Complete Ocean Skipper by Tom Cunliffe

Author:Tom Cunliffe
Language: eng
Format: epub
Publisher: Bloomsbury
Published: 2022-08-15T00:00:00+00:00

Example 2

What is the GMT of local noon in DR longitude 73°E on 21 April?

Mer pass at Greenwich 11h 59m

73E @ 4 minutes per degree early 4h 52m

Local noon 07h 07m GMT

Taking the sight

Once you know the approximate time of local noon, it pays to be on deck 10 minutes early and start shooting the sun. It should still be rising. As it approaches the meridian you’ll be racking it down ever more slowly until, finally, it stands still for a minute or two. The sextant is now reading the noon altitude. Be careful not to start winding the sun back up again as it begins to fall. Wait until the lower limb bites positively into the horizon without altering the sextant and you know you’re there. Noon is past and gone for another day. Note the log; read the sextant, put it away, then work out your latitude.

The theory

Fig 10.16 shows the noon sight setup when viewed from the celestial elevated pole. Fig 10.17 is the same shown from the celestial equator. Note how the celestial horizon makes a right angle with the line dropped from the observer’s zenith, through his geographic position to the centre of the Earth. Zenith Distance (ZD) is the only new concept involved in this calculation and it’s easy. It is the angular distance in degrees between the observer’s zenith and the position of the sun on the celestial sphere. Because the sun is on the meridian, it runs vertically and is therefore a straight-shot angle. Since the line from the observer’s zenith meets the celestial horizon at 90 degrees, the zenith distance must equal 90 degrees minus the sun’s altitude:

ZD = 90° – Altitude

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