Robin2
Well-Known Member
Re: Close ...
so what is the figure of 8 analemna?
so what is the figure of 8 analemna?
Assume (Knowing where you aren't)
- Thread starter NigeCh
- Start date
Spacewaist
Well-Known Member
Thought I knew about this sort of stuff...
...but you've lost me.
I'd got to where James was, but now......?
Qu'est ce que c'est analemna?
Why do I need 8 and where can I find the model answer?
It rarely rains for longer than four hours........
...but you've lost me.
I'd got to where James was, but now......?
Qu'est ce que c'est analemna?
Why do I need 8 and where can I find the model answer?
It rarely rains for longer than four hours........
bedouin
Well-Known Member
Re: Love it
Where did that Laptop come from?
Your GPSs are dead, you've lost your sextant and your nautical almanac has mysteriously disappeared. Your VHF and Radio receivers are out too, and all your compasses have failed.
But somehow in the middle of all this carnage and disaster, your laptop is still going strong!
This must be the "willing suspension of disbelief".
BTW - on a serious point, does the potential accuracy depend on the latitude and date?
Where did that Laptop come from?
Your GPSs are dead, you've lost your sextant and your nautical almanac has mysteriously disappeared. Your VHF and Radio receivers are out too, and all your compasses have failed.
But somehow in the middle of all this carnage and disaster, your laptop is still going strong!
This must be the "willing suspension of disbelief".
BTW - on a serious point, does the potential accuracy depend on the latitude and date?
Joe_Cole
Well-Known Member
Re: Assume (Knowing where you aren\'t)
Surely the correct answer is that you wait 20 minutes (that's why you need the watch) and then, out of nowhere, you'll see (that's used your eyes) a PWC racing passed. All you do is ask him.
I wish I could understand all this astro stuff. One day!
Regards
Joe
PS. Before anyone says anything, I know that PWC drivers never know where they are
Surely the correct answer is that you wait 20 minutes (that's why you need the watch) and then, out of nowhere, you'll see (that's used your eyes) a PWC racing passed. All you do is ask him.
I wish I could understand all this astro stuff. One day!
Regards
Joe
PS. Before anyone says anything, I know that PWC drivers never know where they are
ponapay
Well-Known Member
Re: Assume (Knowing where you aren\'t)
Longitude is easy if you have an accurate watch. You just need local noon and compare the time to get longitude at 15 degrees per hour.
Latitude is easy from sun's alt (but not easy to get that without sextant or equivalent), but the bearing at sunset is a great help - if you have the tables or Weirs.
Bearing and time at sunset should give you latitude.
Longitude is easy if you have an accurate watch. You just need local noon and compare the time to get longitude at 15 degrees per hour.
Latitude is easy from sun's alt (but not easy to get that without sextant or equivalent), but the bearing at sunset is a great help - if you have the tables or Weirs.
Bearing and time at sunset should give you latitude.
FlyingSpud
Well-Known Member
Re: I lost the plot years ago ...
.
I am not a mathematician, but presumably you have in mind something like this:-
The analemma formula could be used to determine the position of the sun as observed at the equator, it effectively allowing you to calculate the figure of 8 the sun would trace in a year (I assume your watch gives you the date as well). Once you fix the position of the sun at the equator then, assuming you could get an accurate angle of the sun at your position everything falls into place, as you can compare this angle to the equator angle, draw the line down as a parallel and obtain the latitude?
The longitude is easy of course, once you have a watch
How would that work? I haven’t bothered to actually work it out, because, frankly, life’s a bit too short and I have better things to do with it.
.
I am not a mathematician, but presumably you have in mind something like this:-
The analemma formula could be used to determine the position of the sun as observed at the equator, it effectively allowing you to calculate the figure of 8 the sun would trace in a year (I assume your watch gives you the date as well). Once you fix the position of the sun at the equator then, assuming you could get an accurate angle of the sun at your position everything falls into place, as you can compare this angle to the equator angle, draw the line down as a parallel and obtain the latitude?
The longitude is easy of course, once you have a watch
How would that work? I haven’t bothered to actually work it out, because, frankly, life’s a bit too short and I have better things to do with it.
alex_rogers
Well-Known Member
Re: Close ...
Where can we find a figure of 8 analemna?
Alex Rogers
<A target="_blank" HREF=http://www.YachtsAtSea.com> www.YachtsAtSea.com </A>
Where can we find a figure of 8 analemna?
Alex Rogers
<A target="_blank" HREF=http://www.YachtsAtSea.com> www.YachtsAtSea.com </A>
G
Guest
Guest
Re: Close ...
Who cares?
Who cares?
Jacket
Well-Known Member
Re: Close ...
Any chance that you can run through this in more detail? Or is there a book/website that has what we need to know?
It sounds the sort of thing I should know, given I can't afford a sextant, electronics seem to die as soon as I look at them, and I'm very good at dropping things overboard.
Any chance that you can run through this in more detail? Or is there a book/website that has what we need to know?
