Imaginary Lines of Latitudes
Figure 1 below shows some important imaginary lines of latitude, namely the equator, the Tropic of Cancer, the Tropic of Capricorn, as well as the Arctic and Antarctic Circles. The equator is located at zero degrees latitude. The equator runs through Indonesia, Ecuador, northern Brazil, Columbia, Congo, Somalia, Uganda, Gabon, and Kenya. On the equator, the sun is directly overhead at noon on the two equinoxes, i.e. near March and September 21, see the Figure of the declination angle below. The equator divides the planet into the Northern and Southern Hemispheres. On the equator, the length of day and night are equal every day of the year, i.e. the day is always twelve hours long and night is always twelve hours long.
Fig.1. Imaginary lines of latitude.
Fig.2. Solar declination angle.
The Tropic of Cancer and the Tropic of Capricorn each lie at 23.5 degrees latitude. The Tropic of Cancer is located at 23.5° N of the equator and runs through Mexico, the Bahamas, Egypt, Saudi Arabia, India, and southern China. The Tropic of Capricorn lies at 23.5° South of the equator and runs through Australia, Chile, southern Brazil (Brazil is the only country that passes through both the equator and a tropic), and northern South Africa.
The tropics are the two lines where the sun is directly overhead at noon on the two solstices, i.e. near June and December 21, see Fig.2. The sun is directly overhead at noon on the Tropic of Cancer on June 21 (the beginning of summer in the Northern Hemisphere and the beginning of winter in the Southern Hemisphere) and the sun is directly overhead at noon on the Tropic of Capricorn on December 21 (the beginning of winter in the Northern Hemisphere and the beginning of summer in the Southern Hemisphere).
The reason for the location of the Tropic of Cancer and the Tropic of Capricorn at 23.5° north and south respectively is due to the axial tilt of the Earth. The Earth is titled 23.5° from the plane of the Earth's revolution around the sun each year.
The area bounded by the Tropic of Cancer on the north and Tropic of Capricorn on the south is known as the "tropics." This area does not experience seasons because the sun is always high in the sky. Only higher latitudes, north of the Tropic of Cancer and south of the Tropic of Capricorn, experience significant seasonal variation in climate.
A polar circle is either the Arctic Circle or the Antarctic Circles. The Arctic Circle is located at a latitude of 66° 33' 44'' N, and the Antarctic Circles is located at a latitude of 66° 33' 44'' S. Areas between each polar circle and its associated pole (North Pole or South Pole), known geographically as the "frigid" zones, would theoretically experience at least one 24 hour period when the sun is continuously above the horizon and at least one 24 hour period when the sun is continuously below the horizon annually. However, due to atmospheric refraction and the sun being an extended object rather than a point source, the continuous daylight area is somewhat extended while the continuous darkness area is somewhat reduced.
A phenomenon when the sun is continuously above the horizon for 24 hour period (day and night) is called as midnight sun. This phenomenon is beautifully illustrated in the figure below. When it occurs, strictly speaking only Duhr exists according to general rule in the Shari'ah.
Fig.3. The midnight sun.