NavList:

Bob Bossert, you asked:
"There was a recent discussion about whether a DR (Lat and Long) of each sight should be used to compute a fix from three bodies sighted on a moving vessel over 20 minutes. Let's say a sailboat moving 7-8 knots in a consistent direction."

The short answer: it's a matter of degree... and it's a good case where a navigator's judgment and common sense come into play. Let's make our speed 6 knots, since that yields some nice arithmetic simplifications for the case of a "back of the envelope" analysis. A speed of 6 knots is equivalent to one nautical mile in ten minutes of time. It's not hard to get in three sights in just ten minutes, and if you do your vessel has travelled only a mile over the run of all three sights. That's really nothing to worry about. The normal systematic uncertainty in all celestial navigation is around a mile, so you gain no significant advantage by worrying about motion in that much time. If you take twenty minutes, as in your original setup? That's enough to worry --just a little-- about observer motion. It's a judgement call.

Suppose instead you have a Sun sight taken an hour before sunset and a bright star sight taken 30 minutes after sunset (and no other sights... why? mostly cloudy weather could easily make this realistic). At a separation of 90 minutes between the sights, you could travel 9 nautical miles at 6 knots. That's enough to worry about for almost any navigator, so yes, you bring the earlier Sun sight forward by dead reckoning. Note that this is not the same as using or knowing an actual dead reckoning position. We're talking here about the change in position. So if I am travelling due South at 6 knots for 90 minutes, I pick up my celestial line of position and move each point on it south by 9 nautical miles (graphically, you only need to move one point, since you would quickly discover that the advanced LOP is parallel to the original --advance a single point and then draw the new LOP through that single point, parallel to the original).

You added:
"One proposal is that DR of each sight is NOT needed. Graph a Circle of Position purely based on Hs converted to Ho only. Your three COP plot shows a triangle. Your fix is inside the triangle."

I'm not sure what you're getting at here. Why bring up the circles of position here at all?? You're always working on a scale graphically where the circle of position has been replaced by a line of position (you're "zoomed in"). Also, what's this about "purely based on Hs converted to Ho only"?? What are you getting at? That seems like a non sequitur here. Same for "your fix is inside the triangle". That's not directly relevant here. Where are you getting this from?

You then wrote:
"On the other hand, another person said you need to reduce the sight considering DR of the vessel when each sight was sighted. And advance the Line of Position of Sight 1 to Sight 3 (distance and direction), Sight 2 to Sight 3. And the fix is the intersection of the 3rd sight's LOP and the sight 1 and 2 advanced lines of positions."

You're setting up a dichotomy here that doesn't really exist. This latter description that you have given is simply what you do... Of course, you have to use some common sense, as noted above. Is it worth worrying about advancing earlier celestial LOPs? Sometimes no in a multi-star twilight round. Frequently yes.

You concluded:
"What is considered good practice for three body fixes happening during twilight?"

Good practice is smart practice. You need to use some common sense and some navigational insight. First, if the total time among sights is relatively short and the vessel speed is relatively low, then you don't have to worry about it. But if you're really flying along, maybe at 20 knots (!) or if the time between sights is unusually long, and if you're looking for a best possible, nearest fraction of a mile fix, then you need to advance each earlier line of position. And finally, there is the "good scout" or "hyper-nerd" answer: if you wish to follow exact rules, no thinking, no common sense, then always, always, always advance earlier lines of position. But that's not "navigator thinking"... That's "robot thinking". :)

There is one key trick you can use to make life easier: find one star abeam... perpendicular to your vessel's motion (true course over ground, as nearly as possible). A sight taken abeam yields a line of position running parallel to your vessel's motion. And advancing a line like that lifts it up and drops it right back down on itself --no change! For smaller vessels, which are likely cases where one might be navigating with sextants, you can plan around this. The simplest case is a pure latitude sight, like Noon Sun or Polaris. Since the LOP for either is a line of latitude, running E-W, if you are sailing due East or due West, there is no adjustment required for change in latitude. You can sometimes plan for this in navigation...

Frank Reed
Clockwork Mapping / ReedNavigation.com
Conanicut Island USA