Re: precession of mercury

On Dec 26, 2:41 pm, hw@..(Dr. Henri Wilson) wrote:
On Thu, 25 Dec 2008 18:39:00 -0800 (PST), Jerry
<Cephalobus_alie...@xxxxxxxxxxx> wrote:
On Dec 25, 3:09 pm, hw@..(Dr. Henri Wilson) wrote:
On Thu, 25 Dec 2008 04:22:32 -0800 (PST), Jerry
<Cephalobus_alie...@xxxxxxxxxxx> wrote:
On Dec 24, 3:24 pm, hw@..(Dr. Henri Wilson) wrote:

The whole process could be simplified if the thing was sent into a circular
orbit in the ecliptic plane. However as long as the axis of precession of the
axis of rotation is perpendicular to that plane, a small tilt in the orbit
plane wont cause much of a problem or error.

A worthless suggestion inconsistent with your earlier posts
(you are obviously trying to make it fit the nominal Hipparcos
orbit) and quite clueless.

It should be obvious that what I said is correct. Placing it an ecliptic orbit
would allow for much simpler analysis and greater accuracy.

False. A satellite in ecliptic orbit around the Sun at one of
Earth's stable Lagrangian points will still observe aberration.

Who said anything about Lagrangian points. By "In ecliptic orbit" I meant "in
an Earth orbit in the ecliptic plane". Do you understand now Crank?

I was trying to explain the problem ONE BABY STEP AT A TIME. The
observed aberration is a COMPOSITE CURVE consisting of small
ellipses resulting from the satellite's orbit around the Earth
superimposed on a big ellipse resulting from the Earth's orbit
around the Sun.

Apparently you can't even grasp the problem when presented in
BABY STEPS.

Stars at the Ecliptic poles will apparently move around in
circles, stars on the ecliptic will move back in forth in
straight lines, and stars at intermediate declinations will
apparently move in ellipses.

Well, if the telescope does not spin on its own axis, the whole field of view
moves in an ellipse but does not spin.

How do you propose to arrange for the telescope to be pointed
perpetually in one direction in space? You'd be surprised how
much torque is exerted on the telescope due to the outer fringes
of Earth's atmosphere, solar wind, light pressure, etc.

No, you need at least two distant reference points for getting
the telescope oriented...

The sideways displacement due to
aberration for any particular star is the same throughout out the year no
matter what its emitted light speed wrt Earth. Thus, relative star movement
caused by parallax or proper speed should be easy to measure.

It's easy to measure against the backdrop of distant stars, but
this effort is not assisted in any measure whatsoever by your
worthless proposal.

A satellite in ecliptic orbit around the Earth will see
superimposed on the yearly stellar aberration ellipse of 20.5"
semimajor axis smaller ellipses whose period will correspond to
the satellite's orbital period about the Earth, and whose semi-
major axes will depend on the satellite's orbital velocity.

So there is no benefit at all to an ecliptic orbit.

There are enormous benefits.

None. Nada. Zilch.

What would have been of great benefit to Hipparcos was if the
satellite had been successfully inserted into its nominal orbit.
High above the Earth, it would have experienced eclipses only
around equinox time, its slower, more uniform speed would have
necessitated simpler aberration corrections, and it would not
have experienced as much radiation damage through repeated
passages through the Van Allen belts.

An ecliptic orbit, by the way, would have made eclipses a year-
round affair.

that is what happened.....every orbit....The eclipse didn't last for long and
apart from a small loss of power did not affect the performance.

I was speaking of the NOMINAL orbit. -Your- "nominal" orbit would
cause eclipses every day throughout the year.

An equatorial or
other orbit requires additional and very complcated corrections.
The Hipparcos telescope was meant to be sent into an ecliptic orbit.

Nope. It was meant to be sent into a geostationary orbit.

Well the designers are only human .... and are not infallible.

Speak for yourself. Your proposal is worthless.

Obvoiusly neither you nor Paul can get it into your head that aberration can be
virtually eliminated from parallax measurements if the telescope used does not
rotate around its own axis.

Not only inconsistent with the immediately preceding paragraph,
but utterly false and clueless. In the preceding paragraph, you
propose a telescope whose "axis of precession of the axis of
rotation is perpendicular to [the ecliptic] plane" while here
you propose a telescope that does not rotate, period.

Try thinking occasionally Crank.
As long as it is in the same orientation when specific stars are in view, that
constitutes 'NOT ROTATING'.

That's called making use of guide stars, an old technique.

I should imagine so...but in the Hipparcos case the thing is spinning around a
tilted axis that precesses. ...so whenever it is in EXACTLY the SAME
orientation, it should be looking at the same field slightly offset by
aberration but with relative star positions altered only by parallax.

You seem to think that parallax measurements must be made with
respect to the telescopic field. Hence your crap about needing
to hold the telescope absolutely steady throughout the year.

In reality, parallax measurements are made with respect to the
background of distant stars.

The scanning procedure is clearly quite ingenious and the same stars are viewed
a number of times during the year. They are identified by the direction in
which the telescope is pointing at the time. It is has the same angular
position at the time, aberration effects are avoided.

You haven't the foggiest idea what you propose, do you? It keeps
changing from post to post, and even within this single post, it
changes...

I have to explain it in more basic terms so you can understand what I said.
Aberration CAN EFFECTIVELY BE ELIMINATED.

As I have tried to explain to you, the guide star technique has
distinct limitations as regards the needs of high precision
imaging and high precision astrometry. A suitable guide star
cannot always be found in the immediate vicinity of an object
to be imaged. A mere 1 minute of arc separation between guide
star and target object would result in an approximately 1 mas
tracking discrepancy for the HST. Try working it out.

No...I doubt you can. OK. Figure 7 km/s orbital velocity for HST
works out to about 5" semimajor axis for the aberration ellipse.
At 90 degrees normal to the HST orbital plane, the aberration
takes on a circular pattern, so the semiminor axis is 5". In
the plane of the HST orbit, the semiminor axis is zero. Divide
5" by 5400 minutes of arc in a right angle, equals on average
1 mas stellar aberration discrepancy per minute of arc difference
in declination between guide star and target.

Did you follow? A similar type of estimate can be made for
differences in right ascension between guide star and target.

My idea is not to use a guide star but to keep the telescope pointed in exactly
the same direction without spin...so the whole image is simply displaced in an
elliptical manner but does not spin in the viewing field.

How do you arrange for a spinless satellite? You'd be surprised
how much torque is exerted on the telescope due to the outer
fringes of Earth's atmosphere, solar wind, light pressure, etc.

One way or another you need guide stars. But as I explained
earlier, there are limitations in the basic guide star technique.
All of the major space observatories (Compton, Chandra, Hubble,
Spitzer, etc.) have used computer-guided active steering to
compensate for aberration due to their orbits around the Earth.

Do you understand now why your proposal is worthless as regards
the needs of high precision astrometry and high precision
imaging?

From what you have said, I understand that many Hipparcos figures are probably
in considerable error because they are based on all light traveling to Earth at
speed c.

There is ZERO EVIDENCE for variable speed of light in the
Hipparcos data. As Paul has explained, variable speed of light
would have major consequences which are NOT OBSERVED.

Jerry
.

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