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not a property of the observation location, it was a global
application configuration parameter. I had to hit C<< <F2> >> to open
the configuration menu. In this menu, I selected the C<Plugins> tab,
then C<Time zone>. With this, I selected a UTC display instead of a
display using the computer's local timezone.

=back

=head1 Heliocentrism Or Geocentrism?

Here are two assertions. Are they true of false?

=over 4

=item A

The Sun goes around the Earth.

=item B

The Earth goes around the Sun.

=back

Assertion A is false, everyone agrees. But assertion B is false too.

Oh yes indeed, will you answer, it should read actually:

=over 4

=item C

The Earth runs along an elliptic orbit with the Sun located on
one focus of the ellipse.

=back

This assertion is false too. Each one of the following assertions
is nearer to the truth than assertions B and C (and A).

=over 4

=item D

The center of mass of the Earth-Moon binary system runs along an
elliptic orbit with the center of mass of the Solar System located
on a focus of the ellipse.

And I will point that the center of mass of the Sun is not the same
as the center of mass of the Solar System. There are even times when
the center of mass of the Solar System is I<outside> the surface of the Sun.
The L<webpage|http://hp41programs.yolasite.com/solar-sys-bary.php>
about an HP-41 program states that on 15th March 1983, the distance between
both centers of mass was nearly 2.1 Sun radii.

=item E

The Earth runs along an orbit around the Sun, with noticeable perturbations
caused by the Moon, Jupiter, Saturn, etc.

Which is a formulation equivalent to assertion D.

=item F

The movement of the Earth with the Solar System is a I<n>-body problem,
with I<n> ≥ 3. Therefore, there is no analytical solution.

=item G

The Solar System is a chaotic system. Even if we can predict with a reasonable
accuracy what the various orbits will look like within the next hundred million
years, this prediction is no longer possible for an interval of one milliard years
(one billion years for US).

=item H

The Earth corkscrews in the general direction of the Hercules constellation
with a approximate speed of 220 km/s.

=item I

The Earth runs along an orbit around the center of the Milky Way, with noticeable
perturbations caused by the Sun, the Moon, Jupiter, Saturn, etc.

=back

Assertions B and C are what Terry Pratchett, Jack Cohen and Ian Stewart call
I<lies to children> (I<Science of Discworld>, chapter 4, pages 38 and 39). These
are false assertions, but simple enough to be understood by a child and which, even
if false, leads children to a better understanding of the described phenomena and brings
them closer to truth. You cannot tell assertion C to a child and expect him to understand
it without telling him first assertion B. And it is worse with assertions D and next.

Moreover, these are what I would call I<lies to adults>. In the beginning, people would consider that
the aim of Physics was to build a mathematical representation of the real world,
getting closer and closer to the ultimate truth. Then, there was quantum physics
including especially de Broglie's work
with the duality of wave and particle and the Copenhagen interpretation. Is the ultimate
nature of the electron (for example) a wave? No. Is it a particle? No. So what? We do not
care about the ultimate nature of the electron. The aim of Physics is to no longer
to provide a mathematical I<representation> of the real world, but to build 
several mathematical I<models> of the real world. We know that intrinsically all
models are false, but each one has it usefulness to lead to make computations about
the real world.

Please note that I was talking about scientific methods. I was not dealing with 
electoral campaigns and advertisements. Every sane adult knows for
sure that these are ridden with lies.

Other lies to adults, also known as "simplifying
hypotheses", you will find in the following:

=over 4

=item *

the light propagates instantly from one place to another,

=item *

the celestial bodies outside the Solar System are motionless,

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time. With sidereal time as a time, that would translate as saying
that sidereal time and the principle of timezones are incompatible.
Am I right or am I wrong? I found nothing about that in the 
"sidereal time as a time" explanations.

So, between a simple and comprehensive definition on one side
and a convoluted and incomplete definition on the other side, 
I will stick with "sidereal time as an angle" definition.

=head2 Other Movements

Before I explain the other movements involved with the
Sun and the Earth, let me tell you a little digressive note.

