LONGITUDE (/ˈlɒndʒᵻtjuːd/ or /ˈlɒndʒᵻtuːd/ , Australian
and British also /ˈlɒŋɡᵻtjuːd/ ), 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
and denoted by the Greek letter lambda (λ). Meridians (lines running
from the
A location's north–south position along a meridian is given by its latitude , which is approximately the angle between the local vertical and the plane of the Equator. If the
CONTENTS * 1
* 2 Noting and calculating longitude * 2.1 Singularity and discontinuity of longitude * 3 Plate movement and longitude
* 4 Length of a degree of longitude
* 5
HISTORY Main article:
The measurement of longitude is important both to cartography and for ocean navigation . Mariners and explorers for most of history struggled to determine longitude. Finding a method of determining longitude took centuries, resulting in the history of longitude recording the effort of some of the greatest scientific minds.
As to longitude, I declare that I found so much difficulty in determining it that I was put to great pains to ascertain the east-west distance I had covered. The final result of my labours was that I found nothing better to do than to watch for and take observations at night of the conjunction of one planet with another, and especially of the conjunction of the moon with the other planets, because the moon is swifter in her course than any other planet. I compared my observations with an almanac. After I had made experiments many nights, one night, the twenty-third of August 1499, there was a conjunction of the moon with Mars, which according to the almanac was to occur at midnight or a half hour before. I found that...at midnight Mars's position was three and a half degrees to the east. John Harrison solved the greatest problem of his day. By comparing the positions of the moon and
In 1612
Unlike latitude, which has the equator as a natural starting
position, there is no natural starting position for longitude.
Therefore, a reference meridian had to be chosen. It was a popular
practice to use a nation's capital as the starting point, but other
locations were also used. While British cartographers had long used
the Greenwich meridian in London, other references were used
elsewhere, including
NOTING AND CALCULATING LONGITUDE
Each degree of longitude is sub-divided into 60 minutes , each of which is divided into 60 seconds . A longitude is thus specified in sexagesimal notation as 23° 27′ 30″ E. For higher precision, the seconds are specified with a decimal fraction . An alternative representation uses degrees and minutes, where parts of a minute are expressed in decimal notation with a fraction, thus: 23° 27.5′ E. Degrees may also be expressed as a decimal fraction: 23.45833° E. For calculations, the angular measure may be converted to radians , so longitude may also be expressed in this manner as a signed fraction of π (pi ), or an unsigned fraction of 2π. For calculations, the West/East suffix is replaced by a negative sign
in the western hemisphere . Confusingly, the convention of negative
for East is also sometimes seen. The preferred convention—that East
is positive—is consistent with a right-handed Cartesian coordinate
system , with the
There is no other physical principle determining longitude directly
but with time.
SINGULARITY AND DISCONTINUITY OF LONGITUDE Note that the longitude is singular at the Poles and calculations that are sufficiently accurate for other positions, may be inaccurate at or near the Poles. Also the discontinuity at the ±180° meridian must be handled with care in calculations. An example is a calculation of east displacement by subtracting two longitudes, which gives the wrong answer if the two positions are on either side of this meridian. To avoid these complexities, consider replacing latitude and longitude with another horizontal position representation in calculation. PLATE MOVEMENT AND LONGITUDE The Earth's tectonic plates move relative to one another in different
directions at speeds on the order of 50 to 100mm per year. So points
on the Earth's surface on different plates are always in motion
relative to one another. For example, the longitudinal difference
between a point on the
If a global reference frame (such as
LENGTH OF A DEGREE OF LONGITUDE The length of a degree of longitude (east-west distance) depends only on the radius of a circle of latitude. For a sphere of radius a that radius at latitude φ is _a_ cos _φ_, and the length of a one-degree (or π/180 radian ) arc along a circle of latitude is l o n g 1 = 180 a cos {displaystyle Delta _{rm {long}}^{1}={frac {pi }{180^{circ }}}acos phi } φ Δ1 lat Δ1 long 0° 110.574 km 111.320 km 15° 110.649 km 107.551 km 30° 110.852 km 96.486 km 45° 111.132 km 78.847 km 60° 111.412 km 55.800 km 75° 111.618 km 28.902 km 90° 111.694 km 0.000 km When the
where e, the eccentricity of the ellipsoid, is related to the major and minor axes (the equatorial and polar radii respectively) by e 2 = a 2 b 2 a 2 {displaystyle e^{2}={frac {a^{2}-b^{2}}{a^{2}}}} An alternative formula is l o n g 1 = 180 a cos where tan = b a tan {displaystyle Delta _{rm {long}}^{1}={frac {pi }{180^{circ }}}acos psi quad {mbox{where }}tan psi ={frac {b}{a}}tan phi } Cos φ decreases from 1 at the equator to 0 at the poles, which
measures how circles of latitude shrink from the equator to a point at
the pole, so the length of a degree of longitude decreases likewise.
