A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2022 Jun 27, 18:28 -0700
Francois Tremblay, you wrote:
"16' is allowed for semi-diameter and 34' for horizontal refraction"
Yes, totaling exactly 50' or 0.833°. That's actually a rather arbitrary rule.
"My understanting is that the values for sunrise and sunset in the Almanach are provided not to indicate the moment the center pount of the sun is at 0 deg but the moment the observer will see the sun pointing above the horizon, taking into account refraction."
Mostly yes, but in truth the modern nautical almanacs follow a formal, standardized definition of sunrise and sunset times. That refraction quantity, 34', only applies to observers near sea level at standard temperature and pressure (and entirely ignoring anmoalous refraction, but we can forgive that). And yet, this definition is also used for the times of these phenomena for observers at high altitudes above sea level (it doesn't matter for a nautical almanac, of course). This doesn't make sense, but it's the standard. And where's height of eye? In fact, the standard assumes exactly zero height of eye. If you're 100 feet above the sea surface, you'll see sunrise 10' of arc, or as much as a minute of time in mid-latitudes, earlier than an observer with zero height of eye. The almanacs are not using a rational or scientific definition of sunrise and sunset --just a standardized definition.