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From: George Huxtable <george@hux.me.uk>

On the 29th April, the Sun's declination is increasing
(Northward) by about 0.8 arc-minutes per hour.

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If we define:

d: hourly rate of change of Sun's declination in arcminutes
D: number of days elapsed during the current year
a: "phase angle" ( a~0 at winter solstice, a~90 at vernal equinox, etc. )

a = {D+10} / 365 * 360 degrees

then the following approximate expression for "d" seems to work well enough:

d = 60 * 23.45 * SIN(a) * ( 360 / 365 ) / 24 * ( Pi / 180 )

For April 29 we have D=118 and hence d=0.8' per hour, just as George stated above.


Peter Hakel