Re: Field rotation rate formula
Bill J Gray Sep 22, 2003
Hi Siebren,
(Pause to grab Meeus' _Astronomical Algorithms_ from the
shelf... set a fixed-size font, if needed, to puzzle through
the following formula:)
"The parallactic angle q can be calculated by means of
the formula
sin(H)
tan q = -----------------------------------
tan(phi) cos(dec) - sin(dec)cos(H)
where... phi is the geographical latitude of the observer,
dec the declination of the celestial body, and H its hour
angle at the given instant."
The parallactic angle is basically that at which Guide's
charts in "zenith up" mode are rotated relative to those in
"north up" mode, for a given point in the sky. That is,
draw a line (great circle) from that point to the north
celestial pole, and another one to the zenith. The angle
between those great circles is the parallactic angle.
The above expression is actually not all that ugly to
differentiate analytically, but I'd recommend the numerical
approach anyway; it's simpler still.
-- Bill