What are agonic and isogonic lines?

Think about for a second, after a protracted expedition, you might be on the geographic north pole. You take a look at your compass, however, maybe unsurprisingly, it’s removed from pointing at your toes.

The needle in a compass is attracted in direction of the magnetic north pole, which occurs to be on the transfer. Relying upon your standpoint, this makes for one thing remarkably attention-grabbing or considerably of an inconvenience, and anybody who has navigated with a paper chart is aware of checking the compass rose for the right amount of variation, plus or minus the correction for the years for the reason that chart was revealed, to maneuver between true and compass programs.

You could not have wrestled with the idea of variation a lot since sitting your idea exams, and in the mean time, you may fairly effectively ignore it (within the UK at the least), for causes I’ll come to, however as time goes on, getting variation proper is more likely to develop into increasingly more vital once more when navigating with a compass.

For sailors, understanding the Earth’s magnetic subject is vital for protected and correct navigation. Central to this understanding are what are referred to as agonic and isogonic traces. These play a vital function in figuring out magnetic declination – the angle between true north and magnetic north at a given location.

Whereas the geographic north and south poles are mounted, the magnetic poles are extremely cellular. Picture: Colin Harris / era-images / Alamy Inventory Picture

The Compass

In your yacht, the compass will in idea level to magnetic north all by itself. Nonetheless, there may be one other issue at play right here known as deviation. That is attributable to onboard gadgets that may alter the magnetic subject regionally in your boat – an inventory that features the engine, navigation gear, gasoline bottles, close by wiring and, in fact, a cell phone operating a navigation app positioned subsequent to the compass.

All this stuff will distort the studying on the compass to various extents. Whereas some exert a magnetic affect of their very own, others merely disrupt the Earth’s magnetic subject – an impact that may differ relying on the yacht’s heading.

Utilizing a hand-bearing compass might help spotlight course compass errors. Picture: Theo Stocker

We’ll put apart any concerns of the vessel’s deviation for the second, as that is completely different on each vessel and have to be compensated for accordingly by a compass adjuster. As an alternative, let’s discuss just a little extra about the place the compass factors to and the way that is completely different in varied places around the globe.

True North vs Magnetic North

Traces of longitude are drawn on the charts from the (geographical) north pole to the south pole. True North is the place these traces of longitude – referred to as meridians – all converge collectively within the north, forming the North Pole, which is within the Arctic Ocean.

Equally, the identical is true for the South Pole – the traces of longitude all converge at a single level, forming the South Pole, which is on the Antarctic landmass deep beneath the ice. It’s across the axis connecting these two factors that the Earth rotates and subsequently how angles of tilt are measured, seasons and daytime are recognized.

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If you happen to want to journey from the underside of your chart to the highest, you journey true north, following the traces of longitude. Your compass, nonetheless, factors to magnetic north. If you happen to used it to steer 000 levels regularly, you’d ultimately discover the Earth’s magnetic North Pole, however not essentially the highest of your chart.

At present, magnetic north is at 86º north and 142º east, roughly 280 miles south of the geographical North Pole within the depths of the Arctic Ocean north of Russia, having moved over 400 miles within the final 25 years.

The American Nationwide Oceanic and Atmospheric Administration (NOAA) declination map tracks the motion of the magnetic north pole as much as 2025 (yellow)

If you happen to contemplate that after 60 miles of crusing, a 1° course error will put you one mile away out of your meant vacation spot, you’ll see this will begin to add up – particularly when crusing lengthy distances, reminiscent of a transatlantic.

The British Virgin Islands and Antigua each have lovely seashores, clear waters and wonderful anchorages, however however it’s a sensible concept to know the place you might be. On that crossing, additionally, you will be shifting by areas of larger and lesser variation, growing the potential for error. Subsequently it’s a good suggestion to know what variation you’re coping with.

Moreover, the positions of magnetic north and south transfer, maintaining us on our toes. Over the past 150 years, the magnetic North Pole has moved roughly 620 miles, or round 1,000km. In response to scientists, the magnetic pole migrates round 10km per 12 months, though this elevated to round 40km a 12 months a number of years in the past. The poles can even, in idea, flip from pole to pole – however that is solely thought to happen each 300,000 years, in keeping with NASA.

Currents within the Earth’s molten iron core lead to modifications to its magnetic subject. Picture: www.ncei.noaa.gov/maps/historical_declination

The Earth’s Core

The rationale why the magnetic pole modifications and is continually on the transfer is as a result of composition of our planet. Earth’s core consists of molten iron, the outer core of which strikes and is sort of a big magnet.

Though this magnetic subject is comparatively weak, it extends out into house, influencing issues like photo voltaic radiation and the Aurora Borealis, in addition to the needle or card of your compass.

At any given level on Earth’s floor, there are three predominant magnetic parts: declination, inclination, and depth.

This angle varies relying on the place the observer is situated on the Earth’s floor. Inclination is the angle that the geomagnetic subject is tilted with in relation to the floor of the Earth. This magnetic inclination varies from 90° (perpendicular to the floor) on the magnetic poles to 0° (parallel to the floor) on the magnetic equator. Lastly, the depth is the power of the magnetism at any given level.

