# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: LAN sight help !!
From: Russell Sher
Date: 1999 Aug 13, 7:23 AM

```Welcome to celestial Barry - The LAN prediction time is often a bit
confusing, but needn't be. Perhaps the following explanation and example
will help:

Firstly, it is basically what you stated - the 20h 15m 12s would therefore
be the predicted time UTC (formally referred to as GMT, by the way) of LAN.
So in practice you would start to observe the body say 10minutes before the
time.
So if all the world's timekeepers kept only UTC then a glance at your watch
would agree with the above time being LAN for your DR longitude.

In the Nautical Almanac, the time of mer pass is a Local Meridian Time (LMT)
- This means that the time given for mer pass is the exact time that you
would use IF you were an observer exactly on the 0 degree meridian
(Greenwich). An example - say that you were at Longitude 127 degrees 30
minutes West and mer pass is given as 1206.
You would solve as follows:
1)      convert the 127 deg. 30 minutes to time - (Hint: an easy way is to
divide by 15) This gives 8hours 30minutes in time.
2)      As you correctly stated you would ADD (because it is West long) the
8 hours 30 min in time to 1206. This gives you-
20 36 UTC. For LAN at you longitude.
3)      If you want to express this UTC time of LAN in your local time, then
you need to subtract your Zone offset (i.e. the difference in whole hours
between your local time and UTC) - Say in this case that it is 9 hours
behind UTC, then you would take    20 36 - 09 00 = 11 36 ZT. for LAN.

4)      Note that if you were situated exactly on the central meridian of
your time zone e.g. at say 120 degrees W. Then the ZT of LAN would equal the
LMT of mer pass i.e. 1206. In this case. This is because you would take 1206
- 0900 + 0900 which would give 12 06. In such a case, your watch if set to
local time would agree with the time given for mar pass on the daily page.

The confusing bit is that although we all keep the same local time within a
zone, two observers placed at each end of the  same zone (i.e. 15 degrees
apart) would experience LAN one hour apart in time although their watches

Hope that helps
Russell
```
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