NavList:

Bob,

You asked about median angle, t and LHA and woundered if it had a historic reason.  To expand on what Gary L. replied the t was for time.

It is hard for us in the 21 st C. to get into our heads how different the 18 th and 19 th C navigators thought. The day did not start at midnight but at Noon !  The almanacs of the day had the data based on noon.  Also the navigators did not think in degrees but in time. They would do a sun shot in the morning ( anti-meridian  AM ) to figure the local TIME. They would then use a cronometer ( watch ) and compare the local time to the Greenwich time.  When they did lunars they did them to see what time at Greenwich the  Lunar position would be and compare the lunar derived time to the local time to get their longitude.  The tables of log sin and Haversign, etc would not give a result in degrees but TIME.  At local noon they would do the famous Noon shot to get Latitude.  Very often they needed the noon shot latitude to figure the AM time shot because accurate Latitude was very important to the time shot.

Afternoon ( post-meridian PM ) they would do a timed shot, go into the almanac, and tables requiset to figure the local TIME.  It is hard to think like they did because we now use sight reduction tables to get azmuth and Hc to compare to our sight using the Sumner-St Hilary method.  We think in terms of degrees - LHA , GHA, Zn, etc. are listed and used as degrees.  The old timers thought GHA in terms of 4 hours, 27 minutes, and 13 seconds.

The sun was always East durning the morning shot and always west for the afternoon shot.  No navigator would think that at 9:30 AM that the sun was 22 hours and 30 minutes west of the ship but we, in this century look at a LHA of 300 degrees and know it is east of us.

The use of LHA in degrees did not come about untill the Sumner-St Hilary and sight reduction tables. The old-timers thought in terms of TIME  ie. t or meridan angle expressed a hours, minutes, and seconds.

I hope this helps.

John H.