Q: Why are the days shorter in the winter?

A: Short winter days are one of the seasonal effects resulting from the tilt of the earth's axis. The sun illuminates only one half of earth at a time while the other half is dark. As the earth rotates under the sun, everywhere on its surface crosses the circle of illumination twice a day, experiencing dawn or dusk with each crossing. If the earth's axis were not tilted, there would be no seasons. The sun would be directly over the equator all of the time and it would rise due east and set due west everywhere on earth all of the time. The circle of illumination would pass through both poles and everywhere on earth would have twelve hours of day and twelve hours of night. As a result of the earth's tilt, the circle of illumination is alsotilted and does not pass through the poles. Each pole points away from the sun for half the year and toward it for the other half. Viewed from space the tilted circle of illumination crosses lines of longitude. Locations in the lighted hemisphere will be in daylight while their complements on the same longitude in the shaded half are already in darkness. So in the summer hemisphere days are more than twelve hours long, while in the winter hemisphere days are less. Viewed from earth, the arc of the sun's path through the daytime sky is longer in the summer and shorter in the winter and the noonday sun is higher above the horizon in summer than in winter. As the earth revolves around the sun its rotational axis continues to point in the same direction relative to the stars which causes the direction to the sun to change gradually, about one degree per day. This causes the noonday sun to appear to oscillate northward and southward during the year as it also causes the location of sunrise and sunset to do the same on the horizon. The closer one is to the equator, the less variation between the length of day and night from one season to another. In fact, the equator has equal twelve hour days and nights year round while the poles alternate between six month of daylight in the summer and six months of darkness in winter.

Richard Brill is assistant professor of science at Honolulu Community College where he teaches earth and physical science. Send questions to him at Honolulu Community College, 874 Dillingham Blvd., Honolulu, HI 96817 or email to rickb@hcc.hawaii.edu

Short Winter Days ©1996 Richard C. Brill