It sounds the sort of thing I should know, given I can't afford a sextant, electronics seem to die as soon as I look at them, and I'm very good at dropping things overboard.
sailbadthesinner
Well-Known Member
Re: Assume (Knowing where you aren\'t)
this kept me awake last night
not having yet done any nav other than coastal and not knowing astro nav theory other than being ablt to deduce North and South from the stars ( northern and Hemisphere only)
i assumed that you could calculate by knwing when midday is and when nightfall is you could calcululate where you were. but have no idea whatsoever how you go about calculating it.
isd there a text you can recommend to learn about this?
Come on brain.get this over and i can go back to killing you with beer
this kept me awake last night
not having yet done any nav other than coastal and not knowing astro nav theory other than being ablt to deduce North and South from the stars ( northern and Hemisphere only)
i assumed that you could calculate by knwing when midday is and when nightfall is you could calcululate where you were. but have no idea whatsoever how you go about calculating it.
isd there a text you can recommend to learn about this?
Come on brain.get this over and i can go back to killing you with beer
steveh
Well-Known Member
Re: Close ...
If your laptop is working and you could connect to the internet, you could have a look at this site http://www.analemma.com/Pages/framesPage.html
and spend many hours working it out. Should be able to figure it out between sunrise and sunset maybe!!
If your laptop is working and you could connect to the internet, you could have a look at this site http://www.analemma.com/Pages/framesPage.html
and spend many hours working it out. Should be able to figure it out between sunrise and sunset maybe!!
alex_rogers
Well-Known Member
Re: Close ...
Okay I see what you mean now. Basically taking two sights by eye at sunset and sunrise. You'd still need some kind of almanac and tables to work out the sights - or precalculated plotting sheet if such a thing exists.
I think your accuracies are still a bit optimisitic though. The cut on the sights will be very bad for an accurate lattitude and refraction corrections for very low altitude sights aren't supposed to be very good.
It is a good method though if you drop the sextant overboard!
Alex Rogers
<A target="_blank" HREF=http://www.YachtsAtSea.com> www.YachtsAtSea.com </A>
Okay I see what you mean now. Basically taking two sights by eye at sunset and sunrise. You'd still need some kind of almanac and tables to work out the sights - or precalculated plotting sheet if such a thing exists.
I think your accuracies are still a bit optimisitic though. The cut on the sights will be very bad for an accurate lattitude and refraction corrections for very low altitude sights aren't supposed to be very good.
It is a good method though if you drop the sextant overboard!
Alex Rogers
<A target="_blank" HREF=http://www.YachtsAtSea.com> www.YachtsAtSea.com </A>
jamesjermain
Well-Known Member
Neat solution
A solution to Nige's problem can be found at the following web site - its a rather neat presentation apparently made at a boys school.
I have to admit to copying it from a post by Viceroy who lives at another e-address.
<http://www.cesjds.org/powerpoint/ryanb/sld001.htm>
JJ
A solution to Nige's problem can be found at the following web site - its a rather neat presentation apparently made at a boys school.
I have to admit to copying it from a post by Viceroy who lives at another e-address.
<http://www.cesjds.org/powerpoint/ryanb/sld001.htm>
JJ
Twister_Ken
Well-Known Member
Thanks JJ. Loved it. nm
§
§
Twister_Ken
Well-Known Member
Figure of eight anal enema?
No thanks!
No thanks!
NigeCh
Well-Known Member
I didn\'t use it ...
But it's almost right. It forgets the analemna. Add that into the solution on that site then his answers get better.
But it's almost right. It forgets the analemna. Add that into the solution on that site then his answers get better.
Twister_Ken
Well-Known Member
Although the idea ...
... of a plumb-bob read sextant on a small boat seems a mite dubious. Maybe if one were to use it whilst seated in a fully gimballed chair?
... of a plumb-bob read sextant on a small boat seems a mite dubious. Maybe if one were to use it whilst seated in a fully gimballed chair?
davel
Well-Known Member
Re: Neat solution
NigeCh,
Is this what you were thinking off or do you have some other cunning method that you could share with us?
I suspect the latter since your list of available objects (watch, chart and eyeball) are insufficient for the method referenced in James' post.
Dave L.
NigeCh,
Is this what you were thinking off or do you have some other cunning method that you could share with us?
I suspect the latter since your list of available objects (watch, chart and eyeball) are insufficient for the method referenced in James' post.
Dave L.
NigeCh
Well-Known Member
Re: Neat solution
It's simpler although the maths involved are more complex.
I'm trying to put together an article on this which will be posted on the BYM furum that will explain it all. I'm currently testing the method with the help of others around the world to see if it works. So far it's accuracy seems to be within 2 miles based on a static position. (6 results so far)
It's simpler although the maths involved are more complex.
I'm trying to put together an article on this which will be posted on the BYM furum that will explain it all. I'm currently testing the method with the help of others around the world to see if it works. So far it's accuracy seems to be within 2 miles based on a static position. (6 results so far)
davel
Well-Known Member
Re: Neat solution
AHH! So we'll also need a brain ! That rules me out then.
Can you let us know when you've made your posting (or even post it here as well).
Dave L.
AHH! So we'll also need a brain ! That rules me out then.
Can you let us know when you've made your posting (or even post it here as well).
Dave L.