The Meridian. Accessed by Jim Johnson on January 31, 2015.The meridian is a special line in the sky that is related to an observer’s location. It is defined by three points: 1) the north point on the horizon, 2) the zenith, and 3) the south point on the horizon. By extending an arm horizontally and pointing north, then swinging the arm upward until it is pointing straight up overhead, and continuing in that direction, which is now a downward motion, until reaching the south point on the horizon, an observer has traced the meridian.

The meridian remains stationary while the celestial sphere appears to rotate past it as the earth rotates about its axis. All objects that we observe on the celestial sphere appear to move perpendicular to the meridian. When an object on the same declination as the observer’s latitude rises from the east and ascends to the meridian, it has reached its zenith. From then on, the object descends toward the western horizon. Objects that are at a greater or lesser declination than the observer’s latitude have also reached their highest point when crossing the meridian, but will cross the meridian either north or south of the zenith.

The knowing what time an object reaches the meridian is important for an observer wanting to get the very best possible view of an object. The view toward the zenith is the clearest possible view, because the observer is looking through the thinnest possible cross section of the earth’s atmosphere. The view toward the horizon, however, is through the thickest possible cross section of the atmosphere, and this is where atmospheric haze will degrade viewing conditions the most. No matter the object’s declination, the time that it crosses the meridian is its closest approach to the zenith, and the very best possible time to observe.

Image Credit:
– The Meridian. Accessed by Jim Johnson on January 31, 2015.

© James R. Johnson, 2015.

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