Noon sight, Longitude by local noon

[Yellow Ballad]

Lockdown challenge is learning how to use my sextant! I've sure I've got the theory and process but I'm trying to write a bit of a to do sheet/notes for various sights and to remind myself in a few weeks what's what. There's a lot to take in.

So basics, noon sight for latitude nice and easy, but reading the Davis instructions it talks about finding longitude which again I get, till I come to the equation of time correction, they're student tables in the back of the book give values but where or how do I work these out from the Almanac?

Does anyone care to spoon feed me an explanation of true sun and mean sun? I'm thinking mean sun is the time the sun passes the meridian of the time zone your in?

Yours, frazzled brain.

 

[knuterikt]

I will give it a try
The sun move around the earth at approximately 15° per hour, meaning that this speed is not constant over the year.
Your clock has got a constant speed that matches the average speed of sun movement (in fact earth rotation), so during a year it evens out.
So when you observer the sun due south and read the time on your clock (converted to UTC/GMT) you must correct the longitude you get from your clock with a correction factor from the almanac.

If I observe the sun due south at 13:00 UTC, I should theoretically by at 15° W but since the sun and my clock is out of sync I would have to correct that longitude to get the right value.

 

[Skylark]

The sun does not pass Greenwich Meridian each day at 12h00. The equation of time shows the difference between 12h00 and when it actually passes overhead.

In the Nautical Almanac, Meridian Passage is generally shown in whole minutes but the column next to this shows equation of time in minutes and seconds, if required.

In 2018 (I have the almanac open), in January, MP was 12h04. It increased to 12h14 in the middle of Feb, Back to 12h00 in April, 11h56 in May, 12h07 in July, and 11h44 in November. Some Nautical Almanacs show this graphically on the first few pages.

Presumably, you've determined the precise time (in UT/GMT) of MP at your location. Add or subtract the days equation of time and then use Arc to Time to give Long .

For example, your maximum sextant altitude was measured at 12h32m00s. On the day in question the equation of time for MP was 12h02.00. You are therefore 30 minutes of time away. Arc to Time 30 minutes is 007 deg 30 mins arc, hence your Long position.

Does this help?

 

[Yellow Ballad]

Thanks both, yea I'm there with the sun may not pass over Greenwich at exactly noon, it could be + - and to take this into consideration but if it was say 12.04 that would equate to

Ping, it's just dropped. I was thinking it would equate to 1 minute of arc, it's one degree so two minutes would equate to 30 minutes of arc close to the example in the Davis instructions.

 

[Refueler]

Lovely in theory ... useless in practice.

Eq of Time is mostly used to ensure you don't miss time of Meridional Passage .... quick calculation in the morning to see what time MP occurs so you are out there before .. through ... and after ... having found max altitude to be able to work out your latitude.
Posts above mention whole minutes ... and reason alamanac has listing whole minutes is precisely for the reason I give. To even start to approach any accuracy with this - you must use the minutes and seconds Eq of Time ...
If you are taking altitude and recording time for MP ... then you will be adding an action to Noon sight that in truth is unnecessary .. providing you have done your morning sights.

If you want to determine Noon position by just MP using altitude vs time ............. then your longitude in practical reality will likely NOT be as accurate as that provided by morning sights crossed with Noon latitude. Luckily middle of the ocean will allow you that luxury.

Its one of those theoretical exercises - which most ships navigators will not use ... because it will be morning sights run up to noon and MP to provide Lat. That position will then be run fwd or back to ships noon for daily run data.

No doubt someone will now question how I navigated ships across oceans for over 17 years ...

 

[earlybird]

The sun move around the earth at approximately 15° per hour, meaning that this speed is not constant over the year.

???That's what Ptolemy said.

 

[Yellow Ballad]

Lovely in theory ... useless in practice.

