NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: marine sextant on land
From: Will Ross
Date: 2008 Oct 29, 14:14 -0700
From: Will Ross
Date: 2008 Oct 29, 14:14 -0700
Why dont you use the dip short tables? On Oct 27, 3:12�pm, Paul Hirosewrote: > I want to use my Astra Mark III at home, but don't want to bother with > an artificial horizon. My solution is to estimate the horizon by > eye. > > In some directions I have a reasonable horizon -- mountains in the > background but flat land nearby. But mostly the view is obstructed by > buildings, trees, and a fence. That's all right. I already know my > position, so the altitude intercept directly indicates the error in my > horizon estimate. From that I'm learning where the horizon lies on these > obstructions. > > I often pre-compute the azimuth and elevation, especially for second > magnitude navigational stars. They can be hard to find in twilight > unless the sextant is preset and I know the direction to look. > > Lights on the horizon can make it hard to find the star when the sextant > is preset. It helps to use a horizon shade or simply cover the front of > the horizon glass with my left hand. But once found, there's no problem > keeping track of the star. > > Manmade lighting isn't entirely a bad thing. The additional illumination > provides a horizon reference when it would be be too dark to shoot > otherwise. From a high building in a big city it may be possible to > shoot all night. > > Any method can be used to reduce the sights. I plot the LOPs on an 8.5 > by 11 inch sheet of paper. It helps refine my technique: I can see that > in some directions I have a tendency to shoot a bit high or low. > Currently I'm plotting a scale of 10 millimeters per degree, but that's > too small. For the next sheet I'm going to expand the scale to 20 mm per > degree. > > Much simplification is possible because the observations are of low > accuracy. Time within half a minute is good enough. Refraction may be > ignored except for very low bodies. The center of the Sun or Moon may be > observed to eliminate semidiameter correction. Index correction is > negligible if the sextant is decently adjusted. Dip is negligible if > you're standing on the ground. Angles may be read to only the nearest > tenth degree. > > Sun or Moon shots are best with the scope removed. With the wide field > of view and both eyes I can better estimate the horizon. The scope helps > for star shots in twilight, but due to the narrower field of view, it > helps to take the sextant away from my eye for a moment to check that > the spot where I'm putting the star looks right. Still, I'm sometimes > way off. One predawn Aldebaran shot during the weekend missed by 2.2 > degrees! But that's part of the fun. The sights have low accuracy > compared to what possible at sea, but that doesn't mean they're easy. > > Anyone who wants to practice celestial navigation with a marine sextant > should try this. You're not actually navigating, so there's no need for > a wide spread of azimuth. Just shoot what's visible from your porch or > balcony. Even one window with a good view should give you some bodies. > > -- > I block messages that contain attachments or HTML. --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---