Meridian altitude
Encyclopedia
Meridian Altitude is an astronomical navigation method of calculating an observer's latitude
.
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 point of 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.
. 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 90° to obtain the angular distance from the position directly above to obtain the zenith distance.
Latitude
In geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...
.
Principle
This is the simplest calculation of astronomical navigation and is when an observer determines his latitude by measuring the altitude of a heavenly object at the time of its meridianMeridian (astronomy)
This article is about the astronomical concept. For other uses of the word, see Meridian.In the sky, a meridian is an imaginary great circle on the celestial sphere. It passes through the north point on the horizon, through the celestial pole, up to the zenith, through the south point on the...
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
Zenith
The zenith is an imaginary point directly "above" a particular location, on the imaginary celestial sphere. "Above" means in the vertical direction opposite to the apparent gravitational force at that location. The opposite direction, i.e...
) at a point on the equator
Equator
An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and containing the sphere's center of mass....
(latitude 0°) and observer A is standing at this point, the geographical point of the sun. If he were to measure the height of the sun above the horizon
Horizon
The horizon is the apparent line that separates earth from sky, the line that divides all visible directions into two categories: those that intersect the Earth's surface, and those that do not. At many locations, the true horizon is obscured by trees, buildings, mountains, etc., and the resulting...
with a sextant
Sextant
A sextant is an instrument used to measure the angle between any two visible objects. Its primary use is to determine the angle between a celestial object and the horizon which is known as the altitude. Making this measurement is known as sighting the object, shooting the object, or taking a sight...
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
North Pole
The North Pole, also known as the Geographic North Pole or Terrestrial North Pole, is, subject to the caveats explained below, defined as the point in the northern hemisphere where the Earth's axis of rotation meets its surface...
or 90°S
South Pole
The South Pole, also known as the Geographic South Pole or Terrestrial South Pole, is one of the two points where the Earth's axis of rotation intersects its surface. It is the southernmost point on the surface of the Earth and lies on the opposite side of the Earth from the North Pole...
) 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
20th parallel north
The 20th parallel north is a circle of latitude that is 20 degrees north of the Earth's equatorial plane. It crosses Africa, Asia, the Indian Ocean, the Pacific Ocean, North America, the Caribbean and the Atlantic Ocean....
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.
Methodology
The estimated time of meridian altitude of the heavenly object is extracted from the nautical almanacNautical almanac
A nautical almanac is a publication describing the positions of a selection of celestial bodies for the purpose of enabling navigators to use celestial navigation to determine the position of their ship while at sea...
. 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 90° to obtain the angular distance from the position directly above to obtain the zenith distance.
See also
- Celestial navigationCelestial navigationCelestial navigation, also known as astronavigation, is a position fixing technique that has evolved over several thousand years to help sailors cross oceans without having to rely on estimated calculations, or dead reckoning, to know their position...
- NavigationNavigationNavigation is the process of monitoring and controlling the movement of a craft or vehicle from one place to another. It is also the term of art used for the specialized knowledge used by navigators to perform navigation tasks...
- LatitudeLatitudeIn geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...
- LongitudeLongitudeLongitude is a geographic coordinate that specifies the east-west position of a point on the Earth's surface. It is an angular measurement, usually expressed in degrees, minutes and seconds, and denoted by the Greek letter lambda ....
- Haversine formulaHaversine formulaThe haversine formula is an equation important in navigation, giving great-circle distances between two points on a sphere from their longitudes and latitudes...
- Intercept methodIntercept methodThe "Intercept Method", or "Marcq St Hilaire method", as it is also rather inaccurately known, is an astronomical navigation method of calculating an observer's position on earth. It was originally called the azimuth intercept method because the process involves drawing a line which intercepts the...
- Longitude by chronometerLongitude by chronometerLongitude by chronometer, is an astronomical navigation method of calculating the longitude of an observer's position on Earth. The longitude derived by this method must be combined with the latitude of the observer's position to resolve a "fix" or exact position of the observer on the...
- Ex-meridianEx-meridianEx- Meridian is an astronomical navigation method of calculating an observer’s position on earth. The method gives the observer a position line on which the observer is situated. It is usually used when the sun is obscured at noon and as a result a meridian altitude is not possible...