NavList:

FER- "William H wrote that he had been taught to observe the Sun when it was 1 semi-diameter [sic] or 16' above the sea horizon. Byron said that the rule he learned was 2/3 of the Sun's diameter or about 21' above the sea horizon."

The rule that Maritime Colleges are teaching are right out of Bowditch which is 2/3 of the sun's diameter over the visible horizon. No mention was made of dip in the text, or in any of my classes, or other navigators I have ever spoken with. In short, no one bothers to change the rule of thumb.

FER - "The (presumably) nearest to correct rule would be dip + 34'(for mean refraction) - 16'(for SD) or in other words dip + 18' (and to be yet more accurate, +/- a few minutes of arc for non-standard temperature and pressure). Considering you're shooting from a greater than average height of eye, do you have a rule of thumb that incorporates a higher dip value?"

No change in the rule of thumb. To fix the problem that you will have at "high latitudes," the texts tell us to shoot the amplitude with the center on the visible horizon and then use the table to correct it. By my eyeball of the table, I'd invoke this rule at about 35 degrees which is when the correction hits 0.5 degrees.

FER - "For a height of eye of 100 feet, dip+18' is nearly 1 Sun diameter (especially since the vertical diameter is reduced by refraction). Also, do you normally sail in the tropics? For low latitudes, the specific choice of altitude matters very little. You could use 16' or 32' and in the tropics this should only change the observed azimuth by an eighth of a degree at most (not enough to worry about, right?). But in latitude 45, any difference in altitude yields nearly the same difference in azimuth: half a degree is half a degree. These rules then really only matter in higher latitudes."

I do sail mostly in the tropics, but when I am in the mid-lats I tend to have less opportunity due to weather to observe amplitudes. I have shot very few of them over 20 degree Latitude. It can certainly happen, but it is not as often as the tropics. We can generally determine amplitudes with 0.1 degree precision if you are using an azimuth circle with a scope. You can get pretty close with that using the standard circle as well. Azimuths using mirrors have about 0.3 degree precision due to the width of the "sun bar" reflected onto the compass card. I don't think that the 1/8th of a degree matters too much.

FER - "Do you have your crew get their azimuths by table or by calculator?"

Most do it by computer. Software allows you to input time, position, observed bearing, Variation, PGC heading, and PSC heading. It then outputs gyro error and deviation. This actually makes amplitudes, which must be done with tables or manual calculations, more difficult to reduce than azimuths.

FER - "Why not have them measure the Sun's altitude, even with a crude instrument, for a more accurate azimuth? If they can work it up electronically, the clearing process is no more complicated. That way if clouds obscure the horizon for a degree or two, you could still catch it a bit higher, right? I understand that the sighting process becomes much more difficult at somewhat higher angles, but is it difficult at two or three degrees altitude? I'm just asking..."

I am missing why you would take altitudes at all? If you do not shoot the bearing when the sun is either very close to the celestial horizon or on the visible horizon, you are shooting standard azimuths. These do not require altitudes to be taken at all. If you are trying to pinpoint exactly when the sun is a set distance above the horizon, you basically will need a second shooter there with a sextant to measure as none of my devices will read altitude in less than 5 degree increments.

Depending on the device you are using, you can shoot azimuths at any altitude up to the zenith, even if you wouldn't want to. Most sailors know that the rule of thumb is to observe altitudes away from transit but most probably don't know why.

I always stress to take them early in the morning or late in the afternoon so that your change in azimuth is minimal to obtain a more accurate reading. 2-3 degrees is okay if you can see the sun, but often times it is about 7-8 degrees before it clears the distant clouds. In either case, you are going to be using straight azimuths, be them manually reduced by triple interpolation in HO 229, using a calculator and the formula, or with a computer.

The biggest difficulty I find in shooting azimuths or amplitudes is keeping the compass card/azimuth circle level to get an accurate reading. Given that we are looking for big changes in gyro ever over time, a small variation in gyro error from observation to observation is generally ignored. If we want super accurate results, we will generally use ranges or the Franklin method near the coast.

I hope I didn't muddle things up too much.

Jeremy
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