A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Lunar Distance in Wikipedia
From: George Huxtable
Date: 2007 Aug 24, 00:03 +0100
From: George Huxtable
Date: 2007 Aug 24, 00:03 +0100
Renee wrote- | I have recreated the simple summary in METHOD, and put Jim's practical | details in a sub-section called "in practice". Rather than describe | how two sights and a time can be used to find position, I link the | interested reader to the article on celestial navigation. This, I | think, removes George's objection that the reader could be confused | about how time, longitude, and star sights were actually used in the | age of lunars, as opposed to how they might be used by lunarians | today. | | In Errors, I again simplified and reorganized the discussion, moving | the discussion of longitude prize into the History section. I find it | easier to follow in its new form. I did mention the wreck of | Shovell's squadron. I noticed that, though the section says clearing | involves atmospheric corrections, it does not say where, when, or how | the corrections are made. Are the corrected altitudes plugged into | the parallax formula? | | I moved the mention of the longitude prize to "history", and could | have moved the wreck of Shovell's squadron there as well, but I | thought the wreck of the squadron gave context to the subject of | Error. ======================= Renee is giving this topic plenty of necessary attention, but in some respects it seems that the latest changes may be a step back, rather than an improvement. Also, perhaps some earlier statements could do with some revision. I will get out my nit-picking tool, and wield it about. But please, anyone, argue back if you disagree. Figure caption- "The altitudes of the two bodies are used to make corrections and determine the time."
"In Celestial navigation, precise knowledge of the time at Greenwich and the positions of one or more celestial objects are combined with careful observations to calculate latitude and longitude." "But reliable marine chronometers were unavailable or unaffordable until well into the 19th century." What about inserting "to many mariners" after "unaffordable"? The navigator then consults a prepared table of lunar distances and the < insert "Greenwich"?> times at which they will occur Having measured the lunar distance and the heights of the two bodies, the navigator can find Greenwich time in three steps. Step One -- Lunar Semidiameter Almanac tables predict lunar distances between the centre of the Moon and the other body. However, the observer cannot accurately find the centre of the Moon. Instead, lunar distances are always measured to the sharply lit, outer edge ("limb") of the Moon. The first correction to the lunar distance is the distance between the limb of the Moon and its center. Since the Moon's apparant size varies with its varying distance from the Earth, almanacs give the Moon's semidiameter for each day. Because the Moon is so much closer to the Earth than the stars are, the position of the observer on the surface of the Earth shifts the relative position of the Moon by up to an entire degree. The parallax correction is a complicated function of the observed lunar distance and the altitudes of the two bodies. Step Three -- Finding the Time The navigator, having cleared the lunar distance, now consults a prepared table of lunar distances and the times at which they will occur in order to determine the Greenwich time of the observation. Having found the time, and having the altitude of a star as well as the Moon, the navigator can now use the techniques of Celestial Navigation to find the vessel's position at the time of the observation. Errors Effect of Lunar Distance Errors on calculated Longitude A lunar distance changes with time at a rate of roughly half a degree, or 30 arc-minutes, in an hour. Therefore, an error of half an arc-minute will give rise to an error of about 1 minute in Greenwich Time, which (owing to the Earth rotating at 15 degrees per hour) is the same as one quarter degree in longitude (about 15 nautical miles at the equator). Almanac error In the early days of lunars, predictions of the Moon's position were good to approximately half an arc-minute, a source of error of up to approximately 1 minute in Greenwich time, or one quarter degree of longitude. "Knowledge of latitude to within 30 nautical miles would have prevented the wreck of Admiral Cloudesley Shovell's squadron in 1707." < Here, what is referred to is knowledge of latitude, not of longitude. And it's quite true that the Shovell disaster was related to a poor acount of latitude being kept. Made worse by a 10-mile error in the positioning of the Scillies, 10 miles too far North on the chart. Even so, Shovell had no business to be so far North. Although Sobell makes much of the Shovell disater, it had little or nothing to do with longitude, and it's quite misleading to refer to it at all, in the context of longtitude. It could, and should, be scrubbed.> George. contact George Huxtable at email@example.com or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To unsubscribe, send email to NavListfirstname.lastname@example.org -~----------~----~----~----~------~----~------~--~---