Navigation and Longitude

From DQWiki
Jump to: navigation, search

Contents | Glossary | << Previous Chapter | Next Chapter >>

Which treats of the Longitude, etc.

Longitude

Now, there are some who are very inquisitive and wish to have a way to get their Longitude, but that is too tedious. For they must realise that the whole frame of the firmament is carried round from the East to the West every 24 hours, so that there remains neither light nor mark that stands still, except the two Poles of the World, which always stand fast. And there are other opinions as to whether the world or the Heavens do move, but it is all the same for the Navigator, who sees one move and stands on the other. But when travelling directly East or West the lights of the firmament make the same manner of arch according to their latitude or declination, but the travelling alters the Meridian, causing planets to have their aspects of rising and setting at varying hours, altering the time of the changing of the Moon, and also the time of the Eclipses. And by such ways you shall know your Longitude. Now the Latitude comes from the Poles, but the Longitude you must bring from another place, which for us is Tycho City. And why Tycho City has a Longitude of zero, from which everywhere is either East or West, I do not know, but I suppose because it was then the easternmost place known by the ancients, or maybe for other reasons.

A Degree of Longitude

While each degree of Latitude is the same, being 60 nautical miles or 20 sea leagues, the degrees of Longitude start the same under the Equinoctial, but get less and less as they approach the Poles, but whatever distance a degree of Longitude is, travelling West that degree gains you one minute in the day, and travelling East loses that same minute. And therefore 15 degrees of Longitude is an hour, and an entire day is 360 degrees of Longitude, which returns you to the spot where you first started. And in our Latitude of Seagate at 37° every 48 miles is a degree and 720 miles is an hour of time, but under the Equinoctial it is 900 miles to an hour, and at 60°N, it is 450 miles to an hour.

Longitude at the Eclipse

And now to get the Longitude, you may do it at the time of the Eclipse of the Moon, for that Eclipse occurs at one point in time for all places on the earth or the sea such that she is above your Horizon. And consider your Almanac which states what hour and minute the Eclipse shall occur, and also place where the Almanac was prepared, that done, with a precise instrument you shall take the time of the Eclipse, and from that you shall know the alteration of your Longitude from where the Almanac was printed, by the rule of every 4 minutes being a degree, and every hour being 15 degrees of Longitude.

Avoid the Ephemeredes

And furthermore, you might know your Longitude with the Ephemeredes, by the conjunction of the Moon with other fixed Stars, if it was not for one great problem, that being the parallax of the Moon, which the semi-diameter of the Earth causes by the proximity of the Moon to the Earth, so that the Moon is viewed from a different angle as it rises from when it sets; wherefore I would not want any Seafarer to be of the opinion that they might get their Longitude with instruments. Therefore let no Navigator trouble themselves with any such rule, but let them keep a perfect account or reckoning of the way and course of their ship, whether the ship goes to leeward or makes good, considering always what things be with them or against them, such as tides, currents, winds, or such like.