Exploring the World Map
Earth map positions are defined by latitude and longitude values in the spherical coordinate system. As opposed to the Cartesian coordinate system in which the horizontal (or x) value is specified initially followed by the vertical (or y) value the spherical system specifies the vertical, north-south, value first and the horizontal, east-west, value second. Further, the values specified in the world map system are angular distance as contrasted with the Cartesian system’s linear distance. Angular distance for latitude, the north-south value, measures the angle between the equatorial plane and lines from points below or above the equator to the center of the earth. For longitude, the east-west value, angular distance is measured between lines from an arbitrarily defined zero line called the Prime Meridian to the center of the earth and a second line from the center of the earth to a point on the equatorial circle (or a point projected onto it).
There are three self-evident possibilities for starting and ending points for latitude, the north-south coordinate value. The poles are natural starting and ending points as they are the furthest vertical points on the world map. Halfway between them at the equator is another natural candidate. This is the chosen starting point with latitudinal value zero.
On the other hand, the east-west orientated value, longitude, has no obvious beginning or ending points. Since longitude at its essence measures angles around a circle (the equatorial circle) and circles have infinite numbers of bilateral system about their centers, how is one to choose a starting and ending point? The world was forced to choose an arbitrary point on the circle and did so in 1884 choosing the line running between the poles and through Greenwich, England as zero degrees longitude. This line is called the Prime Meridian or occasionally the Greenwich Meridian.
As mentioned previously, the equator has been mandated as the line of zero degrees latitude. The angle between a line from the equator to the earth’s center and back to any point north or south of the equator is that point’s latitude. Given a latitude value with angle greater than 90 degrees would mean passing the pole (north or south) and proceeding back towards the equator in the opposite hemisphere. This is because the poles have latitudes of 90 degrees. Positions south of the equator are designated as negative latitudes. Positions north of the equator are positive latitudes.
The angular distance around the axis of the Earth is called longitude. This is, in practical terms, the angle about a circle defined by the equator. The angle from any point on the Earth, projected onto the equatorial circle, to the Prime Meridian is that point’s longitude. The center of the circle, the Earth’s axis, is the vertex of the angle of longitude. Lines of longitude are great circle arcs stretching from pole to pole. All points along their paths share the same longitudinal value since these lines project as points onto the equatorial circle. As stated above, zero degrees longitude, our longitudinal origin, has been chosen to be that line of longitude that passes through Greenwich, England. This line of longitude is named the Prime Meridian. It is also referred to as the Greenwich Meridian. Points west of the Prime Meridian are noted as negative, while points east of it are noted as positive. However, these negative and positive qualities are not natural. They are merely agreed to conventions. The system could just as easily use negative values traveling east and positive traveling west.
The meridian diametrically opposite the Prime Meridian (that line of longitude which together with the Prime Meridian would make a circle) is called the Ante Meridian or sometimes the 180th Meridian. This meridian is exactly 180 degrees both east and west of the Prime Meridian due to the fact that we use a system of 360 degrees around a circle. So moving 360 degrees around the globe, without moving north or south will bring us back to our starting point.
Degrees of arc is the unit used in measuring latitude and longitude. There are sub-units of degrees of arc, called minutes of arc, 60 of which comprise one degree of arc. Likewise minutes of arc have sub-units referred to as seconds of arc. This collection of units is referred to as the degrees-minutes-seconds system or the DMS system. Your common GPS (Global Positioning System) instrument will display positions in degrees-minutes-seconds, but will also use a simpler system of decimal degrees. This system foregoes the complication of dividing by sixty and instead represents fractional degrees of arc as decimal degrees.
If there is any doubt as to what quantities are being reported, measured or addressed you should use the “of arc” specifier to avoid confusion. Of course, there will be many situations in which context provides sufficient clarity and “of arc” can be avoided. If quantities of time or temperature are also being addressed then one should take care to specify “of temperature,” “of time,” or “of arc.”
As opposed to the world map coordinate system the Cartesian coordinate system reports horizontal (i.e. X) components initially and vertical (i.e. Y) components secondly. Latitude, the north-south (i.e. vertical) component is reported initially in the earth map system while longitude, the east-west (i.e. horizontal) component is reported secondarily.
Longitudinal lines meet up and intersect at the poles. They are furthest apart from each other at the equator. Therefore, their greatest distance per degree of arc is at the equator. The distance per degree of longitude at the equator is about 69 miles. The distance per degree of longitude at the poles is zero, since there is no separation at all there. Contrast this with latitude, whose linear distance per angular degree of arc is pretty close to 69 miles all over the earth map.
