On Oct 24, 10:24*am, oriel36 wrote:
The original explanation by
Copernicus is quite clear on axial/equatorial 'tilt' and has never
been tested in any meaningful way since its appearance in De
Revolutionibus -

*"To this circle, which goes through the middle of the signs, and to
its plane, the equator and the earth's axis must be understood to have
a variable inclination. For if they stayed at a constant angle, and
were affected exclusively by the motion of the centre, no inequality
of days and nights would be observed. On the contrary,it day or the
day of equal daylight and darkness, or summer or winter, or whatever
the character of the *season, it would remain identical and
unchanged." Copernicus

As we know, the inclination of the Earth's axis to the ecliptic (which
is "this circle" in the quote above) has a fixed and unalterable
inclination. Otherwise, the celestial pole would not remain fixed in
its position close to the star Polaris.

Does this then mean Copernicus was wrong?

I do not think so. Because his conclusion only follows from his
premise within a system such as that of Tycho Brahe. That is, if the
inclination of the Earth's axis, when compared to the line from the
Earth to the Sun, does not change, then the length of the day could
not alter in the course of a year.

But if the orientation of the Earth's axis remains constant, while the
direction from the Earth to the Sun changes, then an axis not
perpendicular to the plane of the orbit will cause areas close to the
poles to be always in sunlight for part of the year, and always in
darkness for another part.

Thus, I believe that the apparent difference between what Copernicus
has stated and what modern astronomy accepts is not real, and the
mystery would be resolved through the context of what you have quoted.

John Savard