A planet’s meridian adopted as the zero of longitude.
- That's the instant the sun is aligned with the degree line of longitude, or the prime meridian - also called the Greenwich Meridian.
- Zero degrees longitude, the prime meridian could be anywhere - and has been, from the designated home ports of Rome to Paris to Jerusalem to Philadelphia to St. Petersburg.
- On the earliest charts displaying a longitude scale, the prime meridian passed through the Canaries.
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