Pete,
The slow clockwise rotation of the Earth's axis describes a cone
with a half angle of approx. 23.5 deg. that is perpendicular to the
Ecliptic. There is a small scale change as well termed Nodding or
Nutation that is superimposed and has a period of 18.6 years.
Although we like to believe we are geared to the stars, we must
also except that there is quite a bit of slop in the drive train! The
effect caused by precession causes the equinox direction to shift
westward approx. 50 arc-secs per yr. This offset is apparent in the
position of the Zodiac signs in our current epoch and which were
originally positioned by the Babylonians. The cardinal point of the
Vernal Equinox in Aries isn't in Aries at all. An interesting aside is
the popular term Age of Aquarius, which is defined as when the vernal
Equinox is in Aquarius. The same offset is true for all Zodiac signs
relative to their original epoch. The Seasons as you mentioned would of
course experience the same offset to the original epoch in which they
were described.
The Equation of Time is driven by the effects of Obliquity and
Eccentricity and most importantly by there phase relationship. The
attached graph shows how the combination of the two effects produce the
Analemma in our current epoch. The Precession of The Equinoxes(shifting
west) as well as the shift in the Longitude of Perihelion(shifting east)
effect the phase relationship of the two effects. However, the value of
Obliquity and especially Eccenetricity do change in time as well.
I'd recommend Meeus for discussion of the Effects.
Regards,
Luke Coletti
Pete Swanstrom wrote:
> Danny Hillis' project for a Millennium Clock sounds very interesting.
> His mention of precession really has me wondering now what long term
> effect the earth's precession will have on current sundials.
> Specifically,
>
> How is the earthÂ’s precessional axis oriented relative to the
> earthÂ’s polar axis and to the ecliptic?
>
> If the earth's axis precesses once every 25,860 years, does that
> mean that 13,000 years from now it will be summer in the northern
> hemisphere in January?
>
> How does this affect the Equation of Time?
>
> Any answers to these questions, or any idea where I can find FAQ's on
> precession?
>
> Thanks much! Pete Swanstrom.
Attachment converted: MAC Hard Disk:ANALEMMA.GIF (GIFf/JVWR) (000068C7)
