NavList:

William Hawes, you wrote:
"Observation of the sun at sunrise seem to be getting a bit of a bad wrap on the NavList these days. I must come clean and admit that I am a keen advocate of observing the sun at sunrise and have made many dozens of such observations during my time with the Royal Navy and Royal Canadian Navy as a submarine navigator. Before anyone fires back a nastygram, let me hasten to add that I never actually obtained or tried to obtain an LOP from a sunrise observation. In fact the sextant was back in its box by the time I made the sunrise observation. What I used at sunrise to observe the sun was not a sextant, but rather the Azimuth Circle on the Pelorus bridge gyro repeater."

Oh, yes, indeed. That's traditionally called "taking an amplitude". In case the expression is unfamiliar to anyone reading along, an amplitude is any observation of a celestial object at or near the horizon used to check AZIMUTH. These observations are still important today. Celestial observations are the only independent observation that can determine true orientation. In fact, the star trackers found on nearly all spacecraft perform the same function. Ground tracking can determine a spacecraft's location (as can various other technologies), but there is really nothing better than looking at the stars to figure out which way you're pointing.

Amplitudes at sea were daily observations a couple of centuries ago for a much larger correction than small instrumental error. In that period, there were no charts of magnetic variation (a.k.a. magnetic declination). If you were in the middle of the Pacific, the needle of a magnetic compass could be pointing 10 or 20 degrees from true north, and the ONLY way to check it was to watch the stars. And while a star at some higher altitude can yield an azimuth (when other inputs are known), a star or other celestial object near the horizon is much more useful. Procedures for this have varied over the centuries. Should the Sun be observed just as its limb rises or sets? Should an attempt be made to observe it when it is on the true celestial horizon (after correcting for dip and refraction)? There are options.

You added:
"By using the Azimuth Circle prism to observe the bearing of the sun at the true time (not the visible time) of sunrise and compare it with the calculated bearing, one is able to determine the gyro error. A very useful bit of navigation information."

Exceedingly useful, widely used even today for checking gyro error. You suggested that you observed at the "true time" of sunrise. I am guessing that you mean you observed the time when the Sun was at the true horizon (90 degrees from the zenith). As noted above, this is one option. Or does that mean something else?

And you wrote:
"There was a satisfaction in knowing that navigation was somewhat safer once the gyro error had been determined - especially if it was a 1/2 degree or less"

The azimuth of the Sun at sunrise/sunset can generally be determined and counted on to a fraction of a degree. This, of course, is because dip and refraction are vertical corrections, so in latitudes below 45 degrees, they affect the altitude more than the azimuth.

But note that this is VERY different from getting an altitude of the Sun for a standard LOP from an observation at sunrise or sunset. The accuracy of an altitude observation is lower due to the uncertainty of refraction, but more importantly, we require much greater accuracy in altitudes than in azimuths. And in fact, even if you decided to proceed with such an observation, a surface navigator doesn't normally have the tables to do it. Suppose you're on a large modern vessel and you're 100 feet above the waves. You observe the lower limb of the Sun touch the sea horizon. You note the exact GMT and decide to clear this as a normal celestial sight to get a line of position. The Sun LL "Hs" you would record as 0° 00.0'. There's no IC (since there's no instrument). You correct for dip which is 9.7'. Then you open the Nautical Almanac and enter the refraction table for -0° 9.7'. And... and... there's nothing. There IS NO correction tabulated for an altitude below the true horizon. Air navigation tables from the 1940s onward did allow for observations at negative altitudes taken from very high altitudes, but a surface navigator today simply does not normally have the tools to clear such sights. This by itself is merely a hint that you should avoid such observations. The refraction, even if you have tables for negative altitudes, is significantly variable on the order of 10 to 15 minutes of arc right at the horizon, and there is no way to correct for it. In an emergency situation, or for the entertainment of a navigation enthusiast, they're an option when nothing else is available. Almost any other altitude observation would be preferable.

And it bears repeating that there is no observable horizon for aircraft at high altitude. That's why bubble horizons were invented. Even at 2500 feet, the sea horizon is some 60 nautical miles away and is usually lost to extinction and haze or clouds. One cannot observe sunrise or sunset from an aircraft unless it is flying very low. It IS possible to observe the Sun at some low altitudes near and even below the true horizon with a bubble sextant (if you're flying in the right direction and can see out the observation window or dome down to low altitudes), but these are not distinct in any way from any other Sun observations. They're just generic altitudes. And IF you have tables that extend to such low altitudes, you clear them in the usual fashion.

-FER
PS: Note-- it is not "the NavList". It's just "NavList".

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