=head3 Weather And Climate

I hate these people who, each time snow falls, cry
"Where is this global warming scientists talk about
again and again?" These people seem to ignore that
climate and weather are two different things. When
the temperature from a meteorologic station varies
by 10 degrees C from one day to the following, this
is a mundane meteorological event. When the I<average>
temperature for a decade varies by 2 degrees C from
one century to the next, this is a catastrophic
climate event.

The movements I explain below are more "climatic" and
less "meteorogical" than Earth's spin and orbital
rotation. Their values over a short timespan are so
low that the algorithms computing astronomical positions
over a short timespan do not care about them.

Note: weather (but not climate) will come back in a few
chapters as a real phenomenon, not as a metaphor.

=head3 Equinox Precession

The best known movement with a long timescale is the
equinox precession. Presently, the vernal point lies
within the constellation of Pisces. But actually, it moves
all along the ecliptic, making a whole turn in about
26,000 years.

=head3 Nutation

The angle between the equatorial plane and the ecliptic
plane varies slightly. In Paul Schlyter's C program, the
angle decreases by 356 nanodegrees per day (3.56e-7 °/d,
1.3e-4 °/yr).

=head3 Perihelion Precession

There is also the perihelion precession. This movement
is best known for Mercury, because it is the most apparent,
but all other planets have a perihelion precession, including
the Earth.

=head3 Other Drifts And Fluctuations

The formulas computing the positions of celestial bodies
use some constants. But these values are constant only
on a short timespan (astronomically speaking; or, with the
metaphor above, on a "meteorological" timespan). But they are
variable on a longer timespan (or a "climatic" timespan) For example,
everybody knows that the day lasts 24 hours (the mean
solar day, not the sidereal day). Yet, as I have read it
somewhere, in paleontological times, it used to last
22 hours or so.

The variation of the duration of the day is a tiny
variation, but with our modern measure instruments, we
can measure it. Since the time when scientists abolished the
astronomical standard of time for an atomic
standard, it has been necessary to add 27 leap seconds
over 47 years to synchronise the atomic timescale with
the Earth's spin.

For the moment, all adjustments have consisted in
adding a leap second. But it can happen that we
would have to synchronise in the other direction by removing
a second. So this phenomenon produces fluctuations
rather than a slow drift in a single direction.

=head3 The Equation Of Time

There are other fluctuations, easier to measure and with a more
"meteorological" and less "climatic" timescale. The I<true> solar noon
does not occur on the same precise time as the I<mean> solar noon.
There are two reasons.

=head4 Obliquity of the Earth

First, there is an angle between the ecliptical plane and the equatorial plane,
therefore, a constant-speed rotation on the ecliptical plane does not translate
to a constant-speed rotation when measured by right ascension on the equatorial 
plane. The rate of variation of the right ascension is a variable rate.

If we use the same units for the ecliptic longitude and the right ascension
(either degrees or hours), then both values are nearly equal, but still different.
So, when the ecliptic longitude is 46°20'31", the right ascension is 43°52'36",
that is, a 2°27'54" gap. The same happens at longitude 226°20'31". And at
longitude 313°32'52", the right ascension is 316°47", that is a gap of 2°27'54",
but in the other direction. And the same happens at 133°32'52". These are
the maximum values for the gap when using an obliquity of 23°26'.
And if you prefer hours, here are the values:

  .   longitude   right ascension      gap     longitude   right ascension   gap
  .   3h05mn22s      2h55mn30s       -9mn51s   46°20'31"     43°52'36"     -2°27'54"
  .   8h54mn11s      9h04mn03s        9mn51s  133°32'52"    136°00'47"      2°27'54"
  .  15h05mn22s     14h55mn30s       -9mn51s  226°20'31"    223°52'36"     -2°27'54"
  .  20h54mn11s     21h04mn03s        9mn51s  313°32'52"    316°00'47"      2°27'54"

=head4 Kepler's Second Law

Second, the rotational speed of Sun itself on the ecliptical plane is not a constant.
It obeys Kepler's second law, with a rotational speed more or less inversely 
proportional to the Earth-Sun distance.

Q: You cannot apply Kepler's second law to a geocentric model!