W * TE used to be told that the Eskimos live in a land where there are six months of day and six months of night. Even today many are doubtless uncertain how long an Eskimo night really is, though air routes across the Arctic are making this question important, not only to the Eskimos but to all of us. It matters much to a pilot whether daylight will last long enough to see him over a mountain range or across broken ice to a safe landing. He travels so fast that within a few hours he may pass from temperate-zone conditions to the far different ones of the high North. Many activities besides flying also are governed by the length of daylight and darkness in the Arctic-hunting, for example, and sledging, the building of airports and roads, and even farming. Long hours of daylight in the summer enable Northern farmers to grow vegetables of record size. Twelve-pound cabbages have been raised at Good Hope, just below the Arctic Circle, and a 14-pound rutabaga, a I2-pound turnip, and a 7-pound cauliflower at Unalakleet, in about 64' N.' In peacetime the midnight sun was one of the attractions of a trip to the North Cape. In wartime it is one of the hazards of the supply route to Murmansk. Yet a simple chart showing at a glance how many hours of continuous daylight can be expected at a given latitude has never been prepared, so far as the writer is aware. In October, I93I, the Geographical Review published an informative article entitled Seasonal Variations in Daylight, Twilight, and Darkness, by S. W. Boggs of the United States Department of State. Conditions were depicted in separate small charts for nine latitudes both north and south of the equator and for the equator. Information was also given concerning the factors determining the duration of twilight. A different type of diagram was published in the German periodical Arktis in I930.2 It divides the globe seasonally into zones and is useful for determining the general pattem of sunlight, twilight, and darkness, but it does not tell the actual number of hours and minutes of each. This lack caused the writer to undertake the construction of the two accompanying charts. A ruler or some other straightedge is all that is needed to use them. The war has taken thousands of people into the Arctic for the first time. A man working on a new airport or some other defense project may ponder over the calendar to his wit's end without discovering when daylight will become so short as to curtail operations. The mathematical determination is by no means simple or rapid. Suppose you want to know how long the sun will stay above the horizon at Reykjavik, Iceland, on May 20. Reykjavik is at latitude 64'. Find the line for May 20 under the date, and follow it down to the point at which it crosses the 64' curve. From this point lay a straightedge so that it falls across the small X corresponding to the North Pole. The straightedge touches the semicircular scale at the top at I8 hours 50 minutes-the answer.