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    Re: Equinox
    From: Jim Thompson
    Date: 2004 Mar 22, 21:24 -0400
    So is this right then?  This is a draft of my own wording, where I try to phrase this in less technical English for us CN newbies:
    In dry astronomical terms, an equinox occurs when the ecliptic intersects the celestial equator.  In lay terms, as we learned in elementary school, an equinox happens when the sun crosses the equator.  But there is more to this than meets the eye.  First recall that declination is the north/south distance of a celestial body above or below the celestial equator, and that the celestial equator extends out into space from the earth's equator.  Second, consider that the "ecliptic" is the plane of the mean sun's orbit around the earth, not the apparent or real sun.  So the two equinoxes occur each year when the mean sun crosses the equator, not when the apparent (real) sun crosses over. 
    Looking at this another way, the spring and autumn equinoxes occur when the declination of the mean sun is zero.  During an equinox the mean sun crosses north or south of the celestial equator, but the apparent sun often crosses the equator a little before or after the mean sun.  Since the word "ecliptic" refers to the mean sun, then the real (apparent) sun's declination is not often zero during equinox, as one might think, rather it is usually a few arc-seconds different from the point in space and time where the mean sun crosses the equator.
    This difference in time between the biannual equatorial crossings of the mean and apparent suns is analgous to the Equation of Time, which is the name for the length of time that the mean sun follows or precedes the apparent sun throughout the solar day.  For example the Equation of Time expresses the difference in time between the meridian passages of the mean and apparent suns as they cross over your longitude each day at noon.  The Equation of Time can be as long as 16 minutes, but the magnitude of the difference in equatorial crossings between the mean and apparent suns is only a second or two.

    Jim Thompson
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    -----Original Message-----
    From: Navigation Mailing List [mailto:NAVIGATION-L@LISTSERV.WEBKAHUNA.COM]On Behalf Of Michael Dorl
    Sent: Monday, March 22, 2004 12:34 PM
    Subject: Equinox

    Hate to beat a dead horse but it's just such details as this that lead to a better understanding.

    Mr Prinz points out that the equinox occurs when Ecliptic longitude is zero, not when the declination
    is zero.  That definition agrees with the AA (Astronomical Almanac).

    The AA defines
    Equinox - either of two points on the celestial sphere at which the ecliptic intersects the celestial equator;
    also the time at which the Sun passes through either of these intersection points; ie., when the apparent
    longitude of the SUN is 0D or 180D.
    Ecliptic - the mean plane of the Earth's orbit around the Sun.
    Celestial equator - the plane perpendicular to the Celestial ephemeris pole.
    Celestial Ephemeris Pole - the reference pole for nutation and polar motion. ... This pole has no
    nearly-diurnal  (daily) nutation with respect to space fixed or earth fixed coordinate systems.
    Declination - referenced to celestial equator
    So why does the Sun have some non-zero declination at the equinox?  It seems to me that this must be
    because of the difference between the ecliptic (mean plane of the earth's orbit) and the apparent position
    (instantaneous position as viewed from earth) of the Sun.

    Is this it?

    If so, the angle between the ecliptic and the plane of the apparent Sun must be the declination of the Sun
    at the equinox. Is there a name for this?  I make this to be -0.34 arc seconds for this equinox.
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