<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> <P> At and near the poles, the Sun never rises very high above the horizon, even in summer, which is one of reasons why these regions of the world are consistently cold in all seasons (others include the effect of albedo, the relative increased reflection of solar radiation of snow and ice). Even at the summer solstice, when the Sun reaches its highest point above the horizon at noon, it is still only 23.5 ° above the horizon at the poles . Additionally, as one approaches the poles the apparent path of the Sun through the sky each day diverges increasingly from the vertical . As summer approaches, the Sun rises and sets become more northerly in the north and more southerly in the south . At the poles, the path of the Sun is indeed a circle, which is roughly equidistant above the horizon for the entire duration of the daytime period on any given day . The circle gradually sinks below the horizon as winter approaches, and gradually rises above it as summer approaches . At the poles, apparent sunrise and sunset may last for several days . </P> <P> At middle latitudes, far from both the Equator and the poles, variations in the length of daytime are moderate . In the higher middle latitudes where Montreal, Paris and Ushuaia are located, the difference in the length of the day from summer to winter can be very noticeable: the sky may still be lit at 10 pm in summer, but may be dark at 5 pm in winter . In the lower middle latitudes where Southern California, Egypt and South Africa are located, the seasonal difference is smaller, but still results in approximately 4 hours difference in daylight between the winter and summer solstices . The difference becomes less pronounced the closer one gets to the equator . An approximation to the monthly change can be obtained from the rule of 12ths . With 4 hours change over the six months from winter to summer the day lengthens by about 4 * 1 / 12 of an hour (20 mins) in the first month, 4 * 2 / 12 (40 mins) in the second month, 4 * 3 / 12 (1 hr) in the third month, then 1 hr, 40 mins and 20 mins in the fourth, fifth and six months . </P> <P> Also in the middle latitudes, the seasonal climate variations produced by changes in the length of daytime are the most marked, with very distinct periods of cold and heat, and other secondary seasonal changes such as snow and ice in winter that disappear in summer and so on . At high latitudes, it is cold most of the time, with constant snow and ice, so the seasons are less obvious; and in the tropics, it is hot most of the time, with no snow or ice at all, so again the seasons are less obvious . </P>

4. which latitudes experience the greatest annual change in daylight hours