Re: [Phys-L] analemma and equation of time

[David Bowman <David_Bowman@georgetowncollege.edu>]

Regarding JD's comment:
> ...
> At this point you will discover that the earth's axis has
> precessed by a nontrivial amount since the year 2000. The ICRS
> declination of the zenith is not equal to your latitude, and the
> ICRS right ascension of the zenith is not equal to the RA of the
> southern horizon (azimuth 180°), which is just nuts. The
> meridian really should be a great circle of constant RA.
> The discrepancy is not super-huge, less than a minute of arc,
> but it would show up in the analemma diagram. Seriously wrong
> and ugly. Guess how I know.
> ...

I'll decline to opportunity to guess, but would rather point out that besides the above mentioned precession of the north celestial pole (& equinoxes along the ecliptic) the obliquity is also decreasing (albeit at a somewhat smaller rate than the precession mentioned above).  In the 20.71 Julian years since the J2000 epoch the earth's obliquity has dropped by about another 9.694 arc sec (as a secular decrease not including various seasonal and monthly lunar effects).  This makes the current obliquity about 23.4366° (the formula I used has it at 23.436598 degrees).  This is significantly less than the 23½° school kids are typically taught is the case.  It would probably be better to say its 23.4° since that rounding ought to valid for about another 6½ centuries before needing to be rounded down again to 23.3°.

Not only is the Earth's spin axis precessing & bobbing its perihelion is precessing, too, and its orbital eccentricity is decreasing.  And similar changes are concurrently taking place in the motions of the other stuff orbiting the Sun.

David Bowman