NavList:

   

Reply

On 5/4/2013 1:59 PM, William Hawes wrote:
> The true time of sunrise is nominally when the sun's lower limb is one
> half a semi-diameter above the horizon.

I had been pondering this months ago while the summer triangle was
hiding from me, and did a fair amount of research on the topic. What I
found was any number of  rules of thumb for "nominal." Some say 2/3 a
semi-diameter, some 1/2 a semi-diameter etc. Some may assume 60' off the
water, others 10', and some make no mention of height of eye.

As the sun may have a considerable movement in its lateral component
near rise or set, it would seem obtaining a highly accurate azimuth is
time dependent.

Given that dip and refraction are vertical (and there is little the
observer can do to affect abnormal refraction) dip should figure into
the equation, and some target figure a bit more precise than "nominal"
might be in order.

I did a series of calculations based on 40d N with the various rules of
thumb found online and in my library. There were significant differences
if the goal is sub-minute accuracy.

Am I missing something, or just being too anal?

Bill B

   

Reply