A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Lunar mechanics and Double Alts.
From: Herbert Prinz
Date: 2003 Apr 29, 21:43 +0000
From: Herbert Prinz
Date: 2003 Apr 29, 21:43 +0000
"Royer, Doug" wrote: > 04-26 turned out to be the perfect day to attemp this.Atmosphere was crystal > clear and between 50*F at the start and 68*F at the close with not a cloud > all day. Mr. Royer, I am glad that your observations worked out so well. From the data you posted I gather that you adopted the right strategy to overcome the time constraints that you were facing. You shot each object right after it came over your visible horizon at 10 or 11 deg altitude. It reminds me of a clay pigeon shooting in slow motion. Well done! > 1st round of sights at 0458zt.Moon's disk-Mars combination. By this you mean the far limb, right? > 0548zt Moon-Venus. I would probably have chosen to do Mars - Moon again, with the latter now definitely being out of the dangerous low altitudes and Mars being even higher. > 0504zt Moon's upper horn-Enif. > 0610zt Moon's lower horn-Fomalhaut.Could barely > see Fomal. at this time but compleated it. George Huxtable was already wondering about this. I realize that you probably did not mean to measure the distance from the horn, but you just could not help it. The near limb _was_ at the upper horn. This in itself is an indication that Enif was an extremely unsuitable star to use. Enif is actually not a "lunar distance star"; it was not included in the distance tables of olden days almanacs. Enif is flanked by Markab and much brighter Altair, which both were tabulated. All three suffer from their far distance to the ecliptic of ca. 20 deg, which renders them useless in a situation where the Moon is too close to them. If you tried to reduce the sight, you will already have noticed yourself that the true distance between Enif and Moon hardly changed 10' per hour. In order to give observers in both hemispheres a chance, Fomalhaut was included in the list of "LD" stars despite of its SHA being nearly the same as that of Markab. Fomalhaut is similarly problematic, as it is as far to the south of the ecliptic as Markab is to the north. On the day of your observation, Formalhaut was unsuitable for distance observation. You would not have found it in the almanac on that day. Like Enif, it was positioned in a direction perpendicular to the path of the moon. This made it difficult to decide which limb was the correct one. I actually believe that you picked the wrong one by choosing the "lower horn". The correct limb would have been the far limb, near the upper (eastern) horn. (I have not seen the moon lately as it was raining on Saturday here on the East Coast. I am concluding this from almanac data and could have made an error in my computation.) At any rate, the question is irrelevant. The change in distance Moon-Fomalhaut at the time of your observation was around 10' / h and highly non-linear. One does not want to compute GMT from such an observation. I warned you away from Altair, to which all the above comments apply also, but missed out on Fomalhaut, which I did not expect you to see at all. Partly so, because I based my predictions on latitude 40, not knowing yours, and partly because I am not used to visibility conditions like you seem to have had. The sun was already well over the astronomical horizon when you sighted Fomalhaut, but must have been hidden behind the mountains. As I suggested in my previous message, Antares was probably the only suitable star for distance observation last Saturday. Did you get a chance at all to try it? > 0703 and 0849zt Moon-Sun. It appears that you shot the sun as early as possible. May I infer from the timing of the second sight that this was the last time that you could obtain a clear image of the moon for the rest of the day? What we can learn from the unfortunate star sights is that, without an almanac feeding us the "distances of the day", it is upon us to be vigilant about the proper selection of suitable objects. Some criteria will be obvious from almanac data, such as whether a star is visible in the morning, evening, or not at all. Others will be seen easier in the sky itself, such as proper geometry. Around Full Moon, the limb can pose another delicate problem, the correct choice sometimes easier deduced with arithmetic than from appearance. > I spent the rest of the day and evening doing Double Altitudes because I > haven't much experiance doing them. > many people responded that they calculate the alt. of each body.Is this or > actually takeing the Double Alts.the preferred way?If one doesn't really > know precisely where one is wouldn't measureing the alts. give you a better > end result of the Lunar than calculateing the alts.?Is calculateing the > alts. a step saver or am I just not seeing something? I need guidance on > this matter. Ironically, Double Altitudes are the proper solution to the problem. George Huxtable suggests that you are mis-using the term for art. horiz. observations. But if you really mean by it the technique to observe two altitudes (e.g. of the Sun at different times) for the purpose of establishing your latitude and local time, then you are exactly on the right track. In order to compute the altitude of a star, you need neither GMT nor longitude. Latitude and local sidereal time (=LHA Aries) are sufficient. Think of what you do when computing an altitude for an intercept: You get the required LHA of the star by subtracting its GHA from yours (aka longitude). The GHA of Aries cancels out! So the principle of the operation is this: Use the sun altitude observations gathered during the day to establish your position with respect to the sky (NOT: Greenwich) at the moment of one of these observations. Advance this position to the moment of your lunar sight by means of a watch of known rate. Using this information, you can compute the required altitudes. A minor complication comes from the fact that the moon (fortunately!) does not move at the same rate as stars or sun, whence its LHA and Dec is strictly known only after GMT has been established. Therefore it is necessary to make an initial rough guess at GMT. This does not change or invalidate the principle. George Huxtable's objection against computing altitudes is only valid if these altitudes were computed from GMT and known longitude. I recommend this useful technique for practice shots, but it has little to do with the classical problem of finding longitude. Clearly, the whole point of LDs is missed, if you can't finally do it without knowledge of GMT and longitude. But it is important to understand that the question of using GMT for computing altitudes is different and much narrower than that of whether altitudes should or may be computed at all for solving the longitude problem. We have had this discussion of computed versus observed altitudes over and over. Will you indulge me if I put a view on it from yet another angle? In order to properly perform an LD using observed altitudes, we need four(!) officers on deck. Short of that, we have to cheat. Since one person can't measure the altitudes at the same time as the distance, if one is alone, the altitudes at the time of the distance observations have to be inferred (= computed!) from altitudes taken at other times. This is the logical principle, which remains unchanged regardless of the techniques being employed. Now, one way of computing the altitude is to compute it as the arithmetic mean of two altitudes taken before and after the distance observation. Obviously, there are limits to this method, since the altitudes do not change in a linear fashion. Outside the limits of where linear interpolation is permissible, we have to use a fancier method which we call "method of computed altitudes"; inside the limits, we call it "method of observed altitudes". Obviously, working out an arithmetic mean is not enough of a computation for some. Equally obvious is that the difference between the methods is merely a quantitative and very fluid one, depending on one's demands on accuracy. The methods differ only by the amount of sophistication that one puts into the effort of computing the altitudes at the required moment from the altitudes measured at a more convenient moment. It is unshakeable fact that you need to measure two altitudes in the course of the day. Whether you do it when you measure the distance, or at some other moment, is at your discretion, as long as you have a watch. Herbert Prinz