

No, acurate measurements.

One assumption based upon another.

No, one measurement based on tried and tested maths.

There isn't much
information that can be conveyed by shifting position of stars.

Its distance can be.

It
becomes even particularly harder when you can't even place your self in
that space. On earth everything has geographical relatedness. In space
information content is sparse. I'd be nice if we could draw a permanent
grid across the universe.

We have.

If this is done automatically then all we
have to do is measure the squares in our solar system and you can
measure anything that is in view (almost I suppose). But in reality
there is just too much empty space inbetween us and the stars. And, to
be sure, that's the only thing that astronomers really know.

They, and the rest of us, know a lot more than you it seems.

JM
--
Bob.

.

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