# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Basics of computing sunrise/sunset
From: Greg Rudzinski
Date: 2009 Jun 17, 16:41 -0700

```Christian,

Even though the altitude is negative enter it into the formula as
positive. Add the second part of the formula if latitude and
declination are the same name(north or south) and subtract if
contrary. Be sure to apply the equation of time in the correct
direction and the same goes for the arc to time correction for the
angular difference between the longitude of the observed sunset and
the time zone meridian. If sunset is before 1800 LMT add 180� to the
formula angle. If sunset is after 1800 LMT subtract the formula angle
from 360�. Dip is also a factor to corrected for.

Greg

On Jun 17, 1:41�pm, "Christian Scheele"  wrote:
> I'm trying to solve the following sunrise/sunset astronomical triangle and
> am encountering unexpected pitfalls.
>
> cos(LHA) = [sin (a) - sin (lat) sin(dec)]/ cos(lat)cos(dec)
>
> where I am assuming a = -0.83 degrees
>
> It is of course something very basic and I'm almost embarassed to say that
> although I keep rechecking what I'm doing, I remain with an error of about
> 15 minutes by the time I get to the final result in mean zone time. Before I
> drag everybody into it, could somebody please tell me whether there are any
> snags that I should look out for?
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