And you are perfectly making my point...

The only way we could be assured of the laser's accuracy would be to first
measure it with some sort of ruler, then compare the laser with the ruler
measurement. This has not yet been done.

In the US, we have an agency called something like the National Bureau Of
Standards. It's been years, so forgive any minor errors, but at that bureau
they have such things as a metal bar exactly one foot long, a metal weight
that weighs exactly one pound, and similar objects, These objects are made
of various metals and alloys to as best as possible minimize the effects of
expansion and contraction and similar effects.

These are the items that define the various weights and measures we use in
our country; other objects are compared to these to ensure their accuracy.

Now let's look at our laser device...

Something like a laser measuring device (for the purposes we are discussing
here) will be used to measure the distance to something that has been placed
one mile away by a ruler (or similar device). Then it will be used to
measure the distance to something that has been placed ten miles away by a
ruler (or similar device). And this might continue until the curvature of
the earth prohibits additional measurements.

So let's loop back to the original discussion, which had to do with facts
versus consensus.

Our laser measuring device will be examined by various scientists, the
theory behind it will be scrutinized, the testing methodology and results
will be examined, and eventually the scientists will come to a CONSENSUS
that this laser device can accurately measure 250,000 miles. And that's
fine, it probably will be more accurate than previous measurements. But you
will notice that every time a new measurement method comes into play the
distance changes.

But with our metal bar from the NBS, no consensus as to it's accuracy is
required. As it defines the measure, it is defacto correct.






"Edward Todd" wrote in message
...
In article ,
"Bill Denton" wrote:

There is a generally accepted distance between Earth and the moon
(roughly a
quarter of a million miles). This would generally be described as a
"fact".
But no one has ever taken a ruler and actually measured that distance.


One of the Apollo missions left a special prism like mirror set up on
the moon. They then bounced a laser off of it from Earth and were able
to measure the distance with extreme accuracy ( have to look it up but
memory says within a millimeter or so) ... at least the distance at that
particular moment in time (not a perfect circular orbit).


Edward