NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Confused--Can someone answer this question regarding the equation of time
From: Stan K
Date: 2013 Sep 17, 17:09 -0400
From: Stan K
Date: 2013 Sep 17, 17:09 -0400
Andrew,
The time of LAN is seldom required to the nearest second, but if it is, there are better ways.
The least accurate way, which is probably accurate enough to tell the navigator when to get out on deck to start taking sights, is to take the time of meridian passage of the Sun as given on the daily page of the Almanac (which is really for the Greenwich meridian, but is close enough) and apply DLo, the difference between the DR longitude and the zone meridian converted to time, to it. You are starting with a time that is already rounded to whole minutes and applying an approximate DLo, since if you had an exact position you wouldn't need to do any of this anyway. The confusion I have seen among students is whether to add or subtract the DLo.
The next (more accurate) method is to start with 12:00:00, which is always the local apparent time of noon, and apply the Equation of Time for 12h as given on the daily page of the Almanac and the DLo (converted to time) to it. Again, it is a function of the accuracy of your DR longitude, and has the same confusion as to whether to add or subtract, but twofold.
What I find to be the easiest and most accurate, assuming an accurate longitude, is the so-called GHA method. Take your west longitude or 360º minus your east longitude, and find the value of the GHA of the Sun on the date in question that is closest to but does not exceed that value. Record the UT hour corresponding to that value. Subtract that value from your west longitude or 360º minus your east longitude. Use that value to enter the Sun/Planets column of the Increments and Corrections tables "backwards", to find the minutes and seconds that correspond to that value. You now have the UT in hours, minutes, and seconds. Apply the zone description (in reverse) to get the zone time of LAN.
The GHA method takes more words to describe, but is really easy to do, and there is no confusion as to whether to add or subtract anything. Well, IMHO anyway.
Stan
The time of LAN is seldom required to the nearest second, but if it is, there are better ways.
The least accurate way, which is probably accurate enough to tell the navigator when to get out on deck to start taking sights, is to take the time of meridian passage of the Sun as given on the daily page of the Almanac (which is really for the Greenwich meridian, but is close enough) and apply DLo, the difference between the DR longitude and the zone meridian converted to time, to it. You are starting with a time that is already rounded to whole minutes and applying an approximate DLo, since if you had an exact position you wouldn't need to do any of this anyway. The confusion I have seen among students is whether to add or subtract the DLo.
The next (more accurate) method is to start with 12:00:00, which is always the local apparent time of noon, and apply the Equation of Time for 12h as given on the daily page of the Almanac and the DLo (converted to time) to it. Again, it is a function of the accuracy of your DR longitude, and has the same confusion as to whether to add or subtract, but twofold.
What I find to be the easiest and most accurate, assuming an accurate longitude, is the so-called GHA method. Take your west longitude or 360º minus your east longitude, and find the value of the GHA of the Sun on the date in question that is closest to but does not exceed that value. Record the UT hour corresponding to that value. Subtract that value from your west longitude or 360º minus your east longitude. Use that value to enter the Sun/Planets column of the Increments and Corrections tables "backwards", to find the minutes and seconds that correspond to that value. You now have the UT in hours, minutes, and seconds. Apply the zone description (in reverse) to get the zone time of LAN.
The GHA method takes more words to describe, but is really easy to do, and there is no confusion as to whether to add or subtract anything. Well, IMHO anyway.
Stan
-----Original Message-----
From: Andrew Seligman <CaptAndrew@comcast.net>
To: slk1000 <slk1000@aol.com>
Sent: Tue, Sep 17, 2013 3:52 pm
Subject: [NavList] Re: Confused--Can someone answer this question regarding the equation of time
Bowditch did not include that piece of information in the example.
From: Andrew Seligman <CaptAndrew@comcast.net>
To: slk1000 <slk1000@aol.com>
Sent: Tue, Sep 17, 2013 3:52 pm
Subject: [NavList] Re: Confused--Can someone answer this question regarding the equation of time
AAAAAHHHH!!!!!
Bowditch did not include that piece of information in the example.
I thought there was another method I learned in Frank Reed basic cel nav class but maybe I am wrong.
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