5. By the Prime Vertical Instrument.— Here we observe simply the moment when a known star passes the prime vertical on the eastern side, and again upon the western side. Half the interval will give the hoer-angle of the star when on the prime vertical. The dis tance of the star from the pole is the comple ment of the star's declination. The observa tions are not so convenient and easy as in the case of the zenith telescope and the number of stars available is less; but the method presents the great advantage of requiring nothing but an ordinary transit instrument, without any special outfit of micrometer and latitude level. It also evades the difficulties caused by refrac tion.
6. By the The ancients had few or no instruments and of course could not use the preceding methods of finding latitude. They were, however, able to make a very re spectable approximation by means of the sim plest of all astronomical instruments, the gno mon. This is merely a vertical shaft or column of known height erected on a perfectly hori zontal plane; and the observation consists in noting the length of the shadow cast at noon at certain times of the year. It is easy to com plete the sun's zenith distances when farthest north and south; and, since the sun travels equal distances north and south of the celestial equator, the mean of the two results will give the angular distance between the equator and the zenith; i.e., the declination of the zenith,
which is the latitude of the place. This method is an independent one, like that by the observation of circumpolar stars, requiring no data except those which the observer determines for himself. Evidently, however, it does not admit of much accuracy, since the penumbra at the end of the shadow makes it impossible to measure its length precisely. The ancients in stead of designating the position of a place by means of its latitude, used its climate instead —the climate being the slope of the plane of the celestial equator, which is the complement of the latitude. Many of the Egyptian obelisks were erected primarily for use as gnomons and were used for that purpose.
Since the moon and the principal planets keep within the Zodiac, the ecliptic is a most convenient circle of reference and was used as such by the ancients. Consequently great care must be taken to avoid confusion of celestial latitude and longitude with right ascension and declination or with terrestrial trial latitude and longitude. The terms right ascension and decli nation are of comparatively recent introduction, celestial latitude and longitude being much older. See LONGITUDE.