Parallel

The circle formed by the intersection of the terrestrial geoid with an imaginary plane perpendicular to the axis of rotation of the Earth is called parallel.

On the parallels, and from the equator or parallel 0, latitude is measured —the arc of circumference expressed in sexagesimal degrees—, which may be north or south, depending on the sense of measurement of the same. Unlike of the meridians, the parallels are not maximum circumferences, except for the equator, they do not contain the center of the Earth.

The angle formed (with the vertex in the center of the Earth) on any meridian plane by a parallel and the equatorial line is called latitude and is the same for all points of the parallel, which is discriminated between North latitude and South latitude depending on the hemisphere.

Both meridians and parallels make up the geographic coordinate system based on latitude and longitude.

The five main parallels

Enlightenment of the Earth in the December solstice.

There are five notable or main parallels that correspond to a specific position of the Earth in its orbit around the Sun and, therefore, receive a particular name:

• Arctic polar circle (latitude 66° 33' N). It is the northernmost parallel in which the polar night and the midnight sun take place in the northern hemisphere. These events occur in the winter solstices (December) and summer (June) respectively.
• Tropic of Cancer (latitude 23° 27' N). It is the most north parallel in which the Sun reaches the cenit. This happens in the June solstice.
• Ecuador, (latitude 0°). In the equator the Sun culminates in the cenit in the spring equinox (March) and autumn (September).
• Tropic of Capricorn (latitude 23° 27' S). It is the most south parallel in which the Sun reaches the cenit. This happens in the December solstice.
• Antarctic polar circle (latitude 66° 33' S). It is the southernmost parallel in which the polar night and the midnight sun take place in the southern hemisphere. These events occur in the winter solstices (June) and summer (December) respectively.

These angles are determined by the obliquity of the ecliptic (approx. 23° 27').

Zones of the Earth

From these main parallels, the Earth is divided into three zones known as geoastronomical zones:

• One intertropical zoneAlso called zonewhich is the zone between the tropics, and that the equator subdivides in north and south. It coincides with the maximum and minimum decline of the Sun, which reaches great heights and culminates in the cenit twice a year. In this area the solar radiation incites almost perpendicularly and therefore is the warmest.
• Two. temperate areasthe zones between the tropics and the polar circles. The Sun never ends in the cenit. Solar radiation incites more obliquely and therefore are less hot than the previous one.
• Two. glacial areas or Polar zonesthe zones between the polar circles and the poles. In glacial areas, solar radiation incites very obliquely, warming very little.

The intertropical zone

The intertropical zone is the space on the Earth's surface between the two tropics, which is divided in half by the equator or the line and each one being 23° and 27' apart, will be its entire latitude of approx. 47° that reduced to Spanish leagues are 822.5 and in French leagues 940; The longitude of this zone is the entire roundness of the Earth or 360° of the equator, equal to 6,300 Spanish or 7,200 French leagues. The surface and solidity of this area will be found by the precepts of geometry.

The ancients called this torrid zone because its inhabitants having the Sun at its zenith and its rays being perpendicular to them, they judged that it would be largely uninhabited due to its excessive heat, but the moderns have found cool countries in it, temperate and healthy where one enjoys almost perpetual spring and autumn, because the nights are almost 12 hours long and fresh winds blowing during the day that pass over many leagues of sea, temper the sun's rays causing frequent rains and for this reason in many parts of this area have two harvests of fruit each year and the trees have flowers and fruit at all times.

Characteristics of the intertropical torrid zone

The regions located on the equatorial line, because they have their zenith in this circle, have a straight sphere and their properties are the following:

1. Two days in the year have the Sun in the cenit at the point of the noon, these are March 23 and September 23, because the Sun in them makes the day revolution in the equator.
2. The most the Sun departs from its cenit is 23o30'. This happens in 22 June and 22 December, because then the day revolution of the Sun is made by the tropics.
3. The seasons of the year called summer, autumn, winter and spring (according to the movement of the Sun) are generally counted in this way: the stage begins when the Sun is closer to the cenit, the winter when the Sun is more distant, the autumn when departing from the cenit is in the middle distance and the spring when it is in the middle distance approaching to the cenit; with this it is understood that the inhabitants of the equator two The first stage starts on 21st March with the Sun in its cenit and lasts until 21st April that the Sun makes the 11th declination 45' (mit of its maximum decline), then follows a fall of two months until 22nd of June that the Sun has the maximum declination, then follows a winter of two months until 23rd of August that the Sun has the average declination, then follows a spring until 23rd of September; The reason for not being equal is that the declination of the Sun as it is found in the signs of Aries, Libra, Virgo and Piscis, is almost the same as the other 8 signs.

Temperate zones

The temperate zones are the zones between each one of the tropics and their corresponding polar circle in the hemisphere. These areas are characterized by:

1. The Sun never reaches the cenit.
2. The climate is not as hot as in the intertropical zone or as cold as in the polar zones.
3. Throughout the year, the Sun crosses the horizon twice. In other words, days and nights never exceed 24 hours (see differences with polar zones). However, in places close to the polar circle, during the solstices there are days or nights close to 24 hours.
4. The varieties of fauna and flora are greater.

Also called subtropical zones, these present a series of high pressure nuclei, in both hemispheres, aligned following approximately 35° latitude. The axes of each belt experience a weak annual meridian displacement.

The Polar Zones

The polar zones are located north of the Arctic Circle and south of the Antarctic Circle. In the case of the south polar zone, the 58th parallel is usually taken as the starting point, to include the entire Antarctic continent.