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

Adam Chapman writes:

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.

The earth's rotation axis (and equator) gradually change with time.
Some of that change is a constant drift, known as precession. The
earth equator and equinox given by precession alone is known as the
mean equator and mean equinox.

Superimposed on precession is a set of wobbles at different periods,
known as nutation. If both nutation and precession have been
accounted for, then the corresponding equator and equinox are known as
the "true" equator and "true" equinox.

The Astronomical Almanac would have more information about this topic.

Good luck!
Craig

In article .com,
Adam Chapman wrote:

Hi,

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

The "mean equator" is the orientation the Earth's equator would have
if the nutation was averaged out. The orientation of the equator with
nutation effects included is called the "true equator". Where the
equatorial plane of the Earth intersects the celestial sphere, that's
where we have the celestial equator.

this seems to be defined at a certain year.

It's defined at a particular *moment*, not over a whole year.
Those moments are usually selected near the beginning of some
selected year, e.g. 2000.0

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
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thanks everyone