LOPPING

Almanac. The moon's distance from one of these "stars being observed on board a ship, and corrected for refraction and parallax, and the apparent time at the place and.moment of observation being determined in the usual manner by the altitude of the sun or a known star, the difference between the apparent time of the observation and the apparent time at Green wich corresponding to the same distance, interpolated from the Nautical Almanac, gives the longitude of the ship. See Lunar Method. In the projection of maps and charts it is nece ary to a ume a point of the equator as the origin of the longitudes. The meridian pa ing through this point is called the first meridian; and as its selection is perfectly arbitrary, it has been placed by different geographers at various parts of the earth: a circumstance which occasions some inconvenience in consulting works of geography. Modern geographers usually a ume the meridian of the capital of their own country as the first meridian. English writers generally adopt the meridian of the Green wich Observatoryj for which the Nautical Almanac is computed; the French that of the Observatory of Paris. {See Meridian.) Ptolemy, and the most celebrated of the ancient geographers, placed the first meridian at the Fortunate Islands (now the Canaries), which they conceived to be the utmost boundary of the habitable earth. The Arabian astronomers also counted the longitude from the Fortunate Islands; and many of the modern geographers have counted from the island of Ferro, one of the most westerly of the Canaries. The reason for fixing on this point was probably that as there was no land known to the west of the first meridian, the longitudes of all places would be reckoned in the same direction, or there would be no west longitude. The discovery of America destroyed the force of this reason. The inconvenience of counting in two directions is not very great; but it might be avoided by reckoning the longitude all round the circle to 360°, which would undoubtedly be an improvement on the present method. Degrees of Longitude. — The figure of the earth being spheroidal, the degrees of longitude diminish as we proceed from the equator towards either pole. For the law of their variation, and their lengths on the different parallel circles, see Degree. Geocentric Longitude is the longitude of a planet as seen from the earth; that is, the point of the ecliptic to which it perpendicularly corresponds as seen from the centre of the earth. Heliocentric Longitude is the longitude of a planet as seen from the sun. See the terms Geocentric and [s. 696]