The illumination of the Earth is regulated by its daily rotation around its own axis as well as by its yearly elliptical orbit around the Sun.

As it revolves around the Sun, the axis of the Earth maintains an almost constant angle of 66 ½ degrees.

The different positions of the Earth in relation to the Sun, and differences in latitude, cause seasonal differences in radiation.

The division of the Earth into climatic zones - the Frigid (or Arctic) Zones, the Temperate Zones and the Torrid (or Tropical) Zone - comes from these differences in radiation.

Dawn and dusk are transition periods between night and day and day and night.

Twilight arcs are caused by the reflection of the Sun's rays, when the Sun is below the horizon, on various unstable strata in the atmosphere.

The first, or bright twilight arc disappears on the horizon or appears when the Sun's position is 8 degrees below the horizon. This moment is the end of "normal" dusk or the beginning of "normal" dawn.

The stratum that causes this bright twilight arc lies at a height of 11 to 12km and is the boundary of the stratosphere, the tropopause.

When the Sun is in a position 17 degrees to 18 degrees below the horizon, the second, or main twilight arc disappears below the western horizon.

In the morning, this twilight arc appears on the eastern horizon.

This moment is the end or the beginning of "astronomical" dusk or dawn.

Between its end and start, there is complete darkness. During this period, if there is no interference with artificial lighting, stars can be observed with the naked eye.

In spite of this, the disappearance of another twilight arc can still be observed when the position of the Sun is 24 degrees below the horizon.

The twilight strata in the atmosphere that cause these phenomena lie at a height of around 60km (the stratopause) and at 130km.

The daily orbit of the Sun is very steep in the Torrid Zone. Therefore, dawn there is very short, about 20 to 25 minutes. The difference in the length of the days throughout the year is relatively small. Day and night are of almost equal length.

In the latitudes between the Tropic of Cancer and the Tropic of Capricorn (23 degrees North and 23 degrees South) the Sun is at its zenith twice a year.

As the distance from the equator grows, these two zenith points move closer and closer together. This is an important factor in the distribution of the tropical rainy and dry seasons.

The length of day at the polar circles in the summer of the respective hemispheres is twenty-four hours, when the Sun is at its highest position.

Towards the poles, this length of day increases to six months of uninterrupted daylight, while the Sun apparently rotates around the horizon for three months in a rising spiral and after that in a declining spiral for the same length of time.

In the winter, the Sun sinks below the horizon for an almost equally long time.

Certain slight variations are caused by the refraction of the Sun's rays near the horizon. However, the darkness of the winter is shortened considerably by the dawn, especially in areas near the poles where the apparent spiral of the Sun (yearly orbit of the Sun) shows the smallest angle of inclination.

The distance of the Sun from the eastern or western point, the so-called morning or evening distance, is not the same for all places on the same day; it varies with the geographical latitude.

Thus, the length of day and night also changes with latitude.