
NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2025 Jan 22, 11:32 -0800
Charlie McElhill, you wrote:
"Is there any negative impact to the lunar distance measurement of the process if there is a small amount of side error? From your previous note it appears a small amount of side error is tolerable for just altitude sights. However Lunars usually require some degree of horizontal positioning of the sextant for the object to object measurement."
Excellent point. Side offset would certainly damage and possibly ruin any angles measured between small-ish objects. If you're trying to measure the angle between two stars, for example, that are two degrees apart, you might not be able to make convincing "contact" between them if there's a side offset. One object will just "scoot" past the other when you nearly have the angle right. This is of no real-world concern because the problem rapidly evaporates as the distance between the objects increases. Lunars are ruled out on other grounds for distance less than about 15° and certainly below 10°. For angles larger than that, the residual issue of side error at low angles is already reduced to insignificance.
Even in coastal navigation scenarios, though there might appear to be a problem, it's actually easy to get around it...
Suppose I see two buoys a few miles away, separated by about a degree in bearing. I decide to measure the angle between them with my sextant. I know that my sextant has a side offset of about 10 minutes of arc, and the buoys are visibly each three or four minutes in angular height. I hold my sextant sideways to measure the angle between the buoys bringing the direct image of one buoy to line up with the reflected image of the other. But I can't get them to touch. Of course, with no real thought, I can see that all I need to do is slide the reflected buoy over until it is aligned exactly beneath the direct-image buoy. That gives me the angle between the buoys.
Frank Reed