NavList:

   

Reply

----- Original Message -----
From: "CarlZog" 
To: 
Sent: Friday, February 13, 2004 4:21 PM
Subject: Re: [searoom] POB and Jupiter's moons' eclipses


> The specific observations made were eclipses, not occultations or
transits.
> One was to observe the "times of immersions (signifying the instants of
> disappearance of a satellite on entering the shadow of Jupiter) and
> emersions (signifying the reappearances of satellite on emerging from
> Jupiter's shadow)." [History of Nautical Astronomy, Charles Cotter]
>
> It was first proposed by John Flamsteed in the late 1600s and the first
> calculated distances appeared in the British Nautical Almanac of 1765. (I
> don't know what year these calculations were discontinued.)
>
> As John Forrester indicates, there were numerous impracticalities
associated
> with this technique. Compounding those he mentioned are the need to
discern
> the semi-diameter of the lunar body in order to accurately gauge the
> beginning of the eclipse and the need to determine the errors caused by
> atmospheric refraction.
>
> Despite these shortcomings, astronomers viewed this as a better method for
> determining longitude than the use of our own lunar eclipses -- based on
> difficulties prediction our moon's orbit.
>
> As in all astronomical methods, you compare the local time of your sight
> with the time in the almanac. Local time was calculated by observation of
> the sun and a little trig -- determing the angle between the sun's azimuth
> and your meridian provides the time before or after local apparent noon.
>
> Again, it is important to point out that all these efforts were being
> considered many years before POB's Aubrey would have sailed. By the time
> Jack went to sea, lunar distances were the preferred method. This involved
> measuring the angular distance between the moon and another celestial
> body -- the sun or a star. Predicted measurements between the moon and the
> common navigational stars appeared in the nautical almanac.
>
>
> Carl Herzog
>
>

   

Reply