. 202 
HINTS TO TRAVELLERS. 
The method of Moon Culminating Stars, which is entirely independent 
either of local or Greenwich time , includes all that is necessary to find 
the difference of longitude between any two meridians where observa¬ 
tions have been taken, but as the elements in the ‘Nautical Almanac’ 
have been most accurately computed, it is better to take Greenwich as 
the other meridian. 
The principle upon which the longitude is found in this method is 
similar to that which is used in a common lunar observation, and depends 
on the observed motion of the moon; but in the present problem, this 
motion is ascertained by observing the time when the moon’s bright limb 
passes the meridian, instead of measuring the angular distance of the 
moon from the sun, star, or planet. The variation of the moon’s right 
ascension, corresponding to a change of 15° in the longitude, is given 
very accurately by the ‘ Nautical Almanac ’ for every transit of the 
moon’s limb at Greenwich. This variation is about 2m. in time for Ih. 
of longitude, and when the difference of the times of transit under 
different meridians has been found by observation, it is easy to obtain the 
corresponding longitude. 
To find the Longitude by Eclipses of Jupiter's Satellites.* 
In the ‘Nautical Almanac ’ will be found the configuration of Jupiter’s 
satellites for every day in the year, except when Jupiter is so close to the 
sun that his satellites are invisible; these diagrams are given for north 
latitude, and must be reversed for south latitude. When Jupiter comes 
to the meridian before midnight, the whole eclipse (both immersion and 
emersion) takes place on the east side of the planet; when after midnight, 
on the west side. As an inverting eye-piece must be used, this will 
appear to be reversed. The error of the watch on mean time at place 
should be found from observations of the sun’s, or a fixed star’s altitude; 
but if Jupiter is more than 3 hours from the meridian at the time of 
* “ This method, though easy and convenient, is not very accurate; the eclipse 
is not instantaneous, and the clearness of the air, and the power employed, affect 
considerably the time of the phenomenon. Observers have been found to differ 
40 secs, or 50 secs, in the same eclipse.”- — Baper. 
