Hello All,
I know Fred must be right about the declination being non-zero at the
equinoxes, but I can't figure out why. As I understand, solar celestial
right ascension must equal solar ecliptic longitude (Lambda) on the equinoxes
(0 degrees spring and 180 degrees fall). The formulas (see addendum below)
for declination and right ascension that I have (and which I suspect are also
used in the Dialist Companion) relate these values respectively to the sine
and tangent of Lambda, such that they should equal zero when Lambda equals
zero, and 180 when Lambda equals 180. Perhaps someone can help me find my
error.
-Bill
Addendum:
Find Lambda the ecliptic longitude of the sun from this formula:
lambda = L[=mean longitude of the sun] + (1.919 - 0.005 x
T[=fraction of Julian Century]) x sin(M[=mean anomaly of the sun]) + 0.020 x
sin(2M)
Find the right ascension of the sun from this formula:
Right Ascension = arctan (tan(lambda) x cos(epsilon)) in same
quadrant as lambda
Find the declination of the sun from this formula:
declination = arcsin (sin(lambda) x sin(epsilon))
---------------------------------------
Subj: Re: Equinox discrepancy
Date: 8/10/2001 11:37:09 PM Eastern Daylight Time
From: [EMAIL PROTECTED] (Fred Sawyer)
Sender: [EMAIL PROTECTED]
Reply-to: <A HREF="mailto:[EMAIL PROTECTED]">[EMAIL PROTECTED]</A>
(Fred Sawyer)
To: [EMAIL PROTECTED] (Bill Gottesman), [email protected] (Sundial
List)
Bill,
As the help file with the Dialist's Companion points out, this measurement
is only approximate in this software package. In fact, however, equinoxes
and solstices are not defined astronomically in terms of the sun's
declination - they are defined in terms of its true longitude. This is
noted in the help file. Using a zero declination to denote equinox only
gives you an approximation. So you should really be viewing the equinox as
the time when the zodiac indicator changes from virgo to libra. According
to the Dialist's Companion, this happens at 19:06:53 which is pretty good
considering that the help note suggests that this measure should only be
considered good to within about ten minutes (solstices are more difficult
than equinoxes).
Fred Sawyer
