NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Marine sextant on land
From: Paul Hirose
Date: 2008 Oct 27, 12:12 -0700
From: Paul Hirose
Date: 2008 Oct 27, 12:12 -0700
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 -~----------~----~----~----~------~----~------~--~---