mean equator

Hi,

Im reading about satellite navigation and there is a term which i
cannot find a definition for- "mean equator"


this seems to be defined at a certain year.


My giuess is that the mean equator defines the inclination of the
sun's apparent elliptic path around the celestial equator.


???


any help is appreciated.


Regards,
Adam

In article . com,
Adam Chapman wrote:

Im reading about satellite navigation and there is a term which i
cannot find a definition for- "mean equator"

this seems to be defined at a certain year.

My giuess is that the mean equator defines the inclination of the
sun's apparent elliptic path around the celestial equator.

No, that would be the "(mean) ecliptic". The inclinations of the
ecliptic and the equator both can be referred to the "fixed stars", an
ideal or absolute orientation, which in practice could be determined
from the positions of distant galaxies. Since the Earth's axis 'wobbles'
slightly, the plane of the celestial equator with respect to the fixed
stars (and, not quite identically, to the ecliptic) is continually
changing. There are a variety of motions involved, including large,
long-term, periodic ones like "lunisolar precession", shorter-term
periodic ones like "nutation", and smaller, random or chaotic ones due
to the influences of the planets (mainly Jupiter), continental drift,
and so on.

The mean equator is given for a particular epoch in order to account for
secular changes and slow oscillations while averaging out the more rapid
or transient effects. The main practical implication for navigation, or
other applications of ephemerides and almanacs, is that you can ignore
precession (which has a period of ~26 ka) but you still have to correct
for nutation (whose period is 18.6 a, matching the draconic cycle).

--
Odysseus

Brilliant. Thanks vey much