NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Astronomy: Calculating Positions
From: Marcel Tschudin
Date: 2008 Apr 6, 16:45 +0300
From: Marcel Tschudin
Date: 2008 Apr 6, 16:45 +0300
Dear Group Members I have some questions related to Parallax and flattened earth shape when calculating positions of astronomical objects (sun, moon and planets). If I remember right there are certain members in this group having their own programs for such calculations. I start to pose my question here but would be prepared to continue off list if the subject should be beyond the general interest of the group. My question relates to the conversion from geocentric to topocentric coordinates. The program which I'm using for the sun claims an accuracy of about 1 sec of arc. Comparing apparent geocentric sun positions with those of CalSky (www.calsky.com) show a standard deviation of about 2.5 sec of arc. This indicates that what I'm doing is about right until the conversion from geocentric to topocentric cooordinates. For converting geocentric to topocentric coordinates one has to allow also for the difference in parallax and the difference in latitude due to the flattened earth. The function which performs the coordinate transformation does this for a sphere, it thus does not consider those two effects. To my understanding the two corrections have therefore to be made either before or after the transformation. Unfortunately I couldn't find in any of my textbooks how the two corrections have to be considered in the context of this transformation. The textbooks only indicate how to calculate the (maximal?) size of the effects by showing a figure with a cross-section corresponding to the object (sun) passing the meridian; but how is the situation with the object at an othe altitude, like near the horizon (sunrise or sunset)? I therefore presume that the corrections have to be applied before doing the transformation but I don't know how. May be correcting the declination with the Parallax and the observer's latitude with the difference to the flattened earth before entering the transformation? Any help or a hint to where I might find help is welcome. Thank you. Marcel --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---
