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Lu Abel wrote:

> I always thought declination was the "latitude" of a body, ie, the angle
> with respect to the equator of a line drawn to the center of the body from
> the center of the earth.   I've obviously missed something.  Could Herbert
> or someone else educate me on the difference between celestial latitude and
> declination (and how the earth's wobble makes for a difference?).

Lu,

Unfortunately we use the same word for two totally different concepts. When
talking about the Sun's orbit, I assumed that it was understood from the
context that I meant ecliptic longitude and latitude. Sorry for the confusion.

The relation between terrestrial latitude and declination has recently been
discussed on this list. (See "Sextant position versus map datum" and "On the
mischief of geographical position"). However, this does not come into play here
at all.

Celestial (or ecliptic) longitude and latitude are spherical co-ordinates that
are referenced to the plane of the ecliptic (i.e. the plane of the Earth's
orbit around the Sun). This plane intersects the equatorial plane in a line
that points towards the "equinox", also called "First point of Aries". By
definition, this point has ecliptic longitude 0 and latitude 0. It also has RA
0 (or SHA 0) and Dec 0. Both planes are inclined to each other by ca. 23 1/2
deg.

From this it looks as if "geocentric ecliptic latitude" of the Sun would be an
oxymoron. How could the Sun ever be outside of the plane of the Earth's orbit?
But the Earth's orbit is disturbed by Moon and planets, and even the Sun
wiggles around because of Jupiter's revolution. It would be inconvenient to let
the reference frame wiggle along with those motions. So, ecliptic co-ordinates
are referenced to a mean orbit from which the actual position of the Earth or
Sun can deviate slightly.

The time of equinox would be the very instant when the Sun passes exactly
through the point of equinox as seen from Earth. But the Sun could miss that
point and pass it to the N or S. That's why in the definition of the time of
equinox one merely requires the longitude of the Sun to be 0. Latitude and
declination may then differ from 0.

For a successful balancing of the eggs, correct timing at the equinox is of the
essence, but equally helpful is the motivation gained by the consumption of
several egg-nogs.

Herbert

   

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