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> I've been playing with Stellarium and it altitude values to practice the 
math of a sight reduction. I'm assuming I don't have to do any sextant 
corrections if I use the apparent versus the geometric values for 
Stellarium's "Alt" generate my H_o? Is the only difference between the two 
that the apparent incorporates refraction (A2 Tables of the NA)?

In astronomy, "geometric" refers to where a body actually is at a given
moment. "Apparent" refers to where it *appears* to be. The difference is
due to:

1) The time for light to travel to the observer. This is ignored for
stars since light time is implicitly included in star catalog data.

2) Light deflection by the gravitation of other bodies. These bodies are
often (usually?) ignored except the Sun, and that too is ignored unless
great accuracy is necessary.

3) Aberration of light due to the observer's velocity through space.
This causes an cyclic annual variation in the apparent coordinates of
every body by a maximum of 20″ from the "un-aberrated" coordinates. The
effect can be ignored when modest accuracy is adequate, as in a long
term almanac.

In addition to geometric and apparent, there are also astrometric
coordinates, which are adjusted for light time only. (In the case of a
star, the catalog coordinates are adjusted for parallax and proper
motion only.)

None of these includes refraction. In fact, The Astronomical Almanac
definition of "topocentric place" specifically includes aberration and
excludes refraction.

I don't know if Stellarium uses these terms in the way I described.

   

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