Sextant: Time of Sunrise / Sunset?

[nigelmercier]

OK, newbie question about sun sights.

I'm reading a book Sextant: A Voyage Guided by the Stars and the Men Who Mapped the World's Oceans by Barrie, David (2014). It is very entertaining, and complements other recent books I have read regarding finding Longitude.

However, if the following passage in the book makes sense, then I need to start again:

Am I correct and this is the wrong way round? In other words he would be 30° East?

 

[bedouin]

Am I correct and this is the wrong way round? In other words he would be 30° East?

Yes - you are right about that he has it the wrong way round

Anyway it is a pretty inaccurate - only works if you are close to the same latitude

 

[oldbilbo]

You are indeed correct. The quoted text is - as described in Northern vernacular - 'arse about face'. Lament the demise of technical proof readers, and find a rather more reliable source of ideas.

There are many approaches to finding the ( very ) approximate time/longitude, but that's the first I've encountered that's >60° in error. Even DR would be better than that....

You'd be a bit closer identifying the chronometer time of Mer(idian)Pass(age) of the Sun, applying the tabulated Equation of Time, and using that to determine the Diff between Local Apparent Time and chronometer's GMT.... which is, if I recall, something close to what you seek.

Also, there are several ways of better identifying the time of MerPassSun. One is to 'shoot' the ascending Sun an hour and a bit before expected Local Noon while noting the Chronometer Time, leaving the sextant set at that value, then identifying the ChronTime when that reading recurs with the descending Sun. The median time is closer to MerPass.

Of perhaps more practical significance, and while you are out of your bunk early enough to witness sunrise, you could arrange to use that reliable phenomenon to check for error in your steering compass, using tabulated 'Sun's Amplitudes'. Far from being esoteric, this is ( or was ) an important feature of the marine navigator's duties and remains ( ? ) a part of the RYA Ocean Yachtmeister which is ( was ) examined. After all, if your compass is thus known to be accurate, and you record your courses steered/distance run with some diligence, you are rather less likely to steer yourself and your modern-day 'HMS Association' onto the IoS Western Rocks than the magnificently--named Sir Cloudesley Shovell.

With nearly 2,000 sailors lost that night, the Scilly naval disaster was recorded as one of the greatest maritime disasters in British history.[7] The cause of the disaster has often been represented as the navigators' inability accurately to calculate their longitude, although no public discussion of the events specifically raising the question of longitude is known, prior to a pamphlet published on the eve of Parliament's vote on the Longitude Act, seven years later.

 

[nigelmercier]

You are indeed correct. The quoted text is - as described in Northern vernacular - 'arse about face'. Lament the demise of technical proof readers, and find a rather more reliable source of ideas...

Thanks for the replies, I thought I was going crazy!

I've done quite a lot of proof-reading, both technical and non-technical. I can't actually turn it off, I think it's the way my brain works.

By the way, anyone interested in historical methods of determining longitude will know there are some excellent books on the subject, but may not know about the resources at the Cambridge Digital Library

 

[oldbilbo]

No crit levied at you, 'nigelmercier'. Just a turn of phrase....

Should anyone here have aspirations to becoming a full-on Celestial Nav Nerd ( CNN ), then this is the place to go.... http://fer3.com/arc/

Be warned! There is no known cure....



Edit: adding in some more 'tripped over' in NavList.... relating to B-52 standards....

We tried to be perfect but to be within about five miles at night and 10 miles of a daytime celestial navigation termination point was pretty fair. That’s on just your run of the mill average sortie.
During daylight we used the sun and resulting MPPs and took two night celestial fixes prior to our termination point. We arrived at the end celestial navigation position within two miles and within 30 seconds of our estimated time of arrival. Not bad, not bad at all.
The winners of the Strategic Air Command’s annual navigation competitions usually took it to another level though. The winner’s average was frequently within a couple of miles. Sometimes much closer.

The British were always tough competitors.

I'll say! Most RAF V-Force squadron crews were expected routinely to turn in NavEx results, each month, of Tracking within 2 nm and Timing within 30 secs of plan, after a 2000nm flog around the North Atlantic. Those selected for NATO Bombing Competitions - and those hinted at against the USAF in Nevada - would be aiming at performing reliably to 'an order of magnitude better'.

 

[johnalison]

It's an old book and the earth used to rotate the other way in those days.

 

[nigelmercier]

Should anyone here have aspirations to becoming a full-on Celestial Nav Nerd ( CNN ), then this is the place to go.... http://fer3.com/arc/

Thank you, looks interesting. But is there a way to get the site to display like a conventional forum?

 

[Strolls]

Thank you, looks interesting. But is there a way to get the site to display like a conventional forum?

I'd find the mailing-list interface convenient:

Uniquely, the NavList message boards also permit full interaction entirely by email. You can optionally receive individual posts or daily digests by email, and any member can post messages by email (bypassing the web site) by sending to our posting address which is "NavList@fer3.com". This functionality is similar to a traditional Internet mailing list: post by email, read by email, reply by email. Most members will prefer the web interface here for posting and replying to messages.​

 

[Orion Jim]

It's an old book and the earth used to rotate the other way in those days.

You may have something there. Some mornings when I wake up and read the news I think I've awoken on the wrong planet.

 

[John Barry]

On the original question:
Yes, he has it the wrong way round.
Yes, the method is very inaccurate.

The problem is that at very low altitudes the refraction is very large and very variable (and in practice unpredictable). The closer to the horizon the worse the error. You're better with a method of timing the when the Sun is at a fixed height (1 diameter, for example) and using that. It gives an error on the order of 15-20 miles, depending on latitude and azimuth.

Nerdy maths bit available if required.

John

 

[nigelmercier]

On the original question:
Yes, he has it the wrong way round.
Yes, the method is very inaccurate.

For sure, but it made me doubt my basic understanding.

 

[Uricanejack]

I found the book sextant interesting and quite enjoyable. I never noticed the error. Perhaps it was or will be corrected in later printings.
Even with the error It's a good book. It's not an instruction or text book. Which is probably why it is interesting. The writer is telling a story about a voyage from his youth and combining it with a bit of the history of the development of navigation.
The technique described is not one I have ever used. Though in principle it works.
If you apply the time difference correctly.
In practice on a small boat in the north Atlantic almost 50 years ago it was a good as it got at the time.
I generally used it the other more conventional way to predict the time of twilight to prepare my choice of stars. where a rough idea is good enough.
The NA only gives the time for the middle of the three days on a page and only for major latitudes so some interpolation is required.
Combining the time of sunrise, sunset and the amplitude will improve accuracy. again though an amplitude is usually used to check the compass error but a rough position can be determined by working back from an amplitude.

There used to be a set of tables published for use in lifeboats for emergency navigation from which a rough position could be determined in an emergency based on sunrise sunset and amplitudes.

Another alternative emergency navigation technique is to use the suns upper and lower limb times on the horizon as observed altitudes and work the solution out as per a sight. The problems of refraction and parallax are at the maximum and accuracy is affected.
Though If I had the almanac, compass chronometer, norries tables, calculator, note book, pencil and perhaps a dairy with map of the world to plot it on, I might have remembered my sextant.
There is a book I should by one day by a chap about emergency navigation methods. If I see in a used book shop.