If you are after a really precise EOT calculation then you will have to use
astronomical formula to calculate the Sun's position and compare it to the
solar mean time, for example the algorithms outlined in Meeus' book.
However if you are happy to ignore the 4 year leap-year cycle and just use
a day number (and accept the precision loss) then the following formula may
be of interest to you.
I used the Meeus algorithms to compute the EOT for each day at 00UTC during
a mid leap-year cycle (2002) and then produced the following fourier
series. I chose not to average over four years as this introduces a phase
error, I found just using one year in mid-leap-cycle appears to be the best
way to minimise the error.
In the equation below "J" is the day number of the year in UTC time, which
can be in fractions for parts of the day, eg 164.5 is 12UTC on 13 June. EOT
is in minutes.
T = 2*PI*(J - 1)/365.25
EOT = -0.000062368659
+ 7.3636768 * Cos(T + 1.5076785)
+ 9.9351605 * Cos(2 * T + 1.9332853)
+ 0.32924369 * Cos(3 * T + 1.8934472)
+ 0.21208209 * Cos(4 * T + 2.3032843)
you can compute J = dn() by the following VisualBasic code where variables
appended by & are long integers
Function dn(ByVal y&, ByVal m&, ByVal d&)
dn = g(y&, m&, d&) - g(y&,1,1) + 1
End Function
Function g(ByVal y&, ByVal m&, ByVal d&) As Long
Dim n&
m& = (m& + 9) Mod 12
y& = y& - m& / 10
g = 365 * y& + y& / 4 - y& / 100 + y& / 400 + (m& * 306 + 5) / 10 + d& - 1
End Function
At 02:14 PM 12/06/2003 -0700, you wrote:
Hello all,
I'm looking for a handy precise program that calculates EOT values in
minutes and seconds at particular times during the day. In other words, I
don't want the average EOT value for the day as found in the printed tabular
EOT values in Mayall & Waugh. Instead, it would give the exact EOT value at
a particular time during the day.
I've found several EOT calculator programs (applets) on the "Current Data"
link at sundials.org. Does anybody know if the values given in these applets
are daily averages or precise values?
Thanks for any suggestions.
John
John L. Carmichael Jr.
Sundial Sculptures
925 E. Foothills Dr.
Tucson Arizona 85718
USA
Tel: 520-696-1709
Email: [EMAIL PROTECTED]
Website: <http://www.sundialsculptures.com>
-
Hank de Wit
Regional Computer Manager
Bureau of Meteorology, SA
ph: 08 8366 2674
Email: [EMAIL PROTECTED]
-
