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Nautical Know How
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      All our courses are engaging, interactive and fun! Whether formal training or simply looking to brush up, we take online learning to the next level.

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      Understanding the language of the marine world is like trying to learn a pirate’s secret code while juggling fish. Let us explain why a sheet isn’t for your bed, why “heave ho” doesn’t mean throw up (most of the time), and why “dead reckoning” is surprisingly less ominous than it sounds.
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Published:3rd February 20253rd February 2025 Last Updated:3rd February 2025
Did you know? | General Nautical Terminology | Navigation

International Date Line (IDL)

An imaginary longitudinal line located at approximately 180 degrees longitude serving as the demarcation where the calendar date changes by one day when crossed.

The International Date Line (IDL) is a conceptual boundary on the Earth’s surface, primarily situated along the 180th meridian in the mid-Pacific Ocean. It is not a straight line but zigzags to accommodate political and territorial boundaries, ensuring that countries and territories remain within the same calendar day. The IDL plays a crucial role in the global timekeeping system, as it marks the point where the date changes by one day. Travelling eastward across the line results in moving back one calendar day, while travelling westward adds one day to the calendar.

The line is not defined by international law but is recognised by convention and is integral to the Coordinated Universal Time (UTC) system. It ensures consistency in global time zones and helps maintain a standardised framework for navigation, communication, and international agreements. For mariners, the IDL is particularly significant when planning voyages across the Pacific Ocean, as it affects logbook entries, scheduling, and coordination with ports of call. By understanding the IDL, sailors can accurately adjust their ship’s clocks and calendars, ensuring seamless integration with global timekeeping practices.

Related Content:
  • Time Travel is Real, and It’s in the Bering Strait

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The IDL runs between Big Diomede (Russia) and Little Diomede (USA) in the Bering Strait, creating the ultimate time travel paradox—they’re only 2.4 miles apart, yet 21 hours apart in time!

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