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I'm surprised that nobody has mentioned Brad Schaefer's work in this
context (and in many other related fields concerned with atmospheric
effects on arriving light).

A search for Schaefer, combined with extinction, should take you to
anything you need to know.

George.

contact George Huxtable, at  george@hux.me.uk
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
----- Original Message -----
From: "Apache Runner" 
To: 
Sent: Wednesday, August 17, 2011 4:44 PM
Subject: [NavList] Heliacal rising - definition?


|I have a question: can someone define heliacal rising in a quantitative
| fashion?   The reason I ask is that I'm interested in writing a piece of
| code to identify heliacal risings, but when I think about the
| implementation, many questions arise.
|
| Roughly speaking, the heliacal rising is the first time a star is visible
| just before sunrise.    If I think about what might actually be visible
at a
| given latitude, this seems to me to depend on the magnitude of the star,
and
| even atmospheric effects, and even the height of the observer, if I were
to
| get persnickety.
|
| On the other hand, if I define some group of stars with a magnitude
greater
| than X, and say it's 'first visible' before local sunrise at, say 6
degrees
| above the horizon at the start of astronomical twilight, I can get a
precise
| definition that I could implement in a piece of code - although this
would
| probably yield some stars that would be several days past the 'first'
| observation.
|
| The latter definition is straightforward to implement, while the former
is
| difficult for me to see how it would work.
|
| Any thoughts?
|

   

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