# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: JPL ephemeresis and Nautical Almanac - speed of light question
From: Rafal O.
Date: 2018 Apr 4, 00:08 -0700

Hello,

The goal is to use JPL ephemeresis as it is used currently to generate NA (http://aa.usno.navy.mil/publications/docs/na_history.php). As these data provides me information about position of objects in Solar system at time t. But due to finite value of speed of light I cannot see these objects at the same moment, but after some time interval dt. If I understand the problem correctly the procedure is as follows:
(a) Calculate position of Earth and i.e. Saturn at time t, rE(t) and rS(t), respectively.
(b) The front of light wave travels at speed c (299792458 m/s), both Earth and Saturn go thought their orbits, too.
(c) Approximately after time interval dt = |rE(t) - rS(t)| / c the reflected light from Saturn hits the eye's of the observer placed at Earth.
(d) I use some iterations around this second time moment t1=t + dt to precisely (+/-1s) determine when this signal hits Earth.
(e) The position of Earth is now rE(t1) but I see Saturn where it was at time t.
(f) This new vector r(t1,t) = rE(t1) - rS(t) gives me information about position of Saturn relative to Earth is the speed of light is infinite.
(g) I assume that having information from point (f) allows me to calculate Saturn's apparent position.

At least this is how I image how it should work. If I am wrong, please correct me. The whole project is just for fun. Maybe it becomes something useful for me and others. I hope I learn something new.

Best wishes,
Rafal

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