This contrasts with the small (1%) increase in the length of a degree
of latitude (north-south distance), equator to pole. The table shows
both for the
A geographical mile is defined to be the length of one minute of arc along the equator (one equatorial minute of longitude), so a degree of longitude along the equator is exactly 60 geographical miles, as there are 60 minutes in a degree. LONGITUDE ON BODIES OTHER THAN EARTH See also:
Planetary co-ordinate systems are defined relative to their mean axis
of rotation and various definitions of longitude depending on the
body. The longitude systems of most of those bodies with observable
rigid surfaces have been defined by references to a surface feature
such as a crater . The north pole is that pole of rotation that lies
on the north side of the invariable plane of the solar system (near
the ecliptic ). The location of the
In the absence of other information, the axis of rotation is assumed to be normal to the mean orbital plane ; Mercury and most of the satellites are in this category. For many of the satellites, it is assumed that the rotation rate is equal to the mean orbital period . In the case of the giant planets , since their surface features are constantly changing and moving at various rates, the rotation of their magnetic fields is used as a reference instead. In the case of the Sun , even this criterion fails (because its magnetosphere is very complex and does not really rotate in a steady fashion), and an agreed-upon value for the rotation of its equator is used instead. For _planetographic longitude_, west longitudes (i.e., longitudes
measured positively to the west) are used when the rotation is
prograde, and east longitudes (i.e., longitudes measured positively to
the east) when the rotation is retrograde. In simpler terms, imagine a
distant, non-orbiting observer viewing a planet as it rotates. Also
suppose that this observer is within the plane of the planet's
equator. A point on the
However, _planetocentric longitude_ is always measured positively to the east, regardless of which way the planet rotates. _East_ is defined as the counter-clockwise direction around the planet, as seen from above its north pole, and the north pole is whichever pole more closely aligns with the Earth's north pole. Longitudes traditionally have been written using "E" or "W" instead of "+" or "−" to indicate this polarity. For example, the following all mean the same thing: * −91° * 91°W * +269° * 269°E. The reference surfaces for some planets (such as
The modern standard for maps of
Tidally-locked bodies have a natural reference longitude passing through the point nearest to their parent body: 0° the center of the primary-facing hemisphere, 90° the center of the leading hemisphere, 180° the center of the anti-primary hemisphere, and 270° the center of the trailing hemisphere. However, libration due to non-circular orbits or axial tilts causes this point to move around any fixed point on the celestial body like an analemma . SEE ALSO *
REFERENCES * ^ http://www.merriam-webster.com/dictionary/longitude
* ^ Oxford English Dictionary
* ^ Vespucci, Amerigo. "Letter from Seville to Lorenzo di Pier
Francesco de' Medici, 1500." Pohl, Frederick J. AMERIGO VESPUCCI:
PILOT MAJOR. New York: Columbia University Press, 1945. 76–90. Page
80.
* ^ _A_ _B_ "
* ^ "λ =
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