In astronomy, declination is one of the two angles that locate a point on the celestial sphere in the equatorial coordinate system, the other being hour angle. The declination angle is measured north (positive) or south (negative) of the celestial equator, along the hour circle passing through the point in question.
A solstice is the time when the Sun reaches its most northerly or southerly excursion relative to the celestial equator on the celestial sphere. Two solstices occur annually, around 20-22 June and 20-22 December. In many countries, the seasons of the year are defined by reference to the solstices and the equinoxes.
The Tropic of Cancer, also known as the Northern Tropic, is the Earth's northernmost circle of latitude where the Sun can be seen directly overhead. This occurs on the June solstice, when the Northern Hemisphere is tilted toward the Sun to its maximum extent. It also reaches 90 degrees below the horizon at solar midnight on the December Solstice. Using a continuously updated formula, the circle is currently 23°26′09.8″ (or 23.43607°) north of the Equator.
The Tropic of Capricorn is the circle of latitude that contains the subsolar point at the December solstice. It is thus the southernmost latitude where the Sun can be seen directly overhead. It also reaches 90 degrees below the horizon at solar midnight on the June Solstice. Its northern equivalent is the Tropic of Cancer.
A circle of latitude or line of latitude on Earth is an abstract east–west small circle connecting all locations around Earth at a given latitude coordinate line.
A polar circle is a geographic term for a conditional circular line (arc) referring either to the Arctic Circle or the Antarctic Circle. These are two of the keynote circles of latitude (parallels). On Earth, the Arctic Circle is currently drifting northwards at a speed of about 14.5 m per year and is now at a mean latitude of 66°33′50.2″ N; the Antarctic Circle is currently drifting southwards at a speed of about 14.5 m per year and is now at a mean latitude of 66°33′50.2″ S. Polar circles are often equated with polar regions of Earth. Due to their inherent climate environment, the bulk of the Arctic Circle, much of which is sea, is sparsely settled whereas this applies to all of Antarctica which is mainly land and sheltered ice shelves.
Midnight sun, also known as polar day, is a natural phenomenon that occurs in the summer months in places north of the Arctic Circle or south of the Antarctic Circle, when the Sun remains visible at the local midnight. When midnight sun is seen in the Arctic, the Sun appears to move from left to right. In Antarctica, the equivalent apparent motion is from right to left. This occurs at latitudes ranging from approximately 65°44' to exactly 90° north or south, and does not stop exactly at the Arctic Circle or the Antarctic Circle, due to refraction.
Polar night is a phenomenon that occurs in the northernmost and southernmost regions of Earth when the Sun remains below the horizon for more than 24 hours. This only occurs inside the polar circles. The opposite phenomenon, polar day or midnight sun, occurs when the Sun remains above the horizon for more than 24 hours.
The five main latitude regions of Earth's surface comprise geographical zones, divided by the major circles of latitude. The differences between them relate to climate. They are as follows:
- The North Frigid Zone, between the North Pole at 90° N and the Arctic Circle at 66°33′50.2″ N, covers 4.12% of Earth's surface.
- The North Temperate Zone, between the Arctic Circle at 66°33′50.2″ N and the Tropic of Cancer at 23°26′09.8″ N, covers 25.99% of Earth's surface.
- The Torrid Zone, between the Tropic of Cancer at 23°26′09.8″ N and the Tropic of Capricorn at 23°26′09.8″ S, covers 39.78% of Earth's surface.
- The South Temperate Zone, between the Tropic of Capricorn at 23°26′09.8″ S and the Antarctic Circle at 66°33′50.2″ S, covers 25.99% of Earth's surface.
- The South Frigid Zone, from the Antarctic Circle at 66°33′50.2″ S and the South Pole at 90° S, covers 4.12% of Earth's surface.
Daytime or day as observed on Earth is the period of the day during which a given location experiences natural illumination from direct sunlight. Daytime occurs when the Sun appears above the local horizon, that is, anywhere on the globe's hemisphere facing the Sun. In direct sunlight the movement of the sun can be recorded and observed using a sundial that casts a shadow that slowly moves during the day. Other planets and natural satellites that rotate relative to a luminous primary body, such as a local star, also experience daytime, but this article primarily discusses daytime on Earth.
Sun path, sometimes also called day arc, refers to the daily and seasonal arc-like path that the Sun appears to follow across the sky as the Earth rotates and orbits the Sun. The Sun's path affects the length of daytime experienced and amount of daylight received along a certain latitude during a given season.
The 66th parallel north is a circle of latitude that is 66 degrees north of the Earth's equatorial plane, about 61 kilometres (38 mi) south of the Arctic Circle. It crosses the Atlantic Ocean, Europe, Asia and North America.
The 72nd parallel north is a circle of latitude that is 72 degrees north of the Earth's equatorial plane, in the Arctic. It crosses the Atlantic Ocean, Europe, Asia, the Arctic Ocean and North America.
The equator is a circle of latitude that divides a spheroid, such as Earth, into the Northern and Southern hemispheres. On Earth, the Equator is an imaginary line located at 0 degrees latitude, about 40,075 km (24,901 mi) in circumference, halfway between the North and South poles. The term can also be used for any other celestial body that is roughly spherical.
The 23rd parallel north is a circle of latitude that is 23 degrees north of the Earth's equatorial plane, about 50 kilometres (31 mi) south of the Tropic of Cancer. It crosses Africa, Asia, the Indian Ocean, the Pacific Ocean, North America, the Caribbean, and the Atlantic Ocean.
The 66th parallel south is a circle of latitude that is 66 degrees south of the Earth's equatorial plane, about 61 km north of the Antarctic Circle. It crosses the Southern Ocean and Antarctica.
The 67th parallel south is a circle of latitude that is 67 degrees south of the Earth's equatorial plane, about 50 km south of the Antarctic Circle. It crosses the Southern Ocean and Antarctica.
The 68th parallel south is a circle of latitude that is 68 degrees south of the Earth's equatorial plane, in the Antarctic. It crosses the Southern Ocean and Antarctica. At this latitude, the sun is visible for 24 hours, 0 minutes during the December Solstice, and civil twilight during the June Solstice.
The 72nd parallel south is a circle of latitude that is 72 degrees south of the Earth's equatorial plane in the Antarctic. The parallel passes through the Southern Ocean and Antarctica.
The Arctic Circle is one of the two polar circles, and the most northerly of the five major circles of latitude as shown on maps of Earth at about 66° 34' N. Its southern equivalent is the Antarctic Circle.
