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Posted by on Tuesday, October 31, 2006 at 1:30pm.

The reason for this post is I think I understand it and am seeking reassurance.

I am just starting to study celestial navigation and am trying to get to grips with some of the fundamentals. I need to use the Equation of Time of the Sun to establish Longitude.

Longitude is time and if it were not for the equation of time noon would occur at noon at the Prime Meridian (Greenwich) throughout the year every day. Longitude is established by judging when noon occurs at your location. This is when the Sun reaches its highest point in the sky from your location and is called Local Apparent Noon LAN. When this occurs you consider it is 1200 hrs at your location irrespective of what your watch says. This time (1200) is compared to GMT (UTC) and your longitude can be calculated from the difference, 4 minutes of time for every 1 degree of longitude. Just work out if you are East or West of the Prime Meridian.

Back to the Equation of Time.

When I first read about the Equation of Time I printed a couple of tables off the internet and read how it should be applied. I initially understood the figures given in the tables if stated as + or F (FAST) should be added to UTC. (Today the Equation of Time is + 16 minutes 18 seconds). My new understanding is that the + or F indicates that the Sun is FAST therefore noon (highest point of the sun from your location LAN) will occur before the clock time. This means if you wish to synchronise UTC and Sun time any figures given as + or F should be subtracted from UTC.

So at Greenwich today noon (LAN) will have occured at 16 minutes and 18 seconds to 12.

Conclusion. If Equation of Time is given as + subtract it from UTC. If given as - add it!

Anyone agree/disagree with my conclusion?


I suggest you grab a Navigation text, a current one (the EOT changed in the last twenty years). Dutton's if you can find one (it is the one I used as a basic reference for years).
The EOT has to be applied to an observation at Greenwich also. I am uncertain of your meaning on that sentence.
If the Sun is running Fast, LAN then is before local noon.
The use of this is really simple in practice: Grab a text and run through the examples. I could not find any exercises on the internet.

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