> 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