Post by Thomas 'PointedEars' LahnPost by Paul B. AndersenPost by Thomas 'PointedEars' LahnPost by Paul B. AndersenPost by Thomas 'PointedEars' LahnPost by Paul B. AndersenAll astronomers, amateurs as well as professionals, know how
to make the telescope point in the right position when
the right ascension, declination and time are known.
They know that they have to correct for stellar aberration
and parallax because they can't point the telescope to were
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Post by Paul B. AndersenPost by Thomas 'PointedEars' LahnParallax has *nothing* to do with the speed of light.
Who said it had?
You did.
You can't be serious! :-D
See the markings above. There is no point denying what you did, though it
*might* have been just unfortunate wording. Your reaction below belies that
assumption, though: You appear to actually believe that parallax would have
to do with the speed of light.
So you ARE serious. But I still can't understand how it is
possible to interpret what I said that way.
Why not simply admit that you made a minor blunder?
Why is it so important to you to insist that I
said that parallax depend on the speed of light,
which I never said?
Post by Thomas 'PointedEars' LahnPost by Paul B. AndersenYou know of course that when the position of
a star at EPOCH J2000 is right ascension RA and
declination DEC, then the angles RA and DEC give
the direction from the Sun to where the star was when
the light that reached the Sun at January 1, 2000,
was emitted.
You do now this, don't you?
It is quite simple.
When you observe a star, it will, due to stellar aberration,
during one year appear to move along an ellipse. The major axis
of this ellipse will be parallel to the ecliptic plane.
The major axis will be:
arcsin(2v/c) radians ~= 40.98 arcseconds
The minor axis will be:
40.98*cos(ecliptic latitude) arcsecs
(It will be a very small distortion of the ellipse due
to the eccentricity of Earth orbit.)
The parallax will distort this ellipse slightly. The distortion
will always be towards the point in the centre of the ellipse.
If the maximum parallax is p, then the major axis will be
shortened by 2p, while the minor axis will be shortened
by 2p*cos(ecliptic latitude)
But the parallax will always be small relative to 41 arcsecs.
The parallax of the closest star, Proxima Centauri, is p = 0.77".
And for a star 100ly away p = 0.033". It's a reason why
the parallax is given in mas (milli-arcsecs) in the star catalogues.
So the parallax will for most stars be very small relative to
the stellar aberration, and because of its symmetry, it will
not affect the position of the centre of the ellipse.
And the important point is:
The position of the star charted in the star catalogues
is the RA and DEC of the centre of that ellipse.
And this point is where we would see the star from the Sun.
And remember the obvious:
The position charted in the star catalogues is
where the star was relative to the Sun when the light
was emitted.
Post by Thomas 'PointedEars' LahnMy research indicates that this NOT in general so. Some catalogs include
The position of the star, given as RA and DEC doesn't
include stellar aberration and parallax, but there may
be differences between the catalogues due to different
reference systems. Finding the correct position of the vernal
point and the correct angle of the equatorial plane isn't
easy, since both are changing with time due to the precession
and nutation of Earth's axis, and converting from one EPOCH to
another may not be trivial.
But this is nitpicking, and irrelevant to the issue:
Did I claim that stellar parallax depend on the speed of light?
Generally what I said above is correct, and I think you know it.
Post by Thomas 'PointedEars' Lahn<http://star-www.rl.ac.uk/docs/sun67.htx/sun67se4.html>
And none of this has inherently to do with parallax, the point that I am
making which you are still missing.
Parallax is the *apparent* change of position of a (celestial) object (in
the sky) against the background (e.g. fixed stars sky) because it is viewed
from different positions. It does NOT depend on the speed of the observer
relative to the observed object (and therefore NOT on the speed of light),
only on the position of the observer. The assumption there is that the
peculiar motion of stars relative to the observer can be neglected as within
a year the difference is too small), and that stellar aberration is already
considered.
You can observe parallax easily with our own eyes if you focus on an object
with one eye and then with the other eye. If the speed of light would be
* *.* * * * *. * *
\ .---. /
'.___.'
\p: /
\:/
*
/:\
/ :p\
/ : \
/ : \ d
/ : \
/ : \
/ __:___ \
.-''' : '''-.
T S-------T
`-..______...-'
[Annual parallax: The position of a star (*) in front of the star
background (* * *) appears to shift as Terra (T) moves in its orbit and
the star is observed from different positions in the orbit. The farther
away the star, the smaller this shift. For stars that are close enough,
then, trigonometry can be used to estimate their distance
d ≈ (1 AU)/(sinp) ≈ (1 AU)/p.
Surveying missions like Gaia use the same principle on a larger scale due
to much better image resolution.]
I know what parallax is.
This is irrelevant to the issue:
Did I claim that stellar parallax depend on the speed of light?
Since the position charted in the stellar catalogues is where
the star was relative to the Sun when the light from star
was emitted, my statement:
To point the star in the right direction to see a star
"they have to correct for stellar aberration and parallax
because they can't point the telescope to were the star
was when the light was emitted."
Or with slightly different words:
"they have to correct for stellar aberration and parallax
because they can't point the telescope to the RA and DEC
given in the star catalogues."
There is no way this statement can be interpreted to
mean that aberration depend on the speed of light.
And I still can't understand how you managed to interpret
it in such a way.
So I can only repeat my question:
Was it the statement you marked: "where the star was when
the light was emitted" that trigged you to make the strange
blunder that this means that parallax depend on the speed of light?
--
Paul
https://paulba.no/