# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

**Longitude by Sunrise example**

**From:**Jeremy C

**Date:**2010 Feb 4, 10:11 EST

I commented on the article online and asked if he had rough error
expectations as well as asking him for some sort of real world data if it was
available. I suspect that there is little to be had, which just means that
I will have to do some observations this Spring when I am on my ship. I'll
try to take several, and also repeat my LOP experiment.

I will start a new thread with my example of ultra-low altitude
LOP's. I did indeed take into account T/P, which I do for all sights,
especially below 10 degrees Hs. I am at the point in my quest for
precision that I account for any and all factors even if they are 0.1' of arc. I
just looked in my NavLog again and I made an error (I left in my index error,
despite not using a sextant for this observation.) I will correct it and
will get slightly better result, although still not particularly good.

Typing this has gotten me thinking. I can probably re-create a Long by
sunset with my sunset LOP data from this past summer.

If anyone else would like to join me, here is the data.

On 1 July 2009, the 1900 ZT (ZD -9) DR is Lat 17 deg N; Longitude 129
deg E. The course is 190 deg T and the speed is 13.2 knots. Temp is
82 deg F and Pressure is 1008 MB.

1) Determine time of sunset

The lower limb of the sun touches the visible horizon at 18:58:16 ZT and
the upper limb touches at 19:00:48 ZT.

2) Determine Longitude at sunset.

----------------------------------------------------------------

Since I don't have a paper NA, I can't calculate LMT of the sunset since my
program directly gives me ZT. This will not allow me to follow David's
method's directly. I can do it in a round about way by comparing the time
difference in local time.

1) I have a sunset at zone time of 19:01:39 at the DR position.

2) Delta-T is 3m 23 seconds between the calculated Sunset and when the LL
touches the visible horizon. This is equal to 50.8' of arc . That would
give me a Longitude of 129 deg 50.8' East since the sun set earlier we must be
more to the East of our DR. The GPS Longitude was 129 deg 7.5'E That
is a difference of 43.3' of Longitude which is a significant error. Now I
will admit that the DR Latitude is nearly 30 minutes off of the GPS Latitude
which will account for an error in sunset and therefore longitude.

If we correct the DR to the GPS position and recalculate sunset we get
19:00:18. This reduces the delta-T to 2 min 2 seconds. This is 30.5'
arc. This gives us an error of 23' of longitude, which is still
significant.

I am not too confident that I will get much better results
in more observations.

I will be interested to see the results using the tabular methods of sunset
as opposed to the software I use.

Jeremy