>Is there any way to accurately determine the day of a star's heliacal
>rising with Starry Night? I'm having some difficulty in determining on
>just which day a star first becomes visible just before the Sun rises.
>I realize that this is difficult, and possibly somewhat subjective
>without further specified criteria, but so many cultures have employed
>the heliacal risings/settings of bright stars as time-keeping devices
>that it can't be all THAT subjective in real life. (Can't say I've ever
>tried to identify in the real world one myself, though.) Any advice for
>me out there?
You're going to have to guess. Obviously this is not an objective
determination. It depends upon the observer's skill. I encountered
similar problems when I was asked by a local mullah to help him construct
a calendar for his muslim community. He couldn't give me definitions (I
think there are none) for the hillal, the first sighting of the new moon
each month. This varies according to observer, observation elevation, and
smog conditions, and it's not always easy to do in Vancouver! Fortunately
in this case the uncertainty is only at most a day. In the case of
heliacal rising the uncertainty can be much greater.
I do notice that Starry Night has some criterion built in. Sirius will be
invisible at dawn one day and then visible at dawn the next. Why not adopt
their criterion? Then check it out yourself. When I oberve from Cairo in
SNP I get July 24 next year. Be sure to set the Horizon to flat. Sirius
rises behind a tree if you have the shrubery turned on!
Leigh
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