# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

**Re: Luni-Solar Distance**

**From:**George Huxtable

**Date:**2010 Oct 26, 20:52 +0100

Thanks to Lars Bergman, for contributing the following thoughts about calculating the altitudes, to go with a lunar distance observation. Sorrry for the delay in responding, but my old boat has taken some priority just now: but that has provided a bit of useful pondering-time. Lars wrote- =========== George wrote, in reply to Douglas Denny, in 14133: “If the time was known, there would be no point in taking a lunar. That's the purpose of measuring a lunar-distance; to determine the time. So there's a bit of a paradox, in calculating altitudes with which to clear a lunar, for which time and position are both needed. To get round it, you have to assume a trial longitude first, and then use the lunar observation to refine that assumption.” And in 14145: “I don't know of any texts which have considered this question of a need for reiteration when altitudes have been calculated, and wonder whether travellers were even aware of its potential for causing error. Can anyone point to such a publication?” --- In above context, the purpose of measuring a lunar-distance is to determine a standard time, i.e. the time at Greenwich. To calculate an altitude, knowledge of Greenwich time is not necessary, but the local time must be known. Latitude is assumed known and an approximate Greenwich time in order to find the declination from the almanac. Thus, an altitude observation to determine the local time should therefore be executed as near in time to the distance observation as possible. The error in time then depends only on the regularity of the watch and of the ship’s (or traveller’s) reckoning between the two observations. To find the altitude corrections, some rework may be necessary. Normally, parallax and refraction are found from the apparent altitude, thus all tables are arranged with apparent altitude as argument. When altitudes have been calculated, some iterations are necessary to find the proper values. The aim is to end up with an apparent altitude that, reduced in the normal way, gives the same value of true altitude as that found by calculation. In some Swedish manuals for navigation (e.g. Pettersson in 1876) the process of calculating altitudes are described in some detail, involving iteration not of the lunar calculation but of the altitude calculations. ============== On reflection, I think that Lars has got the matter right. As long as an observation to determine local time has been made, at or near to the time of the lunar, that can provide sufficiently good data on which to base the necessary altitude calculations. In which case, the laborious iteration procedure that I had thought necessary could be circumvented. Thanks to Lars for putting me right. However, sometimes a lunar is taken for a different purpose than to establish an immediate longitude. Sometimes, it might just be to set, or correct the error of the chronometer, and it might not be thought necessary, in mid-ocean, to make such a time-sight for local time, there and then. Can the altitudes be successfully calculated under those circumstances, I wonder? And I still wonder about the use of Frank's lunar-distance calculator, at- http://www.historicalatlas.com/lunars/lunars_v4.html , to solve the converse problem from that it was intended for. I would be grateful for any step-by-step guidance on how to go about using that calculator to establish the GMT at the moment of a lunar distance observation, from an unknown longitude, without observing the altitudes. Can it be done? How much iteration is involved? With observed altitudes, there seems to be no such problem. Here is the example I asked about, once again- =========== "A navigator is somewhere on the Equator, at lat = 0º, and knows it from his previous observations. We know (though he doesn't) that at midnight, 00:00 hrs at the start of 26 March 2005, he is exactly on the Greenwich meridian, at long = 0º, at which moment he takes a precise lunar distance between the Moon's near limb (it's full Moon, so either limb will do) and Regulus, of 36º 48.9'. However, not knowing his exact longitude, he guesses it to be 01º 00' East." =========== Can anyone explain to me the most efficient way to go about it, using that lunar calculator? George. contact George Huxtable, at george{at}hux.me.uk or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.