World map locations are accurately specified by latitude and longitude. Longitude is the east-west angular distance from the Prime Meridian, which is an arc running between the poles and through Greenwich, England. Latitude is the angular distance above or below the equator, which is zero degrees latitude. The poles are 90 degrees from the equator and represent the greatest possible latitudes in absolute degrees of arc.
There are three self-evident possibilities for starting and ending points for latitude, the north-south coordinate value. The poles are natural starting and ending points as they are the furthest vertical points on the world map. Halfway between them at the equator is another natural candidate. This is the chosen starting point with latitudinal value zero.
On the other hand, the east-west orientated value, longitude, has no obvious beginning or ending points. Since longitude at its essence measures angles around a circle (the equatorial circle) and circles have infinite numbers of bilateral system about their centers, how is one to choose a starting and ending point? The world was forced to choose an arbitrary point on the circle and did so in 1884 choosing the line running between the poles and through Greenwich, England as zero degrees longitude. This line is called the Prime Meridian or occasionally the Greenwich Meridian.
As mentioned previously, the equator has been mandated as the line of zero degrees latitude. The angle between a line from the equator to the earth’s center and back to any point north or south of the equator is that point’s latitude. Given a latitude value with angle greater than 90 degrees would mean passing the pole (north or south) and proceeding back towards the equator in the opposite hemisphere. This is because the poles have latitudes of 90 degrees. Positions south of the equator are designated as negative latitudes. Positions north of the equator are positive latitudes.
The angular distance around the axis of the Earth is called longitude. This is, in practical terms, the angle about a circle defined by the equator. The angle from any point on the Earth, projected onto the equatorial circle, to the Prime Meridian is that point’s longitude. The center of the circle, the Earth’s axis, is the vertex of the angle of longitude. Lines of longitude are great circle arcs stretching from pole to pole. All points along their paths share the same longitudinal value since these lines project as points onto the equatorial circle. As stated above, zero degrees longitude, our longitudinal origin, has been chosen to be that line of longitude that passes through Greenwich, England. This line of longitude is named the Prime Meridian. It is also referred to as the Greenwich Meridian. Points west of the Prime Meridian are noted as negative, while points east of it are noted as positive. However, these negative and positive qualities are not natural. They are merely agreed to conventions. The system could just as easily use negative values traveling east and positive traveling west.
The meridian diametrically opposite the Prime Meridian (that line of longitude which together with the Prime Meridian would make a circle) is called the Ante Meridian or sometimes the 180th Meridian. This meridian is exactly 180 degrees both east and west of the Prime Meridian due to the fact that we use a system of 360 degrees around a circle. So moving 360 degrees around the globe, without moving north or south will bring us back to our starting point.
Degrees of arc is the unit used in measuring latitude and longitude. There are sub-units of degrees of arc, called minutes of arc, 60 of which comprise one degree of arc. Likewise minutes of arc have sub-units referred to as seconds of arc. This collection of units is referred to as the degrees-minutes-seconds system or the DMS system. Your common GPS (Global Positioning System) instrument will display positions in degrees-minutes-seconds, but will also use a simpler system of decimal degrees. This system foregoes the complication of dividing by sixty and instead represents fractional degrees of arc as decimal degrees.
If there is any doubt as to what quantities are being reported, measured or addressed you should use the “of arc” specifier to avoid confusion. Of course, there will be many situations in which context provides sufficient clarity and “of arc” can be avoided. If quantities of time or temperature are also being addressed then one should take care to specify “of temperature,” “of time,” or “of arc.”
As opposed to the world map coordinate system the Cartesian coordinate system reports horizontal (i.e. X) components initially and vertical (i.e. Y) components secondly. Latitude, the north-south (i.e. vertical) component is reported initially in the earth map system while longitude, the east-west (i.e. horizontal) component is reported secondarily.
Longitudinal lines meet up and intersect at the poles. They are furthest apart from each other at the equator. Therefore, their greatest distance per degree of arc is at the equator. The distance per degree of longitude at the equator is about 69 miles. The distance per degree of longitude at the poles is zero, since there is no separation at all there. Contrast this with latitude, whose linear distance per angular degree of arc is pretty close to 69 miles all over the earth map.
World map locations are accurately specified by latitude and longitude. Longitude is the east-west angular distance from the Prime Meridian, which is an arc running between the poles and through Greenwich, England. Latitude is the angular distance above or below the equator, which is zero degrees latitude. The poles are 90 degrees from the equator and represent the greatest possible latitudes in absolute degrees of arc.