As a warm up, I'll try to throw a little light on what happens to our
view of the sky during a year, as seen from the heliocentric viewpoint.
Several people have raised a point that Earth should be spinning around
two sets of poles if the heliocentric view is correct and the observations
therefore blatantly contradicts this view. This is not correct.
I have included two figures illustrating my points and there is a little
glossary at the bottom. Sorry for the long post, but I hope you will find
it precise, concise and to the point, never-the-less.
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Fig. 1: The Earth at the four seasons.
To the left: Northern summer solstice/Southern winter solstice,
nearest: Northern autumnal equinox/Southern vernal equinox,
to the right: Northern winter solstice/Southern summer solstice,
and farthest: Northern vernal equinox/Southern autumnal equinox.
This figure is far out of proportion, for clarity - that is the sizes of
the Sun, the Earth and the Earth's orbit, are not to scale (see Fig. 2 for
the correct relative proportions). The spin axis, however, is at the correct
angle and all the conclusions about the sky as seen from Earth are unchanged
Point 1)
We see that the four instances of Earth are an integer number of sidereal
days apart, because the Earth is facing the same way. Imagine you are a
star. Juan in Spain is looking at you. As you are sitting about 30cm from
the monitor, Juan is going to see you at slightly different positions in the
sky, during the year - that is what we call parallax. You, as the star, have
to look in slightly different directions to see Juan in Spain. The largest
difference will be between the leftmost and the rightmost part of the orbit
- that is, half a year apart. Now, on my screen, the Earth orbit in that
figure is about 16 cm - that corresponds to you (the star) being only 2 AU
away - that is still well inside the Solar system. The closest star is more
than 100,000 times further away. If you move a mere 10 times further away
from the screen (3 m) you notice that the angular size of the Earth orbit in
the figure is a lot smaller. 10,000 times further away, and you won't be able
to distinguish between the left and the right side of the orbit (or see your
monitor, for that matter...).
Conclusion: Juan will see all the stars in the same place, throughout the
year, if there is an integer (whole) number of sidereal days between his
observations.
Point 2)
The thing that will change, is whether he can see you at all. at the
leftmost and farthest away (northern spring and summer), Juan will not be
able to see you because it is daytime. During the year, the solar time of
the day, corresponding to a given sidereal time, will go through 24 hours.
The sidereal time of day is the same in all four instances in Fig. 1, but
the Solar time for Juan is (something like) 8am, 2am, 8pm and 2pm, going
from Summer, Fall, Winter to Spring in Fig. 1.
Point 3)
The orbit around the Sun is not a spin, but a translational movement.
In other words, the spin axis (with the Earth attached) is moved around in
the orbit, without changing the direction of the spin axis.
It would be very hard to explain, physically, a yearly wobble of the Earth's
spin axis of +/-23 degrees.
Conclusion: There is only one spin axis of Earth and observations of
far-away stars completely agree with the heliocentric picture. Closer stars,
on the other hand, have measurable parallaxes and many of those also move
perceptibly (not to the naked eye) with respect to the Sun.
Point 4)
Precession happens on a timescale of about 26,000 years! It is the Earth's
spin axis that rotates around the ecliptic N/S-poles. This has no perceptible
consequences for the layman (except that your horoscope is about 1 month
off, since they were invented about 2,000 years ago...) - astronomers of
course need to know where to point their telescopes with high precision and
need to account for Precession.
Fig. 1 was found on the web at
http://www.dkimages.com/discover/Home/Science/Earth-Sciences/Atmospheric-Sciences/Climate/Seasons/Seasons-05.html
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Fig. 2: The relative sizes of the Earth's orbit (solid, large ellipse),
the Sun (small circle in the centre). The Moon's orbit would be about 2
pixels in diameter, and the Earth is about 110 times smaller than the Sun -
quite invisible on this scale. I have also plotted a circle of 1 AU, to
compare with the slightly elliptic orbit of the Earth.
Fig. 2 was made by myself.
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Glossary
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Heliocentric viewpoint only means that the Sun is the centre of our Solar
system - not of the whole Universe!
A sidereal day is the time it takes the Earth to spin once around, to the
same location of the stars. 23h56m04s - the stellar day.
A tropical day is the time it takes the Earth to spin once around, to the
same location of the Sun. 24h00m00s - the Solar day.
1 Astronomical Unit (AU) is the mean Earth-Sun distance.
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