The Hindus term longitude lambana, and make it consist of 180°, after the manner of the Greeks. They reckon its beginning (as 0° of longitude) from Yamakoṭi in the farthest east, apparently because following the move­ment of night and day, the nearest point to its origin is selected. The Greeks reckon from the Islands of the Blest. There are six* islands of the western ocean formerly inhabited, but now submerged beneath the sea. From their delightful climate, their choice production of fruits and flowers and the luxuriance of their vegetation, they were accounted a paradise. Men call them the Eternal Islands (<Arabic>) or the Fortunate (<Arabic>). Some assert that the Fortunate Isles are 24 in number between the Eternal Islands and the sea-shore. Of the Greeks, some take the reckoning of lon­gitude from the shore of the western (Atlantic) ocean which they call <Greek>,* which is 10° east of the Eternal Islands. The distance of the shore from the islands in 222@2/9 farsakh according to the system of the ancients, or 189@8/9 farsakh according to the moderns, the latter being guided to this conclusion by observation of the motion of the Zodiacal signs in succession and the proximity of the place. In the longitudinal reckoning of places both are agreed. The longitude is an arc of the equatorial between its point of upper intersection with the meridian measured from the begin­ing of the habitable earth (the first meridian), and its point of upper intersection with the meridian of the given place, and the interval is the distance between the place and the first meridian at its nearest side.*

To find the longitude; at the first meridian or a place whose longitude is known, observe the exact time of the occultation of light in a lunar eclipse, its duration and initial or total reappearance, and let a similar observation be made at the place whose unknown longitude is required. If the time be the same on both, their longitude will be the same. If the time be later at the place required, the city is more to the eastward.* The difference of the times of observation is taken, and an excess in the number of degrees over the place whose longitude is known, is allotted on the calculation of six degrees for every ghaṛí and fifteen degrees for every hour, reckoning 4 minutes to the degree.* If the time be earlier, the city is more westerly and the calculation is the reverse of that for the east. According to the system of the Hindu astronomers who begin their reckoning of longitude from the east, in the first instance, the number of degrees will diminish, and in the second case, increase.