Circle of illumination is an imagined line that divides light from darkness and day from night. Earth’s axis attributes to an imagined line going through the center of the earth from top to bottom. Thus, while circle of illumination divides light from darkness and day from night, axis is a line along which the earth’s rotates.

The circle of illumination partitions the Earth in correctly half. It is a line that is able to be seen from space, and the definite location of the line is dependent on the different seasons. Because the Earth turns on its axis, and the circle of illumination stays in the same normal position, the Earth is actually moving around the circle, but it arrives that the circle is altering positions. The way that the Earth turns in relationship to the circle of illumination is the main conclusive factor behind how many hours it is light or dark out.

Circle Of Illumination
Circle Of Illumination

The circle of illumination also performs a small part on the seasons that are accomplished. Because the Earth is in a disparate place on the circle, there are leaner daylight hours. Winter has the least amount of daylight hours, which means that there is less sunlight to help keep things hot. Places on the Earth that are closer to the north and south poles are more affected by the lower amount of sunlight, making winters colder and more intense.

Circle Of Illumination

Circle of Illumination: The hypothetical line which partitions the parts of the earth with day and night is announced as the circle of illumination. The circle of illumination is at a right angle to the orbital plane. Thus, the circle of illumination is at an angle of 23.5o to the earth’s axis.

Because of the angle between the circle of illumination and the axis; both the hemispheres of the earth do not get the same result of light on most of the days. This causes various seasons at various times of the year.

Motions of Earth

  1. Rotation and Revolution
  2. Summer Solstice
  3. Winter Solstice
  4. Equinox

Rotation: The earth has two categories of motions, particularly rotation and revolution.

  • Rotation is the motion of the earth on its axis.
  • The motion of the earth around the sun in a definite path or orbit is named Revolution.
  • The axis of the earth which is an imagined line makes an angle of 66 1/2o with its orbital plane. The plane created by the orbit is known as the orbital plane.
  • The circle that partitions the day from night on the globe is called the circle of illumination. This circle does not concur with the axis.
  • The earth takes about 24 hours to conclude on rotation around its axis. The period of rotation is noticed as the earthday. This is the daily motion of the earth.

Earth Day: The period of one rotation of the earth is announced as one earthday. One earthday is of 24 hours.

Axis: The hypothetical line around which the earth rotates is named its axis.

Orbit: The hypothetical line on which the earth moves around the sun is called its orbit.

Orbital Plane: The plane passing through the earth (along its orbit) is announced its orbital plane.

The tilt of Axis: Earth’s axis is tilted at an angle of 66.5o on its orbital plane. On the other hand; the earth’s axis is tilted at 23.5o on the line which is vertical to its orbital plane.

Year: The time taken by the earth to achieve one revolution is named one year. One year has 365 and ¼ days (365 days and 6 hours). For the purpose of accessibility, we consider a normal year to have 365 days. The extra six hours are added after every four years to add an additional day to the year. Thus, every fourth year is a leap year which has 366 days.

While the rotation is responsible for day and night on earth, its revolution is responsible for diversities in seasons. The revolution and tilt of the earth’s axis alter the changes in seasons. You have read in your earlier classes that the portion of the earth facing the sunlight encounters day. On the contrary, the portion which is away from the sun experiences night.

Revolution Of Earth And Seasons

The orbit of the earth is elliptical in shape. Due to the elliptical configuration of its orbit, the distance between the sun and the earth carries on changing throughout the year. Sometimes, the earth is too convenient to the sun and at other times it is too far from the sun.

Circle Of Illumination Rotation
Circle Of Illumination Rotation
  • Perihelion: The point at which a planet is nearby to the sun is called perihelion.
  • Aphelion: The point at which a planet is farthest from the sun is called aphelion.


  • When the length of day and night is equal in all parts of the earth, such a day is called equinox. This happens on 21st March and 23rd September.
  • On 21st March, the South Pole of the earth is towards the sun. As a result; this is spring season in the northern hemisphere and autumn in the southern hemisphere.
  • On 23rd September, the North Pole is towards the sun. As a result; this is spring season in the southern hemisphere and autumn in the northern hemisphere.


