In celestial navigation, a meridian is an imaginary line that runs from the North Pole to the South Pole, passing through a particular point on Earth’s surface. Its significance is tied to its relationship with the celestial sphere and helps navigators determine their longitude and position on Earth.
Here’s why the meridian is important in celestial navigation:
- Celestial Sphere Reference: The celestial sphere is an imaginary sphere surrounding Earth, on which celestial bodies (like stars, the Sun, and planets) appear to be located. The celestial meridian is a projection of the Earth’s meridian onto the celestial sphere. It helps define the local coordinate system used in navigation.
- Longitude Determination: The Earth’s prime meridian, located at Greenwich, England, is the reference point for measuring longitude. By comparing the local time of an observation with the time of a known meridian (like Greenwich Mean Time), a navigator can calculate their longitude.
- Altitude and Azimuth Measurements: When using celestial bodies to determine position, navigators often take measurements of the altitude (how high a celestial body is above the horizon) and azimuth (the direction of the celestial body relative to a reference point, typically true north). These measurements are taken relative to the local meridian, allowing the navigator to pinpoint their location.
- Sun’s Meridian Passage: The Sun’s crossing of the local meridian (when it reaches its highest point in the sky) is an important event for determining local noon. This helps navigators accurately measure the time, which can be compared with a known time source (like GMT) to calculate longitude.