# NavList:

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

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Re: Planet positions and light travel time
From: Paul Hirose
Date: 2017 Dec 18, 13:25 -0800

On 2017-12-17 13:33, David Pike wrote:
>
> how do the tables allow for the fact that when you observe a planet, the
light you see comes from where the planet was several minutes before, not
where it was at the time of observation?

The question has to do with the astronomical concept of "place."

Geometric place is where a body actually is. It's what you get from the
JPL planetary ephemerides.

Astrometric place is where a body was, when it emitted the light that
you see now. It's the geometric place adjusted for light time to the
observer.

Apparent place is where a body appears to be. It's astrometric place
adjusted for aberration (the apparent displacement of a body due to the
observer's velocity) and relativistic light deflection due to
gravitation. The coordinates in the Nautical Almanac are apparent places.

Here are geocentric coordinates of Jupiter on 2017 Dec 18 1400 UTC. For
easy comparison, all coordinates are in the same system (the ICRS).
Distance is about 920 million km, and light time 51 minutes.

14h48m36.76s -15°08'28.7 geometric
14h48m36.20s -15°08'26.1 astrometric
14h48m35.18s -15°08'21.8 apparent

Computation of light time is iterative. Begin with the geometric place
of the body at time t. Divide distance to the body by the speed of light
to obtain a first estimate of tau (Greek letter), the light time. Move
the target body to where it was at t-tau. Compute the new distance and
divide by speed of light to get an improved value of tau. Repeat until
the movement of the body between iterations is insignificant for your
purposes. The result is the astrometric place of the body at time t.

The procedure is the Astronomical Almanac includes a tiny correction for
the retardation of light due to the Sun's gravity.

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