How To

Begin by calculating the approximate time of the meridian passage

Look in the Almanac Day Pages for the required date. There should be a small table showing, the date, Sun ( Eqn Of Time, Mer Pass ) and the Moon ( Eqn Of Time, Mer Pass )

Note the time for the Meridian Passage on the required date. i.e. 11:57

This value is the time the Sun will cross the Prime Meridian ( GMT ).

To adjust the time of the passage, use your longitude and add 4 minutes of time for each degree of West in your longitude.

i.e. If our position is 4°10' W and the time of the passage from above is 11:57. We would expect to see our meridian passage 16 minutes later. In this case the passage should be at 12:13 GMT.

Remember this is a GMT time, if your watch or ships clock is set to a zone time, you should also make the required correction.

It is a common simple mistake to forget their clocks may be set to Day Light Savings time :) :)

Taking The Sight

You should be on deck and taking sights a few minutes before the time you calculated above. Normally, 10 to 15 before is plenty ( You should remember, the calculated time is only an estimate).

Taking a Meridian Passage is a little different to taking a normal sighting, in that you will be taking several sights without resetting the sextant to zero in between each sighting.

Take a sighting in the normal way and note the normal readings, i.e. time of sighting, log, heading, sextant angle, etc etc.
(Note: When recording the above information, you should write the values which are changing the fastest first. i.e. When writing a time, write the seconds, then the minutes, then the hour).

After a short time take another sighting. Remember you are starting from the previous sextant reading. Not a sextant reading of zero.
This will allow you to see how far the sun has moved between the two readings.
If the sun has started to fall.

DO NOT BRING THE SEXTANT SETTING DOWN. USE THE VALUE FROM THE PREVIOUS READING.

If the sun is still rising, adjust the sextant as normal.
In the ideal world, the first few readings the sun will be moving quickly, over sucessive sightings, you will see it slow down and if you have guessed your timings in between sightings correctly, you should be able to actually see the sun stop rising, be stationary for a short period and start to fall again.
The time between sightings depend on the speed of the sun. Begin with taking a sighting every minute until you have a better feeling for a more appropriate timing.

Reduce the Sextant Angle and Declination in the normal way.

Sextant Angle Reduction

To reduce a reading from taking using a sextant, you need to obtain the following information.

The sextant Angle - Which is the reading from the actual sextant

The DIP or Height of Eye correction - Estimate the height of your eye above the water and use the "Altitude Correction Tables" to find the value for DIP ( note the sign + or - ).

Use the Almanac to look up any corrections relevant to the heavenly body you are using and also any upper / lower limb corrections. These will be discussed in a more relevant part of the document

The index error of your sextant

In this example we will use the following readings: sextant angle(SA): 33°01'.4 DIP: -2.8 Index Error (IE): +0'.8 Other corrections: 0

Create a table and enter the values we know. The values we calculate using simple addition or subtraction will be left blank.

Sextant Angle 33°01'.4
Index Error +0'.8

Perform the above arithmetic to find the "Observed Altitude" in this case it is 33°02'.2

Use the Observed Altitude calculated above and apply the correction you found for DIP. This value is normally negative so this time use subtraction.
Observed Angle 33°02'.2
DIP -2'.8
Perform the above arithmetic to find the "Apparent Altitude" in this case it is 33°59'.4
Note: The minutes change at 59 / 60 not 100. So when you borrow 1 and pay 1 back, the values are from 60.

In most cases, the Apparent Altitude is not quite enough and we need to apply some other corrections. These vary considerably depending on which heavenly body you are using and if you have taken the sextant reading from the upper or lower limb.

A noon sight should not require any further corrections to change an Apparent Alititude in a True Alititude ( Unless for some reason you used the upper limb of the sun ).

Declination

Declination is simply a case of looking values up in the Almanac Day Tables and applying a correction.

Assume our reading is at 14:17:02

Use the day page to find the value of declination on the required day at the required time for the heavenly body you are using. If it is the Sun, you will normally find the declination in the column next to the GHA.

Note: It may be written in short hand with the N14 a few rows above the hour line you are reading . i.e. N 14°55'.5
You should also make a note if the values are increasing or deceasing as the day progresses. In this example we will assume they are increasing.

At the bottom of the Declination column is a value for d.

Note this down, you will need it later. i.e. 0.7

At this stage we have only looked up the Declination for the hour part of the time of the reading. Now use the Almanac – Increments and Corrections Page for the required minutes of our time. In this example, 17 minutes. Look at the columns 'v or d Corrs' Find the number for the little d we noted in the previous step. In this example, we had a d value of 0.7 so lets assume this references to a value of 0.2

Now do the maths.
Declination ( hours ) N 14°55'.5
Declination (minutes) 00°00'.2
It was an increasing value when we looked it up on the day page so we add the 2 values together.

Giving our final Declination as N 14°55'.7

Putting it all together

The equations used are ZD=90 - True Altitude.

Followed by
LATITUDE= ZD + / - DECLINATION

The rules are:

If Latitude GREATER than declination, same name.
LAT = ZD + DEC

If Latitude OPPOSITE name to declination.
LAT = ZD - DEC

If Latitude LESS than declination, same name.
LAT = DEC - ZD

Example:
90......... 89°60'.0
- True Altitude (TA) : 33°59'.4
Give a ZD of 56°00'.6

Use the ZD above with the Declination to find our latitude.

ZD 56°00'.6
DeclinationN 14°55'.7

Our DR Latitude is 70 N.

Using the rules above we should add ZD and DEC.

Our actual Latitude is 70°56'.3 N

Sadly we do not have permission to include extracts from the Alamancs and Sight Reductions.

All of the numbers used above are ficticious and have only been used to aid your understand.

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