Eq of Time is mostly used to ensure you don't miss time of Meridional Passage .... quick calculation in the morning to see what time MP occurs so you are out there before .. through ... and after ... having found max altitude to be able to work out your latitude.
Posts above mention whole minutes ... and reason alamanac has listing whole minutes is precisely for the reason I give. To even start to approach any accuracy with this - you must use the minutes and seconds Eq of Time ...
If you are taking altitude and recording time for MP ... then you will be adding an action to Noon sight that in truth is unnecessary .. providing you have done your morning sights.

If you want to determine Noon position by just MP using altitude vs time ............. then your longitude in practical reality will likely NOT be as accurate as that provided by morning sights crossed with Noon latitude. Luckily middle of the ocean will allow you that luxury.

Its one of those theoretical exercises - which most ships navigators will not use ... because it will be morning sights run up to noon and MP to provide Lat. That position will then be run fwd or back to ships noon for daily run data.

No doubt someone will now question how I navigated ships across oceans for over 17 years ...

I'm sure it is, I suppose if you manage to grab a noon sight after days of cloud it would do. I just want to understand it and the basics before moving on and noon sights is that. It's obvious a morning sight advanced and afternoon sight is the better way. I'm sure stars and planets when you can take more sights together is even better but no need to run before I can walk!

Slowly slowly in my boat ?

 

[Refueler]

Before I get accused of poo poo'ing ... I'm just putting reality into the arena ...

 

[Yellow Ballad]

I've asked this question on the celestial navigation group on Facebook which has been equally as helpful so I'm batting between two diferent groups.

One thing that was pointed out is that GHA takes the equation of time into account (I should have but didn't realise) so my next question is if you have an almanac to find out the difference from noon at GMT of the meridian at Greenwich you might as well just find the gp point of the sun at LAN by taking a sight 30 minutes before LAN noting the time and noting the time when you get the same sextant reading after noon and working out the mean time. The GP of the sun is your latitude?

The Davis example above is using ships clock GMT to LAN?!

 

[john_morris_uk]

I'm sure it is, I suppose if you manage to grab a noon sight after days of cloud it would do. I just want to understand it and the basics before moving on and noon sights is that. It's obvious a morning sight advanced and afternoon sight is better...

The reason they’re better is your DR position Longitude is likely to be just as accurate as the ‘longitude’ you get from your calculations of the exact time of your noon sun sight.

You’re turn left with the quandary of which to trust more...

There’s a reason why morning and evening stars are popular... You often get a really good fix!

 

[Skylark]

As you’ve no doubt noted, using MP of sun to determine Long is somewhat controversial. I’ll resist the temptation to wander off subject but instead I’ll try to answer your question. It’s then up to you to decide if this is of any practical use.

You’re right to assume that there is symmetry between a rising and falling altitude around MP. However, the likelihood of timing exactly the same sextant reading before and after is small. So to derive the MP becomes a mathematical exercise.

Bowditch, The American Practical Navigator shows a couple of methods.

For a bit of fun (what else is there to do on a long passage), I’ve plotted, say 10 readings before MP and a similar number afterwards.

The objective is to draw the two straight lines and their intersection is as close to MP as you’re likely to find. The Bowditch method does this by simple maths. I’ve also used the equation of a straight line, y=mx+c, to good effect.

 

[Refueler]

The reason they’re better is your DR position Longitude is likely to be just as accurate as the ‘longitude’ you get from your calculations of the exact time of your noon sun sight.

You’re turn left with the quandary of which to trust more...

There’s a reason why morning and evening stars are popular... You often get a really good fix!

The reason Stars are a better position derivative is simply that you are only using a very short run time to move each position line to same point in time ... second you choose good stars that are at good separation in azimuth ... ideally completing 360 deg.

Something rarely mentioned are Moonlit stars .... quite often on a long passage I used to run down a set of moonlit stars as a check ... and they can be surprisingly accurate.

Sun sights and crossing with Noon MP is always at the mercy of time / speed / course made good between morning and noon sights.