NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Lunar Exercise
From: Frank Reed
Date: 2008 Sep 24, 23:25 -0400
From: Frank Reed
Date: 2008 Sep 24, 23:25 -0400
JCA, you wrote: "The lunars were a deliberate exercise, using a wind-up watch that is known to run fast, and a longitude that is truly suspect since I did not know the datum of Washington DC that was used. The Fix of the stone comes from 1881." A longitude in the eastern US from 1881 is likely to be very accurate by the standards of sextant navigation. By then, telegraphic longitude determinations were standard. If you want to determine a longitude there by traditional lunar observation, bear in mind that you will need an observation for local apparent time. Frequently with historical Sun-Moon lunars, the Sun's altitude taken for clearing the lunar distance was pressed into service to give local time, too. A navigator might also take a time sight and set a common watch to local time whenever convenient and then record the apparent time of the lunar distance when that sight was taken. The apparent time has to be adjusted for any change in DR longitude between the setting of the watch and the LD observation. On the other hand, you could do a modern "lunar distance line of position" (take two distances for a fix) if you throw in known GMT. You wrote: "While I like the sextant, I now remember how much I dislike the 3.5x scopes that now come standard with most sextants. On the ship I have a 4x, and I had previously used a 6x scope that is no longer available. I have on order from Celestaire, a 7x scope, but since I haven't received it yet, I am stuck with the standard scope." Yes, I find that this makes a big difference. It's obvious enough when you think about it, but magnification is really important for lunars. A 7x scope gives results twice as good as a 3.5x scope. Geoffrey Kolbe even constructed a 25x scope for his sextant. It's hard to hold the images of the Moon and Sun in the field of view, but for land-based observations, this should yield essentially perfect lunars, limited only by the Moon's uneven, mountainous limb (this assumes that the sextant has been calibrated so that arc errors can be taken into account). -FER --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---