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Regarding assumed longitude using intercept method. - Rumata - 11-14-2024
Gentlemen,
Using intercept method I came to kind of strange situation. Let me be as brief as possible and to the point.
Calculating LHA for observed star. Western longitude, north latitude. Wrote formula to calculate it based on grpaphical representation og GHA Aries, SHA of the star and longitude ( assumed) . I got LHA but depending on the order I added and subtracted data assumed longitude became different. But LHA was the same.
Now, specifics:
LHA of the star = GHA Aries +SHA of the star -360- Longitude assumed = (GHA Aries - Longitude assumed) + SHA - 360. ( I just made a drawing to visualize the data , and from that drawing came those two equations.)
Again, results are identical, but assumed longitude in this particular case is different in about 25 minutes of longitude.
I think, I asked this question before, but can't find the answer.
So when I start to plot this star position, different longitudes assumed made it a little bit difficult to do it right the first time. Of course, I can plot two positions for the same star and see which one makes more sense, but maybe there is a simple way to go through this.
Thank you.
RE: Regarding assumed longitude using intercept method. - PeterB - 11-14-2024
If I understand your current question then I believe that you did ask this question previously. It and my response can be found in the "General Topics" forum under the title "Assumed Longitude" posted by you, (Rumata) on 10/06/2023.
https://thenauticalalmanac.com/Forum/showthread.php?tid=257
In short the GHA of the star when calculated from the GHA Aries and the SHA of the star is very unlikely to have a value that is without any minutes and tenths. You must select your assumed position longitude such that when you calculate the LHA from the GHA of the star the LHA comes out to a whole degree value without any minutes and tenths. If you do this in the wrong order you will get different answers.
For tables you must FIRST calculate the GHA of the star and from that derive the proper assumed longitude -- not the other way around.
Cheers
PeterB
RE: Regarding assumed longitude using intercept method. - Rumata - 11-14-2024
-Thank you, Peter! As I've mentioned in my original post, I thought I asked this question before. Thanks for referring me to it. Let me clarify a few points. I know that I need to get whole number LHA. And yes, you're right, that SHAs almost always have decimal parts. And if I follow your suggestion, I just need to subtract/add longitude to GHA , which gives me definite assumed longitude. And not to try to simplify written equations where assumed longitude eventually will have different value. Will try this approach.
RE: Regarding assumed longitude using intercept method. - PeterB - 11-15-2024
Confusion often arises because people think that the "Assumed Position" is something akin to the dead reckoning position or an estimated position. In fact it is neither. It is a completely contrived position that the navigator is fully aware is not his actual position, it is just a trial position taken up to use with the tables.
This is why you start with determining the GHA of the star at the moment of the sight. That is an unchangeable angle. It is what ever it is for the day and time of the sight. What can be adjusted to get the necessary whole angle LHA from the observer is the locus of calculations -- as if the navigator were to "pretend" he was at at another particular position which happens to yield the whole angle LHA results he requires -- even though he fully knows that he is not at this "pretend" position at all. And this step is only used because the tables do not list every possible angle for LHA, only whole angles for latitude and LHA thereby making them compact enough to carry aboard even a small vessel.
The "assumed" in "assumed position" means deliberately taken up, not estimated or guessed at, or even calculated by dead reckoning. It is probably an unfortunate choice of nomenclature as it is so easily misunderstood. Perhaps it should have been called "the trial position" or something else like "the locus of calculations" but unfortunately it is known the world over as "Assumed Position."
This is why the GHA is found first and then the AP is found from that result, and not in any other order.
PeterB
RE: Regarding assumed longitude using intercept method. - Rumata - 11-16-2024
(11-15-2024, 09:59 PM)PeterB Wrote: Confusion often arises because people think that the "Assumed Position" is something akin to the dead reckoning position or an estimated position. In fact it is neither. It is a completely contrived position that the navigator is fully aware is not his actual position, it is just a trial position taken up to use with the tables.Agree
This is why you start with determining the GHA of the star at the moment of the sight. That is an unchangeable angle. It is what ever it is for the day and time of the sight. What can be adjusted to get the necessary whole angle LHA from the observer is the locus of calculations -- as if the navigator were to "pretend" he was at at another particular position which happens to yield the whole angle LHA results he requires -- even though he fully knows that he is not at this "pretend" position at all. And this step is only used because the tables do not list every possible angle for LHA, only whole angles for latitude and LHA thereby making them compact enough to carry aboard even a small vessel.
The "assumed" in "assumed position" means deliberately taken up, not estimated or guessed at, or even calculated by dead reckoning. It is probably an unfortunate choice of nomenclature as it is so easily misunderstood. Perhaps it should have been called "the trial position" or something else like "the locus of calculations" but unfortunately it is known the world over as "Assumed Position."
This is why the GHA is found first and then the AP is found from that result, and not in any other order.
PeterB