Re: michelson morley experiment

Dear darkknight:

"darkknight" <darkknight.21@xxxxxxxxx> wrote in message
news:j4v5421agbfm9lotd1nj1ge7ndg597bm1b@xxxxxxxxxx
On Sun, 16 Apr 2006 16:29:21 -0700, "N:dlzc D:aol T:com
\(dlzc\)" <N:
dlzc1 D:cox T:net@xxxxxxxxxx> wrote:

Dear darkknight:

"darkknight" <darkknight.21@xxxxxxxxx> wrote in message
news:bbi542pab06k915e2hurc8hotk5i4ktcv3@xxxxxxxxxx
...
In the frame of the earth surface at the location of
the laser, the light leaves the laser straight down
its centerline.

Then why do telescopes have to be "tilted" to
account for stellar aberration? i.e. the line of the
telescope has to be at a very slight angle to the
line of the light.

No. The telescope has to be *inline* with the light.
http://en.wikipedia.org/wiki/Aberration_of_starlight
... just that "inline" varies.

Thanks. I got confused by the analogy with a
narrow bucket and rain, where the bucket has to
be at an angle to the path of the rain for the
raindrops to hit the bottom of the bucket and the
angle changes according to the speed of the bucket.

However, my understanding of stellar aberration is
still zero and I have no clue how astronomers can
even tell the position of a star has "changed" or
what that means. I would have thought the
"position" of a star was equivalent to the angle at
which the light from the star strikes the earth -
which is affected by the tilt of the earth and
the time of day so how could you tell the position
didn't change due to that.

It is an effect *in addition to* that. That is how precisely you
have to measure things.

And apart from that, if the relative positions of
the star and the earth are changing due to motion
of the galaxies or the stars within the galaxies
then that is going to cause the star's position to
change all the time.

Yes, it does.

I don't see how a change from 340 km/second to
400 km/second is going to be the cause of a
change in position

.... "apparent position" ...

of the star - but then I don't know how you measure
the position of a star in the first place.

Likely the same way as surveyors measure(d) the surface of the
Earth (before GPS). Like how they notice very tiny changes in
elevation of important markers due to the depletion of
underground aquifers (subsidence).

I'm supposed to be intelligent but I can hardly
understand anything ...

Patience.

(anyway, no need to waste time trying to explain.
I'm sure I can find a book on it somewhere - yep I
did read the Wikipedia article (for the second
time) but I still don't understand).

The answer will come when you are ready for it. Good luck.

David A. Smith


.

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