Question on Celestial Navigation

[YANDINA]

Despite having GPS I would practice getting location with my sextant and a fancy self contained hand held navigation computer. The closest I ever got to the GPS reading was 50 miles, usually an order of magnitude worse.

It did give me admiration for the map makers for the map I was using in the Bahamas that had been made in the mid 1800's and was only about 30 feet off according to the GPS.

[sailorladd]

Have you tried getting your noon latitude fix?
That was a daily necessity for me after getting my average-of-three sun sightings in the morning...this usually confirmed my DR to some point.

I also learnt that no matter how carefully you take sights, out on a rolling sea on a rolling deck, the most accuracy one can hope for is a fix within a three square mile discrepancy...and for me that was fine when I was way out to sea, and as I started nearing land I would shift into coastal navigation and all that that entails...back then in the mid to late eighties that was the only option open to sailors on a budget... The BEST days of my life though!!!

[GrowleyMonster]

Originally Posted by jeffrwatts

Hi all,

I've been reading and working through a few problem in David Burch's Celestial Navigation Home Study Course.

One thing I'm noticing is that my the fix I calculate by sextant measurement appears highly dependent on my DR.

Since:
1) fix is made on two (or more) lines of position (Zn and a).
2) Each line of position is dependent on Hc and Z
3) Hc and Z are then dependent upon my DR latitude and LHA (which is computed using DR longitude).

Note: I'm using both assumed lat and long for tables, and precise lat and log (for formulas).

Meaning if my DR position is reasonably off, then my fix is likely equally off. So, how does this tell me what my true position really is?

Hoping someone can help me build some better intuition here, as obviously people have been doing this successfully for a few hundred years

Thanks much in advance

Your AP (Assumed Position) is not critical. It only has to be near enough that the LOP can realistically be treated as a straight line.

If you take a half dozen different AP's and calculate your azimuth and intercept for each one, and then plot them on the chart, you will probably find that they all agree nicely. If you shoot two or more bodies, and advance or retard to a single fix time, your fixes should agree, even between two or more different but still reasonably close Assumed Positions. That is the beauty of the system. Yes, it is true that you have to know where you are, in order to find out where you're at. But you don't have to know exactly! It just puts you near enough that the LOP can be used as a straight line. In fact with most tabular methods such as HO229, you fudge the longitude of the Assumed Position to give an even whole minute of LHA, and then plot from that AP rather than from your exact DR position. If this introduced significant error, it would not have been the method of choice for most professional navigators before the age of computers.

There are special cases where the issue that you describe is actually an issue, such as with extremely high HO, something like that. But we generally shoot bodies that are below about 70 degrees high anyway, just because they are easier to shoot than something nearly overhead.

[double u]

Originally Posted by YANDINA

Despite having GPS I would practice getting location with my sextant and a fancy self contained hand held navigation computer. The closest I ever got to the GPS reading was 50 miles, usually an order of magnitude worse.

It did give me admiration for the map makers for the map I was using in the Bahamas that had been made in the mid 1800's and was only about 30 feet off according to the GPS.

did you practice enough? I always thought the sentiment "I'll take the sextant when I need it" hilarious. 3 weeks break from celestial would show me a deteriorated star-fix, 3 months more so, 3 years pause - & I had to start all over.

[Sailor Sailor]

Originally Posted by YANDINA

Despite having GPS I would practice getting location with my sextant and a fancy self contained hand held navigation computer. The closest I ever got to the GPS reading was 50 miles, usually an order of magnitude worse...

Did you realize those navigation calculators are designed for a particular Epoch?

Something is seriously wrong if you are 50 miles off. You might be better off converting to the HO249 method. With the three HO249 volumes (around $75) a current Nautical Almanac, and some Davis work forms, you could be much more accurate in your fixes.

[goboatingnow]

Originally Posted by YANDINA

Despite having GPS I would practice getting location with my sextant and a fancy self contained hand held navigation computer. The closest I ever got to the GPS reading was 50 miles, usually an order of magnitude worse.

It did give me admiration for the map makers for the map I was using in the Bahamas that had been made in the mid 1800's and was only about 30 feet off according to the GPS.

Yes one gets out of practice typically I start around 20-25 ** miles and over a few days it goes to about 7miles. Typically my best is 5 miles.

**This excludes gross errors due to maths errors.