<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> <P> The exact instant of solar noon, when the Sun reaches its highest point in the sky, varies with the seasons . This variation is called the equation of time; the magnitude of variation is about 30 minutes over the course of a year . </P>

A day when the sun is directly over the equator at solar noon is referred to as what