"Rick Mintz" wrote in message
...
Hi.

I am constructing a sundial and want to include a graphic of the "Equation
of Time" on it. I can find graphics of this but I need to have the actual
formula so as to precisely scale a graph in Microsoft Excel.

Anyone have the formula that I need?


Depending upon the required precision there are several
factors required. Primarily it's the Sun's mean
longitude, its apparent right ascention, and the
nutation in longitude and obliquity of the ecliptic.

Meeus, in his book Astronomical Algorithms, provides
the formulae to calculate all of the above and the
Equation of Time.

Coincidentally, we just had an article in The Orrery
newsletter, Issue #50, (http://members.attcanada.ca/~gneill/)
that presented a simple approximate method for calculating
the Equation of Time (Article by David Williams). The
relevant formula is:

ET = 9.8*SIN(2A) + 7.6*SIN(A-0.2)

where A is the angle measured from the December Solstice
to the current position of the Earth, and A is calculated
as:

DC = 2*pi/365
A = DC*(D+10) + 0.033*sin(DC*(D-2))

D is the day number (January 1st is D = 0).

The accuracy is suprisingly good when compared to the
more accurate but arduous formulas of Meeus, or published
values. The constants in the formula are determined
empirically.