On Sep 20, 9:15*pm, oriel36 wrote:
On Sep 21, 2:32*am, RichD wrote:

When and how did astronomers determine that the earth's axis
tilts from the ecliptic, and its value?

--
Rich

Astronomical discoveries are individual rather than communal and the
determination you refer to was first proposed by Copernicus
himself .It might be off-topic in a forum devoted almost exclusively
to magnification in a homocentric setting but here it is anyway -

"..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 center, no inequality of
days and nights would be observed."Copernicus Chapter 11 De
Revolutionibus

http://www.daviddarling.info/images/...gs_changes.jpg

The great astronomer did not have the benefit of 21st century imaging
to modify his approach which comes down to us as the 'no tilt/no
seasons' ideology when effectively he is describing equatorial
conditions as there is only residual variations in daylight/darkness
at the equatorial latitude as opposed to the North and South poles
where the variations are extreme.The greater the distance between the
rotational orientation and the ecliptic axis of any planet the more
polar the conditions or the more equatorial the climate as the
distance shrinks - this uses the Arctic/Antarctic circles as a kind of
a terrestrial boundary between equatorial and polar conditions so that
a reasonable person can see our planet has a largely equatorial
climate.

The North/South poles act like a beacon for the orbital behavior of
the Earth and while it does take a while to become comfortable with
the separate rotation to the central Sun as a component of the orbital
motion of the Earth,it does produce so many things to discuss.Rather
than the awkward *'tilt from the ecliptic', the polar coordinates turn
in a circle to the central Sun as indicative of all locations on the
planet so that axial precession has to be replaced along with the
explanation for the seasons.It is a 100% certainty that a camera
trained on the Earth from Mars will witness the Earth's polar
coordinates turn through the circle of illumination at the equinox in
such a manner -

http://upload.wikimedia.org/wikipedi...precession.svg

Not only have you your answer,you also have the necessary imaging to
demonstrate why that 500 year old explanation needs urgent
modification in an era where climate is such an issue.I could say that
the present climate scientists don't know what they are talking
about ,not for any disagreement I have with them,only that the Earth
does have a pronounced equatorial climate and it has yet to make its
way into the wider community even though it is so much common sense.

If the rotational inclination of Uranus was applied to the Earth,the
Arctic circle would extend almost to the equator as the Earth would
have an almost total polar climate whereas it has,due to its 23 1/2
degree inclination from the ecliptic axis,a largely equatorial
climate.


Your post has made it to alt.astronomy. I have always enjoyed your
informative and thought provoking contributions.

Double-A