NavList:

From: Frank Reed <FrankReed@HistoricalAtlas.com>
Subject: [NavList] Re: Sunrise - the Positive Side
To: garylapook@pacbell.net
Date: Sunday, May 5, 2013, 2:51 PM

---

Byron, you wrote:
"By every standing sunrise/set is
when the instant the upper limb of the sun appears on the horizon
for sun rise and when the upper limb disappears below the visible
horizon, same for the moon."

Exactly. For amplitude tables, because they are easy to compile for a true (corrected) altitude of zero exactly, people sometimes talk about a "true" sunrise, but it's a potentially misleading distinction. All we're looking for when using those tables is an altitude for the Sun such that the corrected zenith distance of its center is exactly 90 degrees (which is what a true altitude of zero actually means). In that sense and that sense only, it's a "true" sunrise or sunset.

The whole concept of the time of sunrise and sunset involves a bit of smoke and mirrors. If you go to a place like Denver, you can easily look up the time of sunrise and sunset. It's calculated and tabulated to the nearest minute. But what does that mean? Will the Sun disappear at that minute? Behind a mountain? Which mountain?? It's really an agreed-upon "nominal" sunrise or sunset. It's calculated AS IF there is an observer at sea level, with zero height of eye, at that latitude and longitude with normal temperature and pressure conditions such that the refraction at the horizon is exactly 34'. This is a useful fictitious observer. How else COULD we define sunrise and sunset for inland observers in complex terrain? The only viable alternative would be to use the "true" horizon, but then the Sun would frequently be found obviously above the visible horizon before sunrise and after sunset. That at least would be consistent with the rules for calculating civil, nautical, and astronomical twilight which are based on the position of the Sun's un-refracted position relative to the true horizon (less thah 6, 12, and 18 degrees respectively).

-FER
PS: You also wrote: "I am also considering to do, sight forms
and navigation problems that the Coast Guard at Newlondon can teach.
I don't know their cut off date?". I think there's plenty of time. And you added "I would like to talk to the Coast Guard person." I will put you in touch with him. We don't really know whether any of these sample problems will be used --if they're just using STELLA (software) for sight reduction, it's all rather irrelevant.

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