The compass rose reveals variation on the time the chart was revealed and the way a lot and by which path it’ll change yearly thereafter

Declination

Magnetic declination is given by the distinction and path between the magnetic pole and true north. Relying upon the place you might be on this planet, the path and quantity of declination differs primarily based on how these two poles align relative to one another. When these two poles align, declination is zero and it’s right here that there’s nothing to be performed to transform magnetic to true since you are located on what is called an agonic line.

The primary chart with declination traces on it’s believed to be from 1701, made by Edmond Halley, a physicist, astronomer and mathematician, Fellow of the Royal Society, of comet fame which he noticed in 1682.

Agonic and isogonic traces
of magnetic variation are proven right here throughout the globe. Picture: www.ncei.noaa.gov/maps/historical_declination

Agonic traces

The phrase agonic is from the Greek for ‘no angle’. An agonic line is outlined as ‘an imaginary line across the earth passing by each the north pole and the north magnetic pole, at any level on which a compass needle factors to true north.’ It’s, nonetheless, very removed from a straight line in the way in which a Nice Circle is, and snakes its manner across the globe.

In case you are west of this agonic line, your compass will level east of true north, giving a optimistic declination. Equally, in case you are east of the agonic line, the compass will level west of true north, giving a destructive declination. For instance, within the English Channel the declination is sort of zero, as of 2024, whereas within the mid-Atlantic it’s over 16° W. This will increase to as much as 24° W within the Southern Atlantic Ocean.

Isogonic traces make calculating variation a lot much less fiddly.

Isogonic traces

All areas with fixed magnetic declination are linked with isogonic traces. Very similar to isobars on a climate chart, these are magnetic ‘contour traces’ becoming a member of areas of the identical magnetic declination. Isogonic traces are illustrated by the magenta traces drawn on small-scale charts in levels and minutes both east or west. On larger-scale charts, you can see compass roses across the chart with extra data – levels and minutes, together with the annual improve of lower on this determine.

These isogonic traces kind curved traces throughout the globe, reflecting the irregularity of the Earth’s magnetic subject. The place isogonic traces converge, declination is zero, and true north aligns with magnetic north. As we transfer away from an agonic line, the declination turns into both east or west.

There’s at the moment an agonic line (inexperienced within the illustration) operating by the UK, crossing the English Channel and coming into the coast simply east of Bridport in Dorset and exiting near the Royal Northumberland Yacht Membership in Blyth.

Anyplace alongside this line, you don’t have any calculation to do to transform Magnetic to True as they’re the identical. Travelling east, you’ll discover 1º E of declination operating from Beachy Head in Sussex to Brancaster Staithe in Norfolk, whereas the 1º W isogonic line skirts Land’s Finish, clips Pembrokeshire, Anglesey and the Isle of Man, slicing Scotland in half and taking pictures off by the Orkneys.

Make a 1° course error and you could possibly be 45 miles out after an Atlantic crossing

Compass course corrections

In sensible phrases, information of agonic and isogonic traces provides an consciousness of how we have to apply calculations to transform magnetic to true for chartwork and in lots of respects, an concept of the place we’re on this planet. That is vital as it’s how our navigation is affected.

Figuring out the magnetic declination at any given location permits us to regulate the compass readings to search out true north precisely. That is important for plotting programs on the chart and figuring out bearings – for instance, acquiring a place utilizing a three-point repair utilizing a hand-held compass after which figuring out what to do to transform magnetic to true.

Having an image in your thoughts’s eye of the place the agonic and isogonic traces at the moment are firstly of every 12 months could possibly be a straightforward option to make fast interpolations of the correct quantity of variation to use, with none fiddly psychological arithmetic.

Hazards and route planning

Incorrectly accounting for magnetic declination would result in navigation errors, ensuing within the incorrect course steered and doubtlessly placing you able you do not need to be in. Understanding the variation in declination alongside the route, we will plan protected passages and keep away from hazards.

Utilizing magnetic somewhat than true north programs in your plotter can prevent psychological arithmetic

Chartplotters

Whereas many fashionable navigation programs, reminiscent of GPS, radar and chartplotters, depend on magnetic bearings for correct positioning, it’s not as straightforward to extract magnetic variation data from an digital chart as it’s a paper one. The settings of some plotters can, nonetheless, be modified to indicate magnetic or true north. If you happen to use magnetic north, you may go straight from chart to compass, accounting just for deviation.

Critical errors

As magnetic declination varies from one location to a different, as an illustration when crossing the Atlantic, you will need to pay attention to these modifications from one location to a different. Within the Canaries, the declination is roughly 3° W, and in Antigua it’s roughly 15° W.

Subsequently, after a few weeks of crusing within the sunshine, we step by step want to vary the calculations to stay heading in the right direction. The odd few levels right here and there can’t be precisely helmed, however a distinction of 15° most likely can.

In conclusion to this, understanding agonic and isogonic traces is vital as they’re basic ideas in magnetic navigation and important for us to navigate precisely and safely the world over’s oceans.

By understanding the background and idea behind these traces, we will apply this data to make sure exact navigation, environment friendly voyages, and the protection of our vessels and crew. With this understanding, we will confidently navigate the great magnetic maze of our planet’s oceans, utilizing the Earth’s magnetic subject to chart a course with precision and talent.

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