technical question on swiss ephemeris and jhora

[Guest guest]

Hare Rama Krishna

 

I have a question about geocentric model. It may be a bit technical.

I've read discussions about settings in Jhora but this question deals

with Swiss Ephemeris itself. Hope Narasimha ji will clarify something.

 

I'm trying to write a jyotish software using java-port of Swiss

ephemeris. I'm following this manual

http://www.astro.com/swisseph/swephprg.htm called " Programming

interface to the Swiss Ephemeris " .

 

Here is what it says about dealing with geocentric mode and

topocentric mode where you can provide geographical coordinates of the

place of birth.

 

"

....

a. True or apparent positions

 

Common ephemerides supply apparent geocentric positions. Since the

journey of the light from a planet to the earth takes some time, the

planets are never seen where they actually are, but where they were a

few minutes or hours before. Astrology uses to work with the positions

we see. ( More precisely: with the positions we would see, if we stood

at the center of the earth and could see the sky. Actually, the

geographical position of the observer could be of importance as well

and topocentric positions could be computed, but this is usually not

taken into account in astrology.). The geocentric position for the

earth (SE_EARTH) is returned as zero.

 

To compute the true geometrical position of a planet, disregarding

light-time, you have to add the flag SEFLG_TRUEPOS.

 

b. Topocentric positions

 

To compute topocentric positions, i.e. positions referred to the place

of the observer (the birth place) rather than to the center of the

earth, do as follows:

 

- call swe_set_topo(geo_lon, geo_lat, altitude_above_sea) (The

longitude and latitude must be in degrees, the altitude in meters.)

 

- add the flag SEFLG_TOPOCTR to iflag

 

- call swe_calc(...)

.... "

 

First I was setting only SEFLG_TOPOCTR and SEFLG_SIDERIAL flags

according to the manual and was setting Lahiri ayanamsa using

swe_set_sid_mode(SE_SIDM_LAHIRI, 0, 0)

function and coordinates of the bith place using

swe_set_topo(glon, glat, alt)

function, and was getting planetary longitudes very similar to those

of JHora and Parashara's Light with default settings.

But longitude of Moon was about 30 arc mins greater. I couldn't

understand what was the problem. I was trying various settings

and after I removed SEFLG_TOPOCTR flag I got the same figures.

But it means that I'm using geocentric positions instead of

topocentric now. Changing geographical coordinates in this mode

however still affects planetary longitudes!

 

Is it correct to use geographical position of birth place in

geocentric mode? Manual says swe_set_topo() function should be used

with SEFLG_TOPOCTR flag. Does this function have any sense without

SEFLG_TOPOCTR flag? If yes then what is the difference between

geocentric and topocentric mode in software?

May be Narasimha ji knows how Swiss Ephemeris works, as I know he is

using it in JHora for a long time.

Thanks.

 

Best wishes,

Govinda Charan