Here's a breakdown of why:

* Zenith: The point directly overhead, 90 degrees above the horizon.

* North: The direction of the celestial north pole, which is roughly aligned with the geographic North Pole.

* South point on the horizon: The point on the horizon directly opposite the north point.

The meridian is a semicircle that extends from the north point on the horizon through the zenith to the south point on the horizon. It's also known as the celestial meridian or the local meridian because it's specific to your location on Earth.

The meridian is important in astronomy because it helps us define:

* Local sidereal time: The time based on the position of a star on the meridian.

* Culmination: The moment when a celestial object reaches its highest point in the sky, which occurs when it crosses the meridian.