Newbie navigation question....

[DRoth]

Hey everyone!

I've just started reading up on navigation techniques. I must be completely missing something (considering how complex navigation actually seems to be), because this technique occurred to me:

Assuming:
1. The sky is clear for measurements
2. You have a good quality watch set to GMT

For longitude, couldn't you just:
1. Take sun sightings to find exactly local noon, when sun is at it's zenith (eg 21:50 GMT)
2. Find exact timezone difference between position and Greenwich (9:50)
3. Translate that to degrees (9.833 x 15), therefore long: 147.495W

There must be some stupid error in my thinking, but I don't see what. Can anyone enlighten me?

[Tom Keffer]

It's called a "noon sight." One of the oldest navigation techniques there is.

[Pblais]

It's called a "noon sight." One of the oldest navigation techniques there is.

It's not that old. It didn't become possible until they invented the ships clock accurate enough not to lose or gain much time. There is a an astronomical method too but it takes quite a bit of math and requires good sightings. In the old days by hand it took about 4 hours to compute.

There is a great little book called "Longitude" that goes through the process and all the politics of developing a reliable method of computing longitude. Another book called "Compass" is also very interesting as well. Accurate compasses were not possible until relatively modern times.

[btrayfors]

Sorry, guys. What DRoth described is NOT the so-called "Noon Sight", which is a sight designed to provide your latitude, not your longitude.

However, his reasoning is correct. Indeed, it is possible to obtain your longitude at about the same time as the noon sight for latitude.

The technique involves taking a sight before local apparent noon (LAN), and recording the time and the sextant reading (Hs). No corrections are necessary to the sextant sight.

Then, you proceed with the noon sight for latitude as normal. Afterwards, you reset your sextant to the reading taken before LAN, and await the descent of the sun. When, in your sextant, it just kisses the horizon you record the exact time.

You then add the two times and divide by two, to give you the exact time of the sun's crossing your meridian.

After that, you simply look up the sun's GHA (Greenwich Hour Angle) in an almanac for the time you calculated, and that IS your longitude.

Simple and effective. If you do it carefully, you can get within a few miles of your longitude....good enough for a ship on the high seas.

I've got a simple form developed for this sight in PDF format, which I'd be glad to share with anyone who wishes to try it. Just shoot me an email at:

bill at wdsg dot com

Bill
S/V Born Free

[Pura Vida]

Basically you are right but what kills is size. A proper sight must compensate for the height of the observer, upper or lower limb of the sun, the difference between the time the sight was taken and the time it was written down, the index error of the sextant... on and on. Some average noon sights on the 5s. You could wait until nightfall and in the northern hemisphere take a sight on polaris and get lattitude too.

A similar problem is in charts. Depending on the scale the line could be several meters wide, or the actual soudings could have been on either side of an obstruction. Anyway I think in that same simple formula whenever I take a sight then add the other stuff in the worksheet.

Dana Sobel's book is a wonderful one. Both as an illustration of the actual problems of navigvation and as an insight into the politics of science at the time. As I recall Newton was against the whole cronometer idea thinking that longitude could be calculated by celestial means alone. Does anybody know how to do lunars?

[SkiprJohn]

I have always used my "noon sight" to determine lat and long. Long is not exact but very close. There are other terms for "noon sight" such as "meridian passage."

DRoth has explained how I've been doing this successfully for at least 25 years. Yes, that was after they got accurate time pieces aboard.

Kind Regards,

JohnL

[btrayfors]

Originally Posted by Pura Vida

A proper sight must compensate for the height of the observer, upper or lower limb of the sun, the difference between the time the sight was taken and the time it was written down, the index error of the sextant... on and on.

Yes, most sextant sights do require these corrections. However, the beauty of the Longitude by Meridian Transit sight I described above is that NO CORRECTIONS ARE NECESSARY. All you're trying to do with the sextant is to determine, as closely as possible, the exact TIME of meridian transit. So, one sight well before and another after MT using the exact same sextant setting will give you the two times when the sun was at the same altitude. Add them up, divide by two, and you have the TIME of meridian transit. Easy, then, to convert to your longitude.

You do have to do the usual corrections for the noon sight for latitude, of course.

Bill

[DRoth]

Thanks for the comments. It seemed like a simple method, and I hadn't come across it in anything I've read about navigation yet, so I thought there might be some fundamental error in my thinking.

[SkiprJohn]

In "Celestial Navigation by H. O. 249" John E. Milligan describes on page 27 exactly what you need to do to obtain an approximate longitude from your noon sight. If you can find his book it really is the simplest to read and understand that I've found and describes sights and examples in my area (Hawaii).
JohnL

[Jenia]

You can also do a Meridian Passage of the moon if the horizon is clear. I did this and the longitude was good, but i can't remember how good the latitude was, however it should be as good as a noon sight lat, except that you need to apply the HP correction.
jenia

[GordMay]

A decent explanation:
http://www.ncsail.org/addendum_v8_3.pdf

[DeepFrz]

Okay Gord, am I missing something here (like my mind) or is there a calculation error on page 30 of the pdf file you posted?

424.5 minutes, + 34 seconds = 425.067 minutes.

Not 425.56 minutes

Damn, almost hit that rock...

[Whimsical]

DeepFrz
424.5 + 1/60x34 = 425.0666666
how did you get more than a minute for 34 secs
Mike

Edit Just realised you were correcting Gord

[GordMay]

Somebody's paying attention!
Yep: 424.5 minutes, + 34 seconds = 426.0666 / 426.07 minutes

BTW: I’m no expert in these arcane arts. It’s been 25 years since I studied the subject; and I’ve NEVER used the methods in earnest.

Originally Posted by NCsail.org

(not GordMay)
Total = 424.5 minutes until Greenwich Noon.
+34 seconds (Equation of Time correction)
= 425.56 minutes until Greenwich Noon.

[DeepFrz]

Hey, someone has to keep an eye on you guys...<gr>