Principle This is the simplest calculation of astronomical navigationand is when an observer determines his latitude by measuring the altitude of a heavenly object at the time of its meridian passage. Meridian passage is when the object passes the observer's meridian, i.e. passes through south or north . This is usually done with the sun for a noon latitude but can be done with any heavenly object. Noon is when the sun passes through the meridian. Let us imagine that the sun is overhead (at the zenith) at a point on the equator (latitude 0°) and observer A is standing at this point , the geographical pointof the sun . If he were to measure the height of the sun above the horizon with a sextant  he would find that the altitude of the sun was 90°. By subtracting this figure from 90°  he would find  the zenith distance of the sun which is 0°, which is the same as his latitude.  Observer B is standing at one of the  poles (latitude 90°N or 90°S)  he would see the sun on the horizon at an altitude of 0°. By subtracting this from 90°  he would find that the zenith distance is 90°, which is his latitude. Observer C at the same time is at latitude 20°N on the same meridian, i.e. on the same longitude as observer A. His measured altitude would be 70° and by subtracting this from 90° gives a 20° zenith distance  which in turn is his latitude. In short, the zenith distance of a heavenly object at meridian altitude is the difference in latitude between it and the observer.
Principle Meridian altitude is the simplest calculation of celestial navigation, in which an observer determines his latitude by measuring the altitude of an astronomical object at the time of its meridian contact. A meridian contact is the moment when the object contacts the observer's meridian, i.e. the imaginary line running north–south and through the zenith, nadir, and celestial poles . This is usually done with the equinox Sun at solar noon to determine the observer's latitude, but can be done with any celestial object. Solar noon is the time when the Sun contacts the meridian. Imagine that the equinox Sun is overhead (at the zenith) at a point on the Equator (latitude 0°) , and Observer A is standing at this point – the subsolar point . If he were to measure the height of the Sun above the horizon with a sextant , he would find that the altitude of the Sun was 90°. By subtracting this figure from 90° , he would find that the zenith distance of the Sun is 0°, which is the same as his latitude. If Observer B is standing at one of the geographical poles (latitude 90°N or 90°S) , he would see the Sun on the horizon at an altitude of 0°. By subtracting this from 90° , he would find that the zenith distance is 90°, which is his latitude. Observer C at the same time is at latitude 20°N on the same meridian, i.e. on the same longitude as Observer A. His measured altitude would be 70° , and subtracting this from 90° gives a 20° zenith distance , which in turn is his latitude. In short, the zenith distance of a celestial object at meridian altitude is the difference in latitude between it and the observer.
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