Q.What is the difference of noon position and meridian passage?

The noon position is the ship's position at 1200 Hrs LT.

The position at the Meridian passage is the position of the ship when the sun is on the observer's celestial meridian. That is when the sun is directly (and exactly) above the observer's head.

The reason that Meridian passage will not occur at exactly 1200 Hrs are

• Because of many factors such as inclination of the earth, Mer-pass may occur at few minutes before (for example at 1156 Hrs LMT) or a few minutes later (for example 1204 Hrs LMT) than noon time.
• The SMT is not always LMT so the time of Mer-pass may be well after or well before noon time depending upon what time we are maintaining on the ship. 

The Mer-Pass time for the Sun is given in the Nautical Almanac as LMT time.

This can be converted to the SMT to get the exact ship's time when the sun will be at the Mer-Pass.