Navigation and Compass Variation

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

Which touches on the variation of the compass, called the Northeasting and Northwesting, and also on how to give a guess to know the Longitude.

Compass Variation

Sometimes in sundry places the compass varies, and especially when sailing long voyages running East and West or when far to the North, and this is called the Northeasting or Northwesting of the compass. I would not wish Mariners to meddle with their compass or whet the side of their needle to make it stand due North, but instead to patiently await the altering of the compass, and observe to what degree it alters, as you may do very accurately by the methods discussed above, and then let your compass well alone. For although it may vary even two or three points, you may make adjustment according to the variation as follows: first admitting that the Northwest point stands due North, and my desire is to go due West, I count the points round in the same direction, and sail Southwest. And if your compass allows, instead you may move the markings of the compass around the circumference so that they align with the Meridian, and this is easiest of all. And thus, by observation and adjustment of how I read my compass, I care not what point stands due North, for it is all one.

Compass Variation by the Sun

While it is possible to know the true meridian, or South bearing by use of a compass or needle, in long voyages going far to the west or east the compass varies, so to find the true Meridian you must adjust how you read your compass. First take a bearing on the Sun with your compass at her rising or first appearance, and then take another bearing on the Sun with your compass at her setting or last sighting. Then you shall perfectly know whether the compass has varied and by how much, for the Sun rises and sets evenly either side of the Meridian. For example, suppose that according to my compass the Sun rises due East, and sets West Northwest. From this I see my compass has varied one full point, that is, my compass reads true North as being North and by East, and the Meridian as South and by West.

And furthermore, as it is seldom that the sun rises and sets clear of all clouds and other impediments near the horizon, you may get the true Meridian by at any time before noon, first take a bearing on the Sun, then take its true height with a Cross-staff or Astrolabe, as you choose. Then in like manner observer the Sun in the afternoon, until you find the Sun at just the height that it was in the morning, at which point you take a bearing on it. Again, this works because the Sun reaches its Zenith at the Meridian, and descends evenly either side, until it reaches the Horizon. For example, suppose that in the morning, the Sun, when exactly South East by the compass, is 20 degrees above the Horizon, and then in the afternoon when the Sun drops to 20 degrees above the Horizon again, the compass reports the Sun is South and by West, then I see that the compass has varied by one half point, that is, my compass reads true North as being North and half East, and the Meridian as South and half West.

Another way to know your true Meridian is by the height of the Sun, that is, take a bearing on the Sun at her greatest height above the Horizon, and so you shall know whether the compass has varied, and how much; but this is least accurate of all the methods. And what you do by the Sun, so you may do by any of the Stars you know perfectly, but you cannot do it so well and truly by the Moon, due to the swiftness of the Moon's motion through the Zodiac, and not by the course of the wanderers or planets, they being fickle and variable as is their name.

Compass Variation by the Stars

You may also find the variation of your compass by the North Star, as follows: take a bearing on the North Star with your compass; if the North point stands with the star then it is not varied, and otherwise the difference in the bearing is the variance. And this bearing must be taken when the two Wain stars, known as the points, are right under or over the North Star, but if the points are directly West of the North Star, then the North Star is a third of a point, or 4° to the East of the North Pole, and if the points are directly East of the North Star, then the North Star is a third of a point, or 4° to the West of the North Pole.

Compass Variation and Longitude

Furthermore, some are of the opinion that by the Northeasting or Northwesting of the compass, you may know the Longitude, but I am not of this opinion, for although I admit that the compass varies, that is, you being 90 degrees to the West from the place your compass was made), your North point should stand Northeast, and in like manner you being 90 degrees East, your North point should stand Northwest, then if the variation be even, the compass should vary by 1 point after 22°30', and that comes to 1350 nautical miles near the equator, and no master or pilot of a ship keeps such a poor account of their journey that they shall know what distance they have to any place better than by the varying of the compass, for any vessel more than a thousand miles off course has problems not with the compass but with the master.

Also I do not know whether it is so that a compass varies in proportion to its distance, and refer them to those that have tried the experience thereof. Wherefore I cease from writing much, although Mariners are very desirous to have some way to get the Longitude. But if it is true that the compass varies in proportion, then it would be very useful for those who travel Northwards, as the degrees are so short in those Latitudes.

Compass Variation and Minerals

Some also are of another opinion, that the compass does not vary in any proportion, but according to the nature of some kind of minerals, that is, there is some country or islands that draws the compass by the lodestone or magnetism that they create their compass with. But if this is the case, then no man has visited that place and returned.