  • The summer solstice falls on 21st June, while the winter solstice falls on 22nd December.
  • On 21st June, the North Pole inclines towards the sun and the Tropic of Cancer accepts direct sunlight. As a result, it is the longest day and shortest night in the northern hemisphere. On the contrary, it is the shortest day and longest night in the southern hemisphere. It is summer season in the northern hemisphere and winter in the southern hemisphere.
  • Between 21st March to 23rd September, the North Pole gains sunlight continuously for about six months. So, it is six months day at the North Pole. On the other hand, the South Pole faces six months’ night during this period.
  • On 22nd December, the South Pole inclinations towards the sun and the Tropic of Capricorn receive direct sunlight. As a result, it is the longest day and shortest night in the southern hemisphere. On the contrary, it is the shortest day and longest night in the northern hemisphere. It is summer season in the southern hemisphere and winter in the northern hemisphere.
  • Between 23rd September to 21st March, the South Pole receives sunlight continuously for about six months. So, it is six months day at the South Pole. On the other hand, the North Pole experienced six months’ night during this period.

What Would Happen if the Earth Did Not Rotate?

If there had been no circumvolution of the earth; one half of the earth would have been regularly in daylight and another half would have been regularly in dark. The portion under sunlight would have been too heated. On the other hand, the portion in dark would have been chilling cold. Such excesses of temperature could have made it impossible for life to thrive on our planet.

Geographic Grid System

Geography is about geographical understanding, which expects an accurate grid system to demonstrate absolute and relative location. Absolute position is the exact x– and y– coordinate on the Earth. Relative location is the position of something relative to other items.

For example, when you use your GPS in your smartphone or car, say Google Maps, you put in an absolute location. But as you initiate driving, the device commends you to turn right or left relative to objects on the ground: “Turn left on exit 202” is relative to the other exit positions. Or if you give directions to your house, you often use relative points to help them understand how to get to your house.

Great and Small Circles

Much of Earth’s grid system is based on the position of the North Pole, South Pole, and Equator. The poles are an imaginary line running from the axis of Earth’s gyration. The plane of the equator is an hypothetical horizontal line that cuts the earth into two equal halves. This brings up the topic of great and small circles. A great circle is any circle that partitions the earth into a circumference of two equal divisions.

It’s also the largest circle that can be drawn on a sphere. The line attaching any points along a great circle is also the shortest distance between those two points. Examples of great circles include the Equator, all lines of longitude, the line that partitions the earth into day and night hailed the circle of illumination, and the plane of the ecliptic, which partitions the earth into equal halves along the equator.

Small circles are circles that cut the earth, but not into equal halves. Examples of small circles consist of all lines of latitude except the equator, the Tropical of Cancer, the Tropic of Capricorn, the Arctic Circle, and the Antarctic Circle.

Latitude and Longitude

Many expect that latitude is a line attaching points on the earth and it’s not. Latitude is really an angular measurement north or south of the equator. So 30 degrees north means a point that is 30 degrees north of the equator. Latitude is also indicated in degrees, minutes, and seconds; 360 degrees in a circle, 60 minutes ( ) in a degree, and 60 seconds ( ) in a minute.

Circle Of Illumination Time Zones
Circle Of Illumination Time Zones

When you use Google Earth, the coordinate locations are in this degrees/minutes/seconds pattern. Latitude varies from 0 degrees (equator) to 90 degrees north and south (the poles). A line connecting all points of the same latitude is hailed a parallel because the lines run parallel to each other. The only parallel that is also a great circle is the equator. All other parallels are small circles. The following are the most important parallel lines:

  • Equator, 0 degrees
  • Tropic of Cancer, 23.5 degrees N
  • Tropic of Capricorn, 23.5 degrees S
  • Arctic Circle, 66.5 degrees N
  • Antarctic Circle, 66.5 degrees S
  • North Pole, 90 degrees N (infinitely small circle)
  • South Pole, 90 degrees S (infinitely small circle)

Latitude is also sometimes described as zones of latitude. Some of these zones of latitude include:

  • Low latitude: generally between the equator and 30 degrees N
  • Midlatitude: between 30 degrees and 60 degrees N and S
  • High latitude: latitudes bigger than about 60 degrees N and S
  • Equatorial: within a petty degrees of the equator
  • Tropical: within the tropics (between 23.5 degrees N and 23.5 degrees S
  • Subtropical: slightly pole-ward of the tropics, generally around 25-30 degrees N and S
  • Polar: within a few degrees of the North or South Pole

Longitude is the angular dimension east and west of the Prime Meridian (image on the right). Like latitude, longitude is measured in degrees, minutes, and seconds. Lines attaching equal points of longitude are hailed meridians. But unlike parallels, meridians do not run parallel to each other. Rather they are most far apart from each other at the equator and merge toward each other toward the poles.

The problem with longitude is that there isn’t a natural standard like the equator is for latitude. For over a hundred years, nations used their own “prime meridian” which proved problematic for trade. But in 1883 an international conference in Washington D.C. was held to determine a global prime meridian.

After weeks of debate, the Royal Observatory at Greenwich, England was resolved as the Greenwich Meridian or also hailed the prime meridian for the world. So today, longitude starts at the Prime Meridian and measures east and west of that line.

At 180 degrees of the Prime Meridian in the Pacific Ocean is the International Date Line. The line resolves where the new day rises in the world. Now because of this, the International Date Line is not actually a straight line, rather it pursues national borders so that a country isn’t divided into two separate days (and we think hour time zones are a pain). If you look at the map below, the International Date Line is to the right in a dark, black line. Note how it is drawn to make sure nations are not divided by the International Date Line.

Standard Timezones of the world. Timezones are, in general, partitioned by longitudinal lines.

Time Zones

This is also a marvelous time to take a look at time areas around the world. If you refer back to the map above, you can examine the different time zones in the various colors. Since the earth rotates 360 degrees in a 24 hour duration, the earth rotates 15 degrees every hour constructing 24 time zones.

In an ideal world, each time zone would follow lines of longitude every 15 degrees (7.5 degrees in each direction from the center of the time zone). But because of political borders, time zones are not partitioned up so perfectly and vary greatly in shape and width. Greenwich, England was chosen in the mid-nineteenth century as the origin point of time worldwide.

The logic was because at the time, England was the superpower of the time both militarily and economically. So the meridian that traveled through Greenwich became zero degrees or the prime meridian. Because of the earth’s rotation in allusion to the prime meridian, locations east of the new meridian meant time was ahead while locations west of the meridian were behind in time in reference to Greenwich, England.

Ultimately, when you combine parallel and meridian lines, you end up with a geographic grid system that allows you to determine your exact location on the planet.

Frequently Asked Questions About Circle Of Illumination

Is The Circle Of Illumination A Great Circle?
Great Circles Include The Equator, All Lines Of Longitude, The Line That Divides The Earth Into Day And Night Called The Circle Of Illumination , And The Plane Of Ecliptic, Which Divides The Earth Into Equal Halves Along The Equator. Small Circles Are Circles That Cut The Earth, But Not Into Equal Halves.
How Would You Differentiate The Axis Of The Earth From Its Circle Of Illumination?
Earth’s Axis Refers To An Imaginary Line Going Through The Center Of The Earth From Top To Bottom. Thus, While Circle Of Illumination Separates Light From Darkness And Day From Night, Axis Is A Line Along Which The Earth’s Rotates.
What Does Illumination Circle Mean?
The Imaginary Line That Separates Day From Night On Earth Is Called The Circle Of Illumination. It Is Basically The Region That Is Experiencing Sunshine. The Circle Of Illumination Cuts All Latitudes Into Half On The Spring And Autumnal Equinoxes.
What Is Circle Of Illumination In Short?
The Imaginary Line That Separates Light From The Darkness And Day From The Night Is Known As The Circle Of Illumination. The Circle Of Illumination Bisects The Equator While The Circle Of Illumination Separates Light From Darkness And Day From Night, The Axis Is A Line Along Which The Earth Rotates.