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Meridian

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Meridian

Definition

The meridian in astronomy is an imaginary great circle on the celestial sphere that passes through the celestial poles and the zenith of a particular location on Earth. It effectively divides the sky into eastern and western halves.

Key Concepts

  • Local Meridian: This is the meridian specific to the observer’s location. It connects the north and south points on the horizon with the zenith directly overhead.
  • Celestial Meridian: This meridian extends beyond the local horizon, intersecting with the celestial poles.
  • Zenith: The point directly above the observer, which the meridian passes through.
  • Celestial Equator: The imaginary projection of Earth’s equator onto the celestial sphere, which crosses the meridian.
  • Transit: A celestial object is said to be in transit when it crosses the local meridian.

Importance in Astronomy

The meridian is a key reference point used for:

  • Timekeeping: The moment when the Sun crosses the local meridian is known as solar noon, the highest point the Sun reaches in the sky for that day.
  • Telescope Alignment: Many telescopes are designed to track celestial objects as they cross the meridian, since objects appear at their highest and clearest point in the sky at that moment.
  • Celestial Navigation: Navigators use the meridian to determine their latitude by measuring the altitude of stars as they cross the meridian.

Types of Meridian

  • Prime Meridian: In geography, this is the meridian at 0° longitude, but in astronomy, it refers to the meridian of an observer’s location.
  • Antimeridian: The great circle opposite the meridian, crossing through the nadir instead of the zenith.

Meridian Passage

When a celestial object (like the Sun or a star) crosses the meridian, it’s called meridian passage or meridional transit. At this point, the object is at its highest altitude in the sky for that day or night.

Additional Notes

  • The Upper Meridian is the part of the meridian above the horizon (passing through the zenith).
  • The Lower Meridian lies below the horizon, passing through the nadir.

References