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    Re: Equinox, eggs and other questions
    From: Herbert Prinz
    Date: 2002 Mar 17, 21:15 +0000

    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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