One week ago, I listened to a lecture about the computation of solar risings
and settings by an astronomer of the Royal Obervatory of Belgium. The
lecturer stated that in France use is made of the center of the solar disk.
He explained that doing so introduces the least discrepancies between
observed and calulated times for sunrise or sunset for observers east or
west of Paris.

I made a check in a copy of the Connaissance des Temps (rather old, I have
to admit: 1978) and indeed it is stated that the risings ans settings are
computed for Paris, for the center of the solar disk, and using a refraction
at the horizon of 36'36".


Laurent Zimmermann
http://users.skynet.be/zmn/cab

----- Original Message -----
From: Paul Schlyter <pausch@...>
To: <guide-user@yahoogroups.com>
Sent: Saturday, April 28, 2001 10:35 AM
Subject: [guide-user] Rise/set/title bar idea


>
> > About rise/set/transit times: these are computed as described by
> > Paul Schlyter, and follow "standard" definitions for each. I don't
> > think you'll find any program that uses a different definition of
"rise",
> > "set", or "twilight".
>
> Then you haven't seen my programs. :-) Some time this year they
> will appear as Javascripts on my web page, and then you'll see...
>
> In Sweden the official almanacs compute the sunrise/set times using
> the center of the solar disk rather than the upper limb. In this
> respect Sweden appears to be rather unique in the world -- all other
> countries (including our Scandinavian neighbours -- except Finland,
> however Finland was Swedish territory a few centuries ago) use the
> Sun's upper limb for sunrise/set computation (should anyone know
> another country). Nevertheless, in my own software which computes
> rise/set times, I usually include an option: center or upper limb of
> solar disk. Usually I choose the upper limb option myself. The
> rise/set times on my web page are also computed using the upper limb,
> and I frequently get emails asking why these times differ a few
> minutes from the times in the printed Swedish almanacs.
>
> Clearly the rise/set of the upper limb more closely resembles the
> visual impression of an observed rise/set: we think the "sunrise"
> occurs when we see the first part of the solar disk, not when we see
> precisely half the solar disk. However, when we use the upper limb
> of the Moon to compute lunar rise/set times, we encouter a problem:
> the upper limb of the Moon is fairly often dark! The "proper" way to
> deal with that is somewhat complex though: one could compute the
> rise/set time of the uppermost part of the ILLUMINATED portion of the
> lunar disk. I've never seen any program or heard about anyone
> actually doing this though. Also it would create problems for
> computing the rise/set time of the new moon, when all of the visible
> lunar disk is in hadow (due to the topography of the Moon, no part of
> the visible lunar disk is illuminated by the Sun when the Moon is
> closer to the Sun in the sky than about 7 degrees).
>
> =======================================================================
>
> Finally, we have twilight. The three standard twilights are:
>
> Solar altitude Twilight
>
> -6 deg Civil twilight
> -12 deg Nautical twilight
> -18 deg Astronomical twilight
>
> To this list I have personally added:
>
> -15 deg Amateur astronomical twilight
>
> because I think the sky is sufficiently dark for all but the most
> demanding observations when the Sun is 15 degrees below the horizon.
>
> Of course I think it would be nice to have the "amateur astronomical
> twilight" option added to some future version of Guide.
>
> At my latitude this effect is easy to observe: I don't need to time
> the twilight at some night(s) to find out. As seen from here, at
> about 12 April the Sun is 18 degrees below the horizon at local
> midnight, and at around 2-4 May the Sun is 15 degrees below the
> horizon at local midnight. At midsummer the Sun is only some 7
> degrees below the horizon at local midnight - then we really don't
> have any night at all.
>
> So I, as well as all other sky observers near my latitude, simply
> note at which date the sky gets too bright for observing -- then
> the "observing season" will take a break over the summer (except
> for bright objects). In the middle of April the spring's observing
> season isn't over yet, but a few days into May there's a "twilight
> dome" in the north which doesn't go away even at astornomical
> midnight. Around 10 May that "twilight dome" has reached the zenith,
> and some days later it also covers the sky towards the south.
>
> ----------------------------------------------------------------
> 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
>
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