A is an imaginary north–south line on Earth’s surface measuring longitude . The Prime Meridian (0°) runs through Greenwich, UK. Meridians converge at the poles, are used for time zones and navigation, and together with parallels (latitude) form the global grid system.

Meridians: The Invisible Lines That Shape Our World When you look at a globe or a digital map, you’ll see a crisp grid of lines crisscrossing the oceans and continents. While the horizontal lines (latitudes) act like rungs on a ladder, the vertical ones—the —are what allow us to pin down our exact location on Earth and, perhaps more importantly, tell the time.

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The word "meridian" comes from the Latin meridies , meaning "midday." This is why we use ( ante meridiem – before midday) and P.M. ( post meridiem – after midday).

The term comes from the Latin meridies , meaning "midday." Historically, this is because the Sun crosses a specific meridian at the same moment for all locations along that line.

Directly opposite the Prime Meridian, at , sits the International Date Line (IDL) . While the Prime Meridian marks the start of the day’s hours, the IDL marks the start of a new calendar day. When you cross this line heading west, you "gain" a day; heading east, you "lose" one. Great Circles