<P> The fact that the Sun is always so close to the vertical at noon on the Equator (never being more than 23.5 ° north or south) explains why equatorial regions are the hottest regions on the planet overall . Additionally, sunrise and sunset always occur very quickly at the Equator, because the Sun's path across the sky is so nearly vertical with respect to the horizon; at the equinox, the Sun requires only two minutes to traverse the horizon at sunrise and sunsets . </P> <P> The tropics occupy a zone of Earth's surface between 23.5 ° north and 23.5 ° south of the Equator . Within this zone, the Sun will pass (or culminate) almost directly overhead on at least one day per year . The line of 23.5 ° north latitude is called the Tropic of Cancer, because when it was named, the Sun passed overhead at this location at the time of year when it was within the zodiac sign of Cancer . The equivalent line of south latitude is called the Tropic of Capricorn, for similar reasons . If they had been named today using the constellation in which the sun is currently in at the time it is directly overhead the tropic line, they would have been called, respectively, the Tropics of Gemini and Sagittarius . The sun enters and leaves each sign of the zodiac slightly later each year at the rate of about 1 day every 72 years . For more information, see precession of the equinoxes . </P> <P> On the Tropical Circles, the Sun is directly overhead only once per year, on the corresponding solstice . At latitudes closer to the Equator and on the Equator itself, it will be overhead twice per year (on the equinoxes in the case of the Equator). Outside the tropics, the Sun never passes directly overhead . </P> <P> Around the poles, which coincide with the rotational axis of Earth as it passes through the surface, the seasonal variations in the length of daytime are extreme . In fact, within 23.5 ° latitude of the poles, there will be at least some days each year during which the sun never goes below the horizon . There will also be days when the Sun never rises above the horizon . This number will be fewer, but close to the number of days in the summer where the sun doesn't set (for example the sunrise is usually a few days before the spring equinox and extends a few days past the fall equinox). This phenomenon of more daylight than night is not unique to the poles . In fact, at any given time slightly more than half of the earth is in daylight . The 24 hours of summer daylight is known as the midnight sun that is famous in some northern countries . To the north, the Arctic Circle marks this 23.5 ° boundary . To the south, the Antarctic Circle marks the boundary . These boundaries correspond to 66.5 ° north or south latitude, respectively . Because the Sun's disc itself is about half a degree in diameter and is very bright, truly dark days during which the sun never seems to rise are only seen beyond 72 ° north or south latitude . </P>

Which days are same everywhere on earth in terms of hours of sunlight and darkness