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Weather Almanac for Autumnal EquinoxComes The Fall Our journey through the month of September brings us the Autumnal Equinox, the day when the sun is overhead at the equator and day/night lengths are equal from pole to pole. This event signifies the start of the Autumn or Fall season. Several years ago I was admonished by an Australian colleague for calling the season Fall. "In many parts of the world," she said, "leaves do not fall in this season." However, my research indicates the term Fall was not applied to the season because of the fall of leaves, but referred to the sun falling below the equator (from a Northern Hemisphere perspective). Similarly, in six months, the sun will spring back above the equator for those of us in the North and we enter the Spring season. For those who read my almanac entry in February on Groundhog Day, you will remember that the length of day changes most rapidly at the equinoxes. Similarly, the sun's position in the heavens -- its altitude above the horizon at solar noon -- also changes most rapidly at this time. Observers in the Northern Latitudes see this as a rapid falling of the Sun in the sky during September. (For those south of the equator, this will happen in March.) Many of us in the North lament that summers are so short and winters so long. From an astronomical perspective, however, this is not true. I won't get into a long, mathematical explanation here, but since the earth's orbit around the sun is elliptical rather than circular, Kepler's laws of planetary motion tell us that a planet will move at different speeds at different parts of its orbit. The more elliptical (the less circular) the greater these differences will be. For Planet Earth, at present -- this changes over geological time -- changes in the orbital speed result in Summer being five days longer than Winter. (Our friends at Southern Latitudes have Winter five days longer than Summer.) As the Earth's axis "wobbles" over thousands of years, the lengths of the seasons will change. Indeed, there are periods in geological history when the Earth's tilt, the cause of the seasons, was near zero (perpendicular to the plane of orbit) and there were no seasons. Some climatologists have suggested that ice ages may be triggered when Northern Hemisphere Winter has the extra days. Next month we will look at why some of those leaves turn gorgeous colors before the fall, and what weather factors assist New England and southeastern Canada in the display of awesome autumnal colors. But later this month, we will look at some life forms which ride the air, and do so without wings, the aeroplankton. By the way, Autumn/Fall is 19 hours longer than Winter. Written by
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