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    Re: Noon Fix--A couple of questions
    From: Frank Reed
    Date: 2011 Sep 11, 00:14 -0700

    Hello Andrew. You wrote:
    "1) Can someone review the procedure for correcting LAN using the equation of time?
    2) How does one deal with daylight savings time when doing a celestial fix?"

    Second question first... Ignore DST and in fact ignore all local clock times and time zones whenever possible. As a modern navigator, you should keep a clock set to Universal Time, or in other words "GMT" as navigators still call it, and be done with it. ALL celestial navigation observations should be recorded with the time in GMT. Incidentally, if you have a smartphone, like an iPhone, you can easily display several clocks for different time zones, but you may discover that there is no easy way to show GMT. This is where the capital of Ghana will become your friend. You can add a separate clock and set its time to "Accra, Ghana". This city is on GMT/UT and since it's near the equator, they do not use Daylight Saving Time. That gives you GMT all year round. Just to remind you, and so you don't have to look it up, GMT is four hours ahead of EDT (which Philadelphia uses for most of the year) and five hours ahead of EST. Again, though, don't worry about it as you go from one time zone to another. Just use GMT for everything.

    Back to your first question... What, did you forget already? :) Actually, that is the main problem with any backup method of navigation --we may not remember it when we actually decide to use it. But here's the scoop: when you determine time by the Sun, you are getting LAT or "Local Apparent Time". This is "sundial time". When the Sun is on the meridian, it is 12:00:00 exactly, in LAT (you remember the graph folding? The fold is 12:00:00 exactly when we use observations around noon to get local time). When you make an observation for LAT, you also look at your chronometer (or any modern wristwatch or smartphone or whatever) and you record the GMT. But those two times don't speak the same language. GMT is a "mean time" --a machine time. So we need to know for that date, is the Sun fast or slow? If the Sun is fast, then that means that the Sun reaches the meridian BEFORE a mean time clock says it should. Now, the trick here is that to compare Local Apparent Time with Greenwich Mean Time, we have a choice. We can either adjust GMT to convert it into an apparent time, or we can adjust LAT and convert it into a mean time. The latter is by far more common, and it is the modern standard. So if you have an observed LAT and the Sun is slow on that date, then you will add some number of minutes and seconds to convert it to LMT (local mean time). If instead the Sun is fast on that date, then you subtract those minutes and seconds from LAT to convert it to LMT. Once you have that LMT, you can compare it directly to GMT. The difference is the longitude in time units. You can finally convert that to degrees at the rate of 15 degrees per hour.

    And finally, just as a reminder, if you have determined Local Apparent Noon by the axis of symmetry of some sights around noon, you have to either correct for your vessel's north/south motion or adjust your course to east or west for the duration of the sights. There is a smaller correction for the Sun's north/south motion which you can ignore on first pass.

    -FER


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