Meridian altitude is a method of celestial navigation to determine the latitude of an observer. It notes the altitude angle of an astronomical object above the horizon at culmination.
Meridian altitude is the simplest calculation of celestial navigation. An observer determines their latitude by measuring the altitude of an astronomical object at the time of its meridian transit. A meridian is the imaginary plane running northâÂÂsouth 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 crosses the meridian.
For example, imagine that the equinox Sun is overhead (at the zenith) at a point on the Equator (latitude 0ð), and Observer A is standing at this point â 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 90ð. By subtracting this figure from 90ð, he would find that the zenith distance of the Sun is 0ð, which is the same as his latitude. If Observer B is standing at one of the geographical poles (latitude 90ðN or 90ðS), he would see the Sun on the horizon at an altitude of 0ð. By subtracting this from 90ð, he would find that the zenith distance is 90ð, which is his latitude. Observer C at the same time is at latitude 20ðN on the same meridian, i.e. on the same longitude as Observer A. His measured altitude would be 70ð, and subtracting this from 90ð gives a 20ð 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.
The estimated time of meridian altitude of the heavenly object is extracted from the nautical almanac. A few minutes before this time the observer starts observing the altitude of the object with a sextant. The altitude of the object will be increasing and the observer will continually adjust the sextant to keep the reflected image of the object on the horizon. As the object passes the meridian a maximum altitude will be observed. The time in UTC of this is observed.
The altitude obtained is corrected for dip (the error caused by the observers height above the sea) and refraction to obtain the true altitude of the object above the horizon. This is then subtracted from 90ð to obtain the angular distance from the position directly above to obtain the zenith distance.
A further correction must then be taken into account to counter the "wobble" of the earth's spin and rotation relative to the sun and planets. This is given in the declination for the body on a particular day in the year (also taken from the nautical almanac). If the declination of the body is in the opposite hemisphere (ie if you are in the northern hemisphere and the declination is in the southern hemisphere) then the declination must be subtracted from your true zenith distance, otherwise the declination is added.