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{{inline, date=June 2024 Meridian altitude is a method of
celestial navigation Celestial navigation, also known as astronavigation, is the practice of position fixing using stars and other celestial bodies that enables a navigator to accurately determine their actual current physical position in space or on the surface ...
to determine the latitude of an observer. It notes the
altitude angle The horizontal coordinate system is a celestial coordinate system that uses the observer's local horizon as the fundamental plane to define two angles of a spherical coordinate system: altitude and ''azimuth''. Therefore, the horizontal coordin ...
of an
astronomical object An astronomical object, celestial object, stellar object or heavenly body is a naturally occurring physical entity, association, or structure that exists within the observable universe. In astronomy, the terms ''object'' and ''body'' are of ...
above the
horizon The horizon is the apparent curve that separates the surface of a celestial body from its sky when viewed from the perspective of an observer on or near the surface of the relevant body. This curve divides all viewing directions based on whethe ...
at
culmination In observational astronomy, culmination is the passage of a celestial object (such as the Sun, the Moon, a planet, a star, constellation or a deep-sky object) across the observer's local meridian. These events are also known as meridian tran ...
.


Principle

Meridian altitude is the simplest calculation of
celestial navigation Celestial navigation, also known as astronavigation, is the practice of position fixing using stars and other celestial bodies that enables a navigator to accurately determine their actual current physical position in space or on the surface ...
. An observer determines their
latitude In geography, latitude is a geographic coordinate system, geographic coordinate that specifies the north-south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from −90° at t ...
by measuring the altitude of an
astronomical object An astronomical object, celestial object, stellar object or heavenly body is a naturally occurring physical entity, association, or structure that exists within the observable universe. In astronomy, the terms ''object'' and ''body'' are of ...
at the time of its meridian transit. A meridian is the imaginary plane running north–south and through the
zenith The zenith (, ) is the imaginary point on the celestial sphere directly "above" a particular location. "Above" means in the vertical direction (Vertical and horizontal, plumb line) opposite to the gravity direction at that location (nadir). The z ...
,
nadir The nadir is the direction pointing directly ''below'' a particular location; that is, it is one of two vertical directions at a specified location, orthogonal to a horizontal flat surface. The direction opposite of the nadir is the zenith. Et ...
, and
celestial pole The north and south celestial poles are the two points in the sky where Earth's axis of rotation, indefinitely extended, intersects the celestial sphere. The north and south celestial poles appear permanently directly overhead to observers at ...
s. This is usually done with the equinox
Sun The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...
at solar
noon Noon (also known as noontime or midday) is 12 o'clock in the daytime. It is written as 12 noon, 12:00 m. (for '' meridiem'', literally 12:00 midday), 12 p.m. (for ''post meridiem'', literally "after midday"), 12 pm, or 12:00 (using a 24-hour cl ...
to determine the observer's latitude, but can be done with any celestial object.
Solar noon Noon (also known as noontime or midday) is 12 o'clock in the daytime. It is written as 12 noon, 12:00 m. (for '' meridiem'', literally 12:00 midday), 12 p.m. (for ''post meridiem'', literally "after midday"), 12 pm, or 12:00 (using a 24-hour clo ...
is the time when the Sun crosses the meridian. For example, imagine that the
equinox A solar equinox is a moment in time when the Sun appears directly above the equator, rather than to its north or south. On the day of the equinox, the Sun appears to rise directly east and set directly west. This occurs twice each year, arou ...
Sun is overhead (at the
zenith The zenith (, ) is the imaginary point on the celestial sphere directly "above" a particular location. "Above" means in the vertical direction (Vertical and horizontal, plumb line) opposite to the gravity direction at that location (nadir). The z ...
) at a point on the
Equator The equator is the circle of latitude that divides Earth into the Northern Hemisphere, Northern and Southern Hemisphere, Southern Hemispheres of Earth, hemispheres. It is an imaginary line located at 0 degrees latitude, about in circumferen ...
(latitude 0°), and Observer A is standing at this point – the
subsolar point The subsolar point on a planet or a moon is the point at which its Sun is perceived to be directly overhead (at the zenith); that is, where the Sun's rays strike the planet exactly perpendicular to its surface. The subsolar point occurs at the ...
. If he were to measure the height of the Sun above the
horizon The horizon is the apparent curve that separates the surface of a celestial body from its sky when viewed from the perspective of an observer on or near the surface of the relevant body. This curve divides all viewing directions based on whethe ...
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 pole A geographical pole or geographic pole is either of the two points on Earth where its axis of rotation intersects its surface. The North Pole lies in the Arctic Ocean while the South Pole is in Antarctica. North and South poles are also defined ...
s (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.


Methodology

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 90° 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.


See also

*
Celestial navigation Celestial navigation, also known as astronavigation, is the practice of position fixing using stars and other celestial bodies that enables a navigator to accurately determine their actual current physical position in space or on the surface ...
* Ex-meridian * Haversine formula * Intercept method * Longitude by chronometer


References

*''Nicholls's Concise Guide, Volume 1'', by Charles H. Brown F.R.S.G.S. Extra Master *''Norie's Nautical Tables'', edited by Capt. A.G. Blance *''The Nautical Almanac 2005'', published by Her Majesty's Nautical Almanac Office *''Navigation for School and College'', by A.C Gardner and W.G. Creelman Navigation