NavList:

   

Reply

>> Bill wrote:
>> First, to calculate the precise moment a body will be at 90d or 270d do I
>> not need to know my latitude exactly?

> Peter responded:
> Ah; precision precision. You are probably quite correct ... to be
> precise. Establishing the latitude is, traditionally, relatively easy.
> You could, for example, have worked it out only a few hours earlier at
> meridian passage of the sun and are now looking for another position
> line to get a fix...

Summer and "few hours" between LAN and a 90/270d azimuth? ;-)

>> Bill:
>> If I am that sure of my latitude, why
>> not use a time site to extract longitude directly?

> Peter:
> You would need a number of bodies to get a fix. Here you need only
> one, usually the sun (although, as with meridian passages, any body
> can be used, assuming you can see it and the horizon).

A number of bodies would give you lat AND lon (at least it once did prior to
the cocked hat discussion ;-) As you pointed out, I would not need a number
of bodies to get a running fix if I were relatively confident of my
latitude, just one set of observations at 90/270d.

After running a number of hypotheticals, it strikes me that your longitude
might be as good (latitude known) or better (latitude unknown) if you deep
six the pain of precalculation and use a time sight near Zn 90/270.  It
gives longitude directly in one set of calculations.

The beauty is:
--No pre-computation
--Only one variable, latitude.  GHA and declination are derived from the
time of the sight.  Elevation is from observation.


For example:
1 May, 2007, Sun
20:41:45
AP N 40, W 60
Az 270 (Actual transit)

1 May, 2007, Sun
20:41:45
Real position N 41, W 60
Az 266.73

1 May, 2007, Sun
20:21:45
AP N 40, W 60
Az 269.56

1 May, 2007, Sun
20:21:45
Real position N 41, W 60
Az 266.20

I correct for variation so I know the compass reading I need to hit 90/270d
true and make my observations from just before I see the body at 90/270d
until a bit after and average. Then I do a time sight to extract longitude.

If I thought I was at N 40 and was at in fact at N 41, and used N 40 in my
time site at 20:41:45. My longitude would be 59d 59!7, off by on 0!3 or 0.2
nautical miles.

Let's say my compass work wasn't spot on, and I missed 90/270 by 3+ degrees.
Again I thought I was at N 40 and was at in fact at N 41, and used N 40 in
my time site from 20:21:45. My longitude would be 59d 55!0, off by on 5!0 or
3.8 nautical miles.

Not too bad for being a degree off latitude. Real world, say your latitude
was 6!0 off over your run from LAN and you missed 90/270d by 3+ d.
Longitude from time sight would be 55d 59!6.  0!4 or 0.3 nautical miles off.

> Peter:
> There is one limitation in practice - its a summer-time only trick. In
> winter months you may not find the sun at 90 or 270d.

For sure the sun will not hit 90/270 until the declination is approx. 0d. We
would want any body we use to have a declination that offers an elevation
where refraction will not become a wild card at 90/270.

Thoughts?

Bill



--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---

   

Reply