July 6, 2022

The equator is the maximum circumference of the surface of a celestial body perpendicular to the axis of rotation and therefore equidistant from the poles. The latitude of the equator is, by definition, equal to 0 ° 0′0 ″. The equator divides a celestial body into two hemispheres commonly called the northern hemisphere and the southern hemisphere.

The terrestrial equator

The Earth's equator is the imaginary line formed by the intersection of the Earth's surface with a plane perpendicular to the Earth's rotation axis and passing through the center. The equator divides the Earth into two hemispheres; the one containing the North Pole is called the northern hemisphere (or terrestrial northern hemisphere), while the one containing the South Pole is called the southern hemisphere (or terrestrial southern hemisphere). Points located on the equator are assigned latitude 0. The terrestrial equator is, with the Arctic Circle, Tropic of Cancer, Tropic of Capricorn and Antarctic Circle, one of the five parallels of reference and is the longest ever. The length of the Earth's equator is approximately 40076 km. The (geodesic) distance between the equator and one of the two poles is therefore approximately 10 000 km. At the equator the Sun is at its zenith at noon on the equinoxes (twice a year) and the day and night always have an almost identical duration. The belt around the equator, bounded by the two tropics, is the one in which the Sun, at certain times of the year, is at its true zenith, i.e. exactly 90 degrees of elevation above the horizon in the precise center of the solar disk ( even if the term zenith is used for the maximum daily and annual solar declination of each locality). Along the imaginary equatorial line the Sun rises every day just before 6:00 and sets just after 18:00 (excluding the exact time for each place always dictated by the longitude within the time zone and possibly by the application of the summer time throughout the year); therefore the day technically lasts imperceptibly more than twelve hours. This is due to the curvature of the Earth and the presence of the atmosphere, in fact in every place in the world the Sun must reach a negative declination on the horizon to set completely behind it. This declination for the apparent diameter of the solar disk together with the presence of the atmosphere that diffuses the light equates on average to −0.833 degrees. The same passage is made at dawn, in the opposite direction, when the Sun passes from −0.833 degrees to 0.27 degrees to rise entirely above the horizon. The day, on the imaginary equatorial line, technically varies from just under 12 hours 6 and a half minutes to just over 12 hours 7 and a half minutes. The maximum inclination of the rays is constantly very strong and varies from 66.56 hundredths of a degree to 90 degrees (reached twice in the exact center of the solar disk during the equinoxes) throughout the year. The apparent parabola of the solar disk is strongly accentuated in height and much less in width within the 360 ​​degrees of the "celestial dome" (or celestial vault). In the meridian hour the Sun extremely high in the sky transits, almost at the same elevation, from the East to the West of the celestial coordinates passing through the azimuth 180 (midday) or the azimuth 0 according to the time of the year (peak in the North ), to then rise again to peak. Only from the equator is the entire sky visible, and there are no circumpolar stars: all the stars, therefore, set.


The climate of the equatorial belt has high and stable average temperatures all year round and generally with a low difference between minimum and maximum temperatures, very high average relative humidity, daily and abundant rainfall. As we move away from the equator, the rains are less frequent. Thanks to the constant rains, the equatorial forest was formed.

Territories crossed by the equator

The equator crosses the lands and / or seas of 13 territories; these are: São Tomé and Prínc