- Lines of Latitude
- Lines of Longitude
- Facts on Earth
- Zenithal Projections
- Simple Conic Projection
- Cylindrical Equal-area Projection
The equator is the circular line created by the intersection of the spherical
earth and a flat plane that is equidistant between the two ends of the
polar axis and perpendicular to the axis. From this definition, two conditions
can be drawn.
Firstly, the center of the circular plane created by the equator is also
the center of the earth.
Secondly, every point on the equator is equidistant from the two poles.
The equator serves many purposes. One of such purposes is to divide the
earth into two equal hemispheres - northern and southern. In addition,
it serves as a reference point and division for all other lines that are
parallel to the equator. Such lines are known as parallels or latitudes.
As such, the equator has been numbered 0° with the latitudes divided
into those that are north and south the equator.
Most importantly, the equator is the largest circle of all the lines
Such characteristics make the equator a 'Great Circle.'
Latitudes or parallels are imaginary lines created by the intersection
of a plane with the earth. Such planes are parallel to the equator and
are at a constant angular distance from the equator.Thus, latitudes are
also perpendicular to the polar axis. Conventionally, the numbering starts
from the equator at 0° and increases to both poles at 90°.When
drawn at an interval of 1°, there will be 89 circles of latitudes
on either side of the equator.
Some characteristics of latitudes:
- The arc between any two parallels a and b will subtend an angle of
(a-b) ° at the centre of the globe.
- There is a complementary arc between the parallel and the pole subtended
by an angle of (90-n) ° at the centre. This is called the angle
of the co-latitude.
The Prime Meridian is the semi-circular line created by the intersection
of the spherical earth and a flat plane. This flat plane lies along the
polar axis and goes through the observatory at Greenwich near London,
England. In other words, it is a special meridian that bisects the sphere
into east and west hemisphere and serves as a reference point for all
other meridians. As a result, it has been numbered 0° with the longitude
divided into those that are east and west of the Prime Meridian.
Longitudes or meridians are imaginary lines created by the intersection
of a plane with the earth. Unlike latitudes, the edge of such planes touches
the polar axis at all points to create a semi-circles (lunes) through
the poles. As such, longitudes possess many of the same characteristics
as the polar axis. For one, all longitudes would be perpendicular to the
equator. Secondly, the centre of the circle created by two opposite meridians
would intersect the centre of the earth. Thus, having equal radius to
those of the earth and are all equal in size. This also means that they
are equal to the equator in size and thus, are also defined to be 'Great
Circles.'Conventionally, the numbering starts from the Prime Meridian
at 0° and increases east and west of the Prime Meridian to 180°.Therefore,
making a complete 360° rotation with 360 equal lunes around the polar
axis, which will cover the entire surface of the earth.
Some characteristics of longitudes:
- The arc of the Equator between any two meridians subtends an angle
at the centre of the earth equal to the difference between their respective
degrees of longitude. Ex. if the meridians are a and b, the angle subtended
would be b-a.
- A similar angle would be made by tangents to these meridians at the
- Opposite halves of a meridian are of supplementary degrees of longitude.
From the characteristics of parallels and meridians, there comes about
several relationships between the two types of lines. Such relationships
are listed as follows:
- Any meridian intersects any parallel at a right angle.
- With 90 latitudes on either side of the equator, the lines of latitudes
will divide the meridian into 180 equal arcs all subtended by 1°
at the centre of the sphere and with a length 360th of the circumference.
In addition, these arcs of the meridians are equal to the arcs of the
equator created by intersections with the meridian.
- The parallels will be divided into 360 equal arcs by the 180 different
meridians on either side of the prime meridian. The length of such arcs
decrease from the equator twoards the pole at which point they vanish.
In reality, the earth is not a perfect sphereical object due to the constant
spinning of the earth on its axis. This motion creates a slight bluge
on the surface midway between the two equator. Thus, flattening the surface
of the polar regions and distorting the meridians to departure from perfect
circles to an extent of 1/297. In addition, the equatorial diameter becomes
26 miles longer than that of the meridian's diameter. The true characteristics
possessed by the earth is listed as follows:
Equatorial radius: 6, 378.2064 km
Equatorial diameter: 12, 756.4128 km
Polar semi-axis: 6, 356.5838 km
Polar axis: 12, 713.1676 km
Equatorial circumference: 40, 057 km
Such characteristics has been defined as geoid, which means earth-shaped
Mainwaring, James. An Introduction to the Study of Map Projection.
London: Macmillan and Co. Ltd, 1942.
Masood, M. Salar. Map Projections. Mysore: Mysore Printing and
Publishing House, 1961.
Steers, J.A. An Introduction to the Study of Map Projections. London:
University of London Press Ltd, 1962.