NavList:

Gary,

You make a good point about the usefulness of time sights which provides the mariner with only two opportunities per day for half the year. I suspect that some voyages were done during the spring and summer only when the declination of the Sun is the same name as the Latitude. A possible fall and winter option would be to time equal altitudes of the Sun AM and PM to estimate the moment of LAN. Converting the moment of LAN to Longitude using arc to time tables gives the mariner a simple means of estimating Longitude. In the tropics there is also the unusual option of plotting an arc of position from the Sun's geographic position using the observed zenith distance.

Greg Rudzinski

Duttons, 1934 says:

"Some of the older methods, which still are in use to some extent in the merchant marine will first be presented very briefly. It should be understood that there will be practically no occasion to make use of the theory, nor of the time sight. They are presented merely in order that the naval officer may be acquainted with the terms in use to some extent by merchant officers..."

Of course, flight navigators never used this method since you need sights more often than once or twice a day.

As long as we are discussing time sights I also have a question. You need to take the observation of the sun when it is on the prime vertical. But, during at least half of the year the sun will never be on the prime vertical. Any body who's declination is opposite to the latitude never crosses the prime vertical and that is the situation for everybody for half of the year. In addition, whenever the sun's declination is the same as the latitude but is greater than the latitude then it also will never be on the prime vertical. This is not a problem unless you are sailing in the tropics but for everybody in the tropics there will be days when the sun's declination is greater than the latitude.

How did the "old navigation" navigators get around this?
\
gl

----------------------------------------------------------------
NavList message boards and member settings: www.fer3.com/NavList
Members may optionally receive posts by email.
To cancel email delivery, send a message to NoMail[at]fer3.com
----------------------------------------------------------------