> Last night, at my observation site, I was checking the sun set at the
> time that GUIDE show me in Quick Info. Even if I had enter the exact
> location, there was a small but significant discrepancy, in term of
> few minutes, between what I observe and the time calculated by GUIDE
> Quick Info. I thing that there is two main reasons for such
> discrepancy.

Apart from the two reasons you listed below, there's a third reason as
well: your local horizon. Do you have an unobstructed sea horizon?
If not, how far away from the zenish is your local horizon at that
sunset point? Most likely not exactly 90 degrees.

> For one, the atmospheric refraction could explain a part of it. The
> second is the reference of the part of the solar disk which is used
> as reference. For example, does GUIDE consider the sun set time when
> the upper part or the lower part of the disk disappear below the
> horizon ? Or does it consider just the center ?

Usually atmospheric refraction is accounted for, and the sunset
refers to the upper limb of the solar disk. The refraction is
accounted for by assuming a constant value for the refraction at the
horizon - in reality the horizontal refraction is dependent on
atmosheric conditions (which are hard to predict), so unusual
atmospheric conditions is yet another possible reason for
discrepancies between predicted and observed sunset time.

> Is the same rationale is use for calculating the nautic, civil and
> astronomic twilight ? Does GUIDE take in account the atmospheric
> refraction and if so, how such correction is made ?

No - when computing twilight times one simply computes the time the
center of the solar disk is 96, 102 and 108 degrees from the zenith,
without applying any correction for refraction.

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Paul Schlyter, Swedish Amateur Astronomer's Society (SAAF)
Grev Turegatan 40, S-114 38 Stockholm, SWEDEN
e-mail: pausch at saaf dot se or paul.schlyter at ausys dot se
WWW: http://hotel04.ausys.se/pausch http://welcome.to/pausch