Variation

The angular difference between true north and magnetic north, also known as magnetic declination, which affects compass readings.

Variation is a crucial concept in marine navigation, as it accounts for the difference between true north (the direction along the Earth’s surface towards the geographic North Pole) and magnetic north (the direction a magnetic compass points, towards the Earth’s magnetic pole). This difference arises because the Earth’s magnetic field is not perfectly aligned with its axis of rotation. The amount of variation differs depending on your geographic location and can change over time due to shifts in the Earth’s magnetic field.

Navigators must adjust for variation when plotting courses or using a compass to ensure accurate navigation. Charts typically indicate the local variation for a specific area, and this value must be added or subtracted from compass readings to convert between magnetic and true bearings. For example, if the variation is 5° west, and your compass shows a bearing of 100°, the true bearing would be 105°. Conversely, if the variation is 5° east, the true bearing would be 95°.

Failure to account for variation can result in significant navigational errors, especially over long distances. Mariners must also be aware that variation can change slightly over time, so it’s important to use up-to-date charts and check for any changes in local variation when planning a voyage.