NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Celestial Navigation without a sextant.
From: Frank Reed
Date: 2008 Mar 09, 19:31 -0400
From: Frank Reed
Date: 2008 Mar 09, 19:31 -0400
You wrote: "Supposing one was in a small boat with an accurate timepiece and the necessary tables, how accurate could you determine your longitude by observing the rising or setting of the sun or any other celestial body?" This would work with the Sun only. The time of sunrise can be trusted to about one minute (worse at higher latitudes). So that would give you your longitude to the nearest quarter of a degree. This could certainly be useful. At minimum, it's a nice quick sanity check. Say you're sailing west across the Pacific in the tropics. Overnight you make sixty nautical miles according to a taffrail log (or as estimated from average speed and he duration of the night). The Sun should then rise about four minutes later than expected. If instead it rises six minutes late, you may have a nice current helping you along... More accurate observations later in the day will tell. You could not easily use the Moon for these observations since the Moon is only rarely clearly visible right at rise or set and even then you have to correct for longitude. It's an iterative process. You can't use the stars or even a bright planet like Venus because they're simply not visible until they're a degree or more above the horizon. Atmospheric extinction at the horizon is 12 magnitudes or more. And you asked: "Also, assuming you had a compass and were north or south of the tropics, would it be possible to estimate your latitude by taking a bearing of the setting or rising of a celestial body?" This is less accurate simply because it's tough to get a good bearing. How about timing the length of the day (after correcting for the longitude change)? A few years back, I mentioned that there are fish tags that work on exactly these principles. These are tags attached to tuna that record underwater brightness every few minutes along with GMT. Peak brightness gives local apparent noon and thus longitude (this is determined by the symmetry of the light curve around noon, not the actual "peak" brightness which would be hard to pin down). Duration of brightness as a surrogate for the length of day gives latitude, though not as accurately. Why do something so "primitive"? In short, because GPS doesn't work underwater. -FER --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To , send email to NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---