Re: How to use VSOP 87

On 2005-04-06, C. Balci <cbalci@xxxxxxxxxxxxxxxxx> wrote:

> I got the VSOP87 data from astrolabe from Bill (thank you Bill) but
> Bill's source code is a little bit difficult to follow (that's my
> fault. i am not good at working with other people's source codes).
> That's why i asked information about using VSOP data.

I think the Meeus book really is your best route. Have you tried buying it
from Willmann-Bell directly?

The VSOP data is arranged in a sort of hierarchy:

Eight planets (no Pluto). Within each planet:
Three dimensions (longitude, latitude, radius). Within each dimension:
Some series. Within each series:
Some terms. For each term:
Three factors.

To find a given planet's dimension at a given time, you have to loop through
the series and the terms, applying a formula to the factors and summing up the
results correctly.

The code to do this is actually very compact. From the astrolabe Python
version:

X = 0.0
tauN = 1.0
tau = jd_to_jcent(jd) / 10.0
c = _planets[(planet,dim)]

for series in range(len(c)):
seriesSum = 0.0
s = c[series]
for t in range(len(s)):
A, B, C = s[t]
seriesSum = seriesSum + A * cos(B + C * tau)
X = X + seriesSum * tauN
tauN = tauN * tau # last one is wasted

if dim == "L":
X = modpi2(X)

return X

-Bill
--
Sattre Press History of Astronomy
http://sattre-press.com/ During the 19th Century
info@xxxxxxxxxxxxxxxx by Agnes M. Clerke
http://sattre-press.com/han.html
.

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