NavList:

Jackson McDonald, you wrote:
"If you were the navigator aboard ship, would you attempt to take a sight, compensating as best as you could for this mirage effect, or would you simply not take a sight?"

There are a couple of ways of handling anomalous dip:
1) Live with it. This is really what the vast majority of navigators do, even if they know little about it. At sea, for the most part, anomalous dip is rare. And it's probably less than a minute of arc. That just makes it one component of the usual "+/- a mile or two" expectation for the accuracy of a celestial fix. I usually contend that "normal" variations in dip (+/- about 0.3-0.5') are almost certainly the biggest source of uncontrollable error in standard celestial navigation when skillfully practiced.
2) Look for clues and work around it. There are clues for circumstances when there may be anomalous refraction at the horizon. There's the strange visible mottling and undulation that I described in this recent post. Seeing this requires some magnification, so scanning the horizon with binoculars might help. Surface fog is also a clue. If there are patches of scudding fog just a few meters to tens of meters deep, this can indicate a surface temperature inversion (even after the fog "burns off"). The conditions that produce these fog patches also yield mirage effects.
3) If we have any reason to suspect anomalous dip, you can try to get a large number of sights in at least three azimuths. This is very difficult in daylight but might occasionally be useful in twilight. You shoot two or three sights each of Venus, Jupiter, and the Moon, for example (you could do this in twilight tonight). When you plots the LOPs, if there is anomalous dip, it would create a constant offset in all LOPs. The lines from each body would cluster together tightly, but the triangle that they form would be open at the center. By subtracting an extra minute or two, if you can get the triangle to close (more of less), then you've eliminated that systematic error. But note: this ONLY applies if you have more than three sights, really at least six, so it's not often useful unless you have two people taking sights. If you only have three sights and no reason to suspect such a systematic error, then there is no valid reason to apply this sort of procedure!
4) As Brad mentioned there are special instruments, like the "Soviet" dip meter that Alex Eremenko showed a few of us, that can measure the actual angle between the sea horizons on opposite sides of the sky (it it's 180°10.4' over the top of the sky from one horizon to the other, then the dip is 5.2'). Instruments like this are especially useful in regions like the Arctic where anomalous dip is common (abnormal is normal), so it makes good sense that it was developed as standard kit for the Soviet Navy